|  | Home | Libraries | People | FAQ | More | 
Most compilers have front ends and back ends. The front end parses the text of an input program into some intermediate form like an abstract syntax tree, and the back end takes the intermediate form and generates an executable from it.
A library built with Proto is essentially a compiler for an embedded domain-specific language (EDSL). It also has a front end, an intermediate form, and a back end. The front end is comprised of the symbols (a.k.a., terminals), members, operators and functions that make up the user-visible aspects of the EDSL. The back end is made of evaluation contexts and transforms that give meaning and behavior to the expression templates generated by the front end. In between is the intermediate form: the expression template itself, which is an abstract syntax tree in a very real sense.
To build a library with Proto, you will first decide what your interface will be; that is, you'll design a programming language for your domain and build the front end with tools provided by Proto. Then you'll design the back end by writing evaluation contexts and/or transforms that accept expression templates and do interesting things with them.
This users' guide is organized as follows. After a Getting Started guide, we'll cover the tools Proto provides for defining and manipulating the three major parts of a compiler:
How to define the aspects of your EDSL with which your users will interact directly.
What Proto expression templates look like, how to discover their structure and access their constituents.
How to define evaluation contexts and transforms that make expression templates do interesting things.
After that, you may be interested in seeing some Examples to get a better idea of how the pieces all fit together.
You can get Proto by downloading Boost (Proto is in version 1.37 and later), or by accessing Boost's SVN repository on SourceForge.net. Just go to http://svn.boost.org/trac/boost/wiki/BoostSubversion and follow the instructions there for anonymous SVN access.
          Proto is a header-only template library, which means you don't need to
          alter your build scripts or link to any separate lib file to use it. All
          you need to do is #include
          <boost/proto/proto.hpp>. Or, you might decide to just include
          the core of Proto (#include
          <boost/proto/core.hpp>) and whichever contexts and transforms
          you happen to use.
        
Proto depends on Boost. You must use either Boost version 1.34.1 or higher, or the version in SVN trunk.
Currently, Boost.Proto is known to work on the following compilers:
| ![[Note]](../../../doc/src/images/note.png) | Note | 
|---|---|
| Please send any questions, comments and bug reports to eric <at> boostpro <dot> com. | 
Proto is a large library and probably quite unlike any library you've used before. Proto uses some consistent naming conventions to make it easier to navigate, and they're described below.
          All of Proto's functions are defined in the boost::proto
          namespace. For example, there is a function called value() defined in boost::proto
          that accepts a terminal expression and returns the terminal's value.
        
          Proto defines metafunctions that correspond to each
          of Proto's free functions. The metafunctions are used to compute the functions'
          return types. All of Proto's metafunctions live in the boost::proto::result_of
          namespace and have the same name as the functions to which they correspond.
          For instance, there is a class template boost::proto::result_of::value<> that you can use to compute the
          return type of the boost::proto::value() function.
        
          Proto defines function object equivalents of all of
          its free functions. (A function object is an instance of a class type that
          defines an operator()
          member function.) All of Proto's function object types are defined in the
          boost::proto::functional namespace and have the same
          name as their corresponding free functions. For example, boost::proto::functional::value is a class that defines a function
          object that does the same thing as the boost::proto::value() free function.
        
          Proto also defines primitive transforms -- class types
          that can be used to compose larger transforms for manipulating expression
          trees. Many of Proto's free functions have corresponding primitive transforms.
          These live in the boost::proto
          namespace and their names have a leading underscore. For instance, the
          transform corresponding to the value() function is called boost::proto::_value.
        
The following table summarizes the discussion above:
Table 32.1. Proto Naming Conventions
| Entity | Example | 
|---|---|
| Free Function | 
                     | 
| Metafunction | 
                     | 
| Function Object | 
                     | 
| Transform | 
                     | 
Below is a very simple program that uses Proto to build an expression template and then execute it.
#include <iostream> #include <boost/proto/proto.hpp> #include <boost/typeof/std/ostream.hpp> using namespace boost; proto::terminal< std::ostream & >::type cout_ = { std::cout }; template< typename Expr > void evaluate( Expr const & expr ) { proto::default_context ctx; proto::eval(expr, ctx); } int main() { evaluate( cout_ << "hello" << ',' << " world" ); return 0; }
This program outputs the following:
hello, world
          This program builds an object representing the output operation and passes
          it to an evaluate()
          function, which then executes it.
        
The basic idea of expression templates is to overload all the operators so that, rather than evaluating the expression immediately, they build a tree-like representation of the expression so that it can be evaluated later. For each operator in an expression, at least one operand must be Protofied in order for Proto's operator overloads to be found. In the expression ...
cout_ << "hello" << ',' << " world"
          ... the Protofied sub-expression is cout_,
          which is the Proto-ification of std::cout.
          The presence of cout_ "infects"
          the expression, and brings Proto's tree-building operator overloads into
          consideration. Any literals in the expression are then Protofied by wrapping
          them in a Proto terminal before they are combined into larger Proto expressions.
        
          Once Proto's operator overloads have built the expression tree, the expression
          can be lazily evaluated later by walking the tree. That is what proto::eval()
          does. It is a general tree-walking expression evaluator, whose behavior
          is customizable via a context parameter. The use of
          proto::default_context
          assigns the standard meanings to the operators in the expression. (By using
          a different context, you could give the operators in your expressions different
          semantics. By default, Proto makes no assumptions about what operators
          actually mean.)
        
Before we continue, let's use the above example to illustrate an important design principle of Proto's. The expression template created in the hello world example is totally general and abstract. It is not tied in any way to any particular domain or application, nor does it have any particular meaning or behavior on its own, until it is evaluated in a context. Expression templates are really just heterogeneous trees, which might mean something in one domain, and something else entirely in a different one.
As we'll see later, there is a way to create Proto expression trees that are not purely abstract, and that have meaning and behaviors independent of any context. There is also a way to control which operators are overloaded for your particular domain. But that is not the default behavior. We'll see later why the default is often a good thing.
          "Hello, world" is nice, but it doesn't get you very far. Let's
          use Proto to build a EDSL (embedded domain-specific language) for a lazily-evaluated
          calculator. We'll see how to define the terminals in your mini-language,
          how to compose them into larger expressions, and how to define an evaluation
          context so that your expressions can do useful work. When we're done, we'll
          have a mini-language that will allow us to declare a lazily-evaluated arithmetic
          expression, such as (_2
          - _1) / _2
          * 100,
          where _1 and _2 are placeholders for values to be
          passed in when the expression is evaluated.
        
          The first order of business is to define the placeholders _1 and _2.
          For that, we'll use the proto::terminal<>
          metafunction.
        
// Define a placeholder type template<int I> struct placeholder {}; // Define the Protofied placeholder terminals proto::terminal<placeholder<0> >::type const _1 = {{}}; proto::terminal<placeholder<1> >::type const _2 = {{}};
          The initialization may look a little odd at first, but there is a good
          reason for doing things this way. The objects _1
          and _2 above do not require
          run-time construction -- they are statically initialized,
          which means they are essentially initialized at compile time. See the
          Static
          Initialization section in the Rationale
          appendix for more information.
        
Now that we have terminals, we can use Proto's operator overloads to combine these terminals into larger expressions. So, for instance, we can immediately say things like:
// This builds an expression template (_2 - _1) / _2 * 100;
This creates an expression tree with a node for each operator. The type of the resulting object is large and complex, but we are not terribly interested in it right now.
So far, the object is just a tree representing the expression. It has no behavior. In particular, it is not yet a calculator. Below we'll see how to make it a calculator by defining an evaluation context.
No doubt you want your expression templates to actually do something. One approach is to define an evaluation context. The context is like a function object that associates behaviors with the node types in your expression tree. The following example should make it clear. It is explained below.
struct calculator_context : proto::callable_context< calculator_context const > { // Values to replace the placeholders std::vector<double> args; // Define the result type of the calculator. // (This makes the calculator_context "callable".) typedef double result_type; // Handle the placeholders: template<int I> double operator()(proto::tag::terminal, placeholder<I>) const { return this->args[I]; } };
          In calculator_context,
          we specify how Proto should evaluate the placeholder terminals by defining
          the appropriate overloads of the function call operator. For any other
          nodes in the expression tree (e.g., arithmetic operations or non-placeholder
          terminals), Proto will evaluate the expression in the "default"
          way. For example, a binary plus node is evaluated by first evaluating the
          left and right operands and adding the results. Proto's default evaluator
          uses the Boost.Typeof
          library to compute return types.
        
Now that we have an evaluation context for our calculator, we can use it to evaluate our arithmetic expressions, as below:
calculator_context ctx; ctx.args.push_back(45); // the value of _1 is 45 ctx.args.push_back(50); // the value of _2 is 50 // Create an arithmetic expression and immediately evaluate it double d = proto::eval( (_2 - _1) / _2 * 100, ctx ); // This prints "10" std::cout << d << std::endl;
Later, we'll see how to define more interesting evaluation contexts and expression transforms that give you total control over how your expressions are evaluated.
          Our calculator EDSL is already pretty useful, and for many EDSL scenarios,
          no more would be needed. But let's keep going. Imagine how much nicer it
          would be if all calculator expressions overloaded operator() so that they could be used as function
          objects. We can do that by creating a calculator domain
          and telling Proto that all expressions in the calculator domain have extra
          members. Here is how to define a calculator domain:
        
// Forward-declare an expression wrapper template<typename Expr> struct calculator; // Define a calculator domain. Expression within // the calculator domain will be wrapped in the // calculator<> expression wrapper. struct calculator_domain : proto::domain< proto::generator<calculator> > {};
          The calculator<>
          type will be an expression wrapper. It will behave just like the expression
          that it wraps, but it will have extra member functions that we will define.
          The calculator_domain is
          what informs Proto about our wrapper. It is used below in the definition
          of calculator<>.
          Read on for a description.
        
// Define a calculator expression wrapper. It behaves just like // the expression it wraps, but with an extra operator() member // function that evaluates the expression. template<typename Expr> struct calculator : proto::extends<Expr, calculator<Expr>, calculator_domain> { typedef proto::extends<Expr, calculator<Expr>, calculator_domain> base_type; calculator(Expr const &expr = Expr()) : base_type(expr) {} typedef double result_type; // Overload operator() to invoke proto::eval() with // our calculator_context. double operator()(double a1 = 0, double a2 = 0) const { calculator_context ctx; ctx.args.push_back(a1); ctx.args.push_back(a2); return proto::eval(*this, ctx); } };
          The calculator<>
          struct is an expression extension. It uses proto::extends<>
          to effectively add additional members to an expression type. When composing
          larger expressions from smaller ones, Proto notes what domain the smaller
          expressions are in. The larger expression is in the same domain and is
          automatically wrapped in the domain's extension wrapper.
        
          All that remains to be done is to put our placeholders in the calculator
          domain. We do that by wrapping them in our calculator<> wrapper, as below:
        
// Define the Protofied placeholder terminals, in the // calculator domain. calculator<proto::terminal<placeholder<0> >::type> const _1; calculator<proto::terminal<placeholder<1> >::type> const _2;
          Any larger expression that contain these placeholders will automatically
          be wrapped in the calculator<> wrapper and have our operator()
          overload. That means we can use them as function objects as follows.
        
double result = ((_2 - _1) / _2 * 100)(45.0, 50.0); assert(result == (50.0 - 45.0) / 50.0 * 100));
Since calculator expressions are now valid function objects, we can use them with standard algorithms, as shown below:
double a1[4] = { 56, 84, 37, 69 }; double a2[4] = { 65, 120, 60, 70 }; double a3[4] = { 0 }; // Use std::transform() and a calculator expression // to calculate percentages given two input sequences: std::transform(a1, a1+4, a2, a3, (_2 - _1) / _2 * 100);
Now, let's use the calculator example to explore some other useful features of Proto.
          You may have noticed that you didn't have to define an overloaded operator-()
          or operator/()
          -- Proto defined them for you. In fact, Proto overloads all
          the operators for you, even though they may not mean anything in your domain-specific
          language. That means it may be possible to create expressions that are
          invalid in your domain. You can detect invalid expressions with Proto by
          defining the grammar of your domain-specific language.
        
For simplicity, assume that our calculator EDSL should only allow addition, subtraction, multiplication and division. Any expression involving any other operator is invalid. Using Proto, we can state this requirement by defining the grammar of the calculator EDSL. It looks as follows:
// Define the grammar of calculator expressions struct calculator_grammar : proto::or_< proto::plus< calculator_grammar, calculator_grammar > , proto::minus< calculator_grammar, calculator_grammar > , proto::multiplies< calculator_grammar, calculator_grammar > , proto::divides< calculator_grammar, calculator_grammar > , proto::terminal< proto::_ > > {};
          You can read the above grammar as follows: an expression tree conforms
          to the calculator grammar if it is a binary plus, minus, multiplies or
          divides node, where both child nodes also conform to the calculator grammar;
          or if it is a terminal. In a Proto grammar, proto::_ is a wildcard that matches
          any type, so proto::terminal<
          proto::_ >
          matches any terminal, whether it is a placeholder or a literal.
        
| ![[Note]](../../../doc/src/images/note.png) | Note | 
|---|---|
| This grammar is actually a little looser than we would like. Only placeholders and literals that are convertible to doubles are valid terminals. Later on we'll see how to express things like that in Proto grammars. | 
          Once you have defined the grammar of your EDSL, you can use the proto::matches<> metafunction to check
          whether a given expression type conforms to the grammar. For instance,
          we might add the following to our calculator::operator() overload:
        
template<typename Expr> struct calculator : proto::extends< /* ... as before ... */ > { /* ... */ double operator()(double a1 = 0, double a2 = 0) const { // Check here that the expression we are about to // evaluate actually conforms to the calculator grammar. BOOST_MPL_ASSERT((proto::matches<Expr, calculator_grammar>)); /* ... */ } };
          The addition of the BOOST_MPL_ASSERT() line enforces at compile time that we
          only evaluate expressions that conform to the calculator EDSL's grammar.
          With Proto grammars, proto::matches<> and BOOST_MPL_ASSERT() it is very easy to give the users of
          your EDSL short and readable compile-time errors when they accidentally
          misuse your EDSL.
        
| ![[Note]](../../../doc/src/images/note.png) | Note | 
|---|---|
| 
             | 
          Grammars and proto::matches<>
          make it possible to detect when a user has created an invalid expression
          and issue a compile-time error. But what if you want to prevent users from
          creating invalid expressions in the first place? By using grammars and
          domains together, you can disable any of Proto's operator overloads that
          would create an invalid expression. It is as simple as specifying the EDSL's
          grammar when you define the domain, as shown below:
        
// Define a calculator domain. Expression within // the calculator domain will be wrapped in the // calculator<> expression wrapper. // NEW: Any operator overloads that would create an // expression that does not conform to the // calculator grammar is automatically disabled. struct calculator_domain : proto::domain< proto::generator<calculator>, calculator_grammar > {};
          The only thing we changed is we added calculator_grammar
          as the second template parameter to the proto::domain<> template when defining calculator_domain. With this simple addition,
          we disable any of Proto's operator overloads that would create an invalid
          calculator expression.
        
Hopefully, this gives you an idea of what sorts of things Proto can do for you. But this only scratches the surface. The rest of this users' guide will describe all these features and others in more detail.
Happy metaprogramming!
Here is the fun part: designing your own mini-programming language. In this section we'll talk about the nuts and bolts of designing an EDSL interface using Proto. We'll cover the definition of terminals and lazy functions that the users of your EDSL will get to program with. We'll also talk about Proto's expression template-building operator overloads, and about ways to add additional members to expressions within your domain.
As we saw with the Calculator example from the Introduction, the simplest way to get an EDSL up and running is simply to define some terminals, as follows.
// Define a literal integer Proto expression. proto::terminal<int>::type i = {0}; // This creates an expression template. i + 1;
With some terminals and Proto's operator overloads, you can immediately start creating expression templates.
          Defining terminals -- with aggregate initialization -- can be a little
          awkward at times. Proto provides an easier-to-use wrapper for literals
          that can be used to construct Protofied terminal expressions. It's called
          proto::literal<>.
        
// Define a literal integer Proto expression. proto::literal<int> i = 0; // Proto literals are really just Proto terminal expressions. // For example, this builds a Proto expression template: i + 1;
          There is also a proto::lit() function for constructing
          a proto::literal<> in-place. The above
          expression can simply be written as:
        
// proto::lit(0) creates an integer terminal expression proto::lit(0) + 1;
          Once we have some Proto terminals, expressions involving those terminals
          build expression trees for us. Proto defines overloads for each of C++'s
          overloadable operators in the boost::proto
          namespace. As long as one operand is a Proto expression, the result of
          the operation is a tree node representing that operation.
        
| ![[Note]](../../../doc/src/images/note.png) | Note | 
|---|---|
| 
            Proto's operator overloads live in the  | 
As a result of Proto's operator overloads, we can say:
-_1; // OK, build a unary-negate tree node _1 + 42; // OK, build a binary-plus tree node
For the most part, this Just Works and you don't need to think about it, but a few operators are special and it can be helpful to know how Proto handles them.
          Proto also overloads operator=, operator[], and operator(), but these operators are member functions
          of the expression template rather than free functions in Proto's namespace.
          The following are valid Proto expressions:
        
_1 = 5; // OK, builds a binary assign tree node _1[6]; // OK, builds a binary subscript tree node _1(); // OK, builds a unary function tree node _1(7); // OK, builds a binary function tree node _1(8,9); // OK, builds a ternary function tree node // ... etc.
          For the first two lines, assignment and subscript, it should be fairly
          unsurprising that the resulting expression node should be binary. After
          all, there are two operands in each expression. It may be surprising at
          first that what appears to be a function call with no arguments, _1(),
          actually creates an expression node with one child. The child is _1 itself. Likewise, the expression
          _1(7) has two
          children: _1 and 7.
        
Because these operators can only be defined as member functions, the following expressions are invalid:
int i; i = _1; // ERROR: cannot assign _1 to an int int *p; p[_1]; // ERROR: cannot use _1 as an index std::sin(_1); // ERROR: cannot call std::sin() with _1
          Also, C++ has special rules for overloads of operator-> that make it useless for building
          expression templates, so Proto does not overload it.
        
Proto overloads the address-of operator for expression types, so that the following code creates a new unary address-of tree node:
&_1; // OK, creates a unary address-of tree node
          It does not return the address of the _1 object. However, there is special
          code in Proto such that a unary address-of node is implicitly convertible
          to a pointer to its child. In other words, the following code works and
          does what you might expect, but not in the obvious way:
        
typedef proto::terminal< placeholder<0> >::type _1_type; _1_type const _1 = {{}}; _1_type const * p = &_1; // OK, &_1 implicitly converted
          If we limited ourselves to nothing but terminals and operator overloads,
          our embedded domain-specific languages wouldn't be very expressive. Imagine
          that we wanted to extend our calculator EDSL with a full suite of math
          functions like sin()
          and pow()
          that we could invoke lazily as follows.
        
// A calculator expression that takes one argument // and takes the sine of it. sin(_1);
          We would like the above to create an expression template representing a
          function invocation. When that expression is evaluated, it should cause
          the function to be invoked. (At least, that's the meaning of function invocation
          we'd like the calculator EDSL to have.) You can define sin
          quite simply as follows.
        
// "sin" is a Proto terminal containing a function pointer proto::terminal< double(*)(double) >::type const sin = {&std::sin};
          In the above, we define sin
          as a Proto terminal containing a pointer to the std::sin() function. Now we can use sin as a lazy function. The default_context that we saw in the Introduction
          knows how to evaluate lazy functions. Consider the following:
        
double pi = 3.1415926535; proto::default_context ctx; // Create a lazy "sin" invocation and immediately evaluate it std::cout << proto::eval( sin(pi/2), ctx ) << std::endl;
The above code prints out:
1
I'm no expert at trigonometry, but that looks right to me.
          We can write sin(pi/2) because the sin
          object, which is a Proto terminal, has an overloaded operator()() that builds a node representing a function
          call invocation. The actual type of sin(pi/2) is actually
          something like this:
        
// The type of the expression sin(pi/2): proto::function< proto::terminal< double(*)(double) >::type const & proto::result_of::as_child< double const >::type >::type
          This type further expands to an unsightly node type with a tag
          type of proto::tag::function and two children: the first
          representing the function to be invoked, and the second representing the
          argument to the function. (Node tag types describe the operation that created
          the node. The difference between a
          + b
          and a -
          b is that the former has tag
          type proto::tag::plus and the latter has tag type proto::tag::minus. Tag types are pure compile-time
          information.)
        
| ![[Note]](../../../doc/src/images/note.png) | Note | 
|---|---|
| 
            In the type computation above,  | 
It is important to note that there is nothing special about terminals that contain function pointers. Any Proto expression has an overloaded function call operator. Consider:
// This compiles! proto::lit(1)(2)(3,4)(5,6,7,8);
          That may look strange at first. It creates an integer terminal with proto::lit(), and then invokes it like
          a function again and again. What does it mean? Who knows?! You get to decide
          when you define an evaluation context or a transform. But more on that
          later.
        
          Now, what if we wanted to add a pow() function to our calculator EDSL that
          users could invoke as follows?
        
// A calculator expression that takes one argument // and raises it to the 2nd power pow< 2 >(_1);
          The simple technique described above of making pow
          a terminal containing a function pointer doesn't work here. If pow is an object, then the expression
          pow<
          2 >(_1) is
          not valid C++. (Well, technically it is; it means, pow
          less than 2, greater than (_1),
          which is nothing at all like what we want.) pow
          should be a real function template. But it must be an unusual function:
          one that returns an expression template.
        
          With sin, we relied on
          Proto to provide an overloaded operator()() to build an expression node with tag
          type proto::tag::function for us. Now we'll need to do
          so ourselves. As before, the node will have two children: the function
          to invoke and the function's argument.
        
          With sin, the function
          to invoke was a raw function pointer wrapped in a Proto terminal. In the
          case of pow, we want it
          to be a terminal containing TR1-style function object. This will allow
          us to parameterize the function on the exponent. Below is the implementation
          of a simple TR1-style wrapper for the std::pow
          function:
        
// Define a pow_fun function object template< int Exp > struct pow_fun { typedef double result_type; double operator()(double d) const { return std::pow(d, Exp); } };
          Following the sin example,
          we want pow<
          1 >(
          pi/2 ) to have
          a type like this:
        
// The type of the expression pow<1>(pi/2): proto::function< proto::terminal< pow_fun<1> >::type proto::result_of::as_child< double const >::type >::type
          We could write a pow()
          function using code like this, but it's verbose and error prone; it's too
          easy to introduce subtle bugs by forgetting to call proto::as_child()
          where necessary, resulting in code that seems to work but sometimes doesn't.
          Proto provides a better way to construct expression nodes: proto::make_expr().
        
make_expr()
        
          Proto provides a helper for building expression templates called proto::make_expr(). We can concisely define
          the pow()
          function with it as below.
        
// Define a lazy pow() function for the calculator EDSL. // Can be used as: pow< 2 >(_1) template< int Exp, typename Arg > typename proto::result_of::make_expr< proto::tag::function // Tag type , pow_fun< Exp > // First child (by value) , Arg const & // Second child (by reference) >::type const pow(Arg const &arg) { return proto::make_expr<proto::tag::function>( pow_fun<Exp>() // First child (by value) , boost::ref(arg) // Second child (by reference) ); }
          There are some things to notice about the above code. We use proto::result_of::make_expr<>
          to calculate the return type. The first template parameter is the tag type
          for the expression node we're building -- in this case, proto::tag::function.
        
          Subsequent template parameters to proto::result_of::make_expr<> represent child nodes. If a child
          type is not already a Proto expression, it is automatically made into a
          terminal with proto::as_child().
          A type such as pow_fun<Exp> results in terminal that is held by
          value, whereas a type like Arg
          const &
          (note the reference) indicates that the result should be held by reference.
        
          In the function body is the runtime invocation of proto::make_expr().
          It closely mirrors the return type calculation. proto::make_expr()
          requires you to specify the node's tag type as a template parameter. The
          arguments to the function become the node's children. When a child should
          be stored by value, nothing special needs to be done. When a child should
          be stored by reference, you must use the boost::ref() function to wrap the argument.
        
          And that's it! proto::make_expr()
          is the lazy person's way to make a lazy funtion.
        
In this section, we'll learn all about domains. In particular, we'll learn:
In the Hello Calculator section, we looked into making calculator expressions directly usable as lambda expressions in calls to STL algorithms, as below:
double data[] = {1., 2., 3., 4.}; // Use the calculator EDSL to square each element ... HOW? std::transform( data, data + 4, data, _1 * _1 );
            The difficulty, if you recall, was that by default Proto expressions
            don't have interesting behaviors of their own. They're just trees. In
            particular, the expression _1
            * _1
            won't have an operator()
            that takes a double and returns a double like std::transform() expects -- unless we give it one. To
            make this work, we needed to define an expression wrapper type that defined
            the operator()
            member function, and we needed to associate the wrapper with the calculator
            domain.
          
In Proto, the term domain refers to a type that associates expressions in that domain to an expression generator. The generator is just a function object that accepts an expression and does something to it, like wrapping it in an expression wrapper.
You can also use a domain to associate expressions with a grammar. When you specify a domain's grammar, Proto ensures that all the expressions it generates in that domain conform to the domain's grammar. It does that by disabling any operator overloads that would create invalid expressions.
The first step to giving your calculator expressions extra behaviors is to define a calculator domain. All expressions within the calculator domain will be imbued with calculator-ness, as we'll see.
// A type to be used as a domain tag (to be defined below) struct calculator_domain;
            We use this domain type when extending the proto::expr<>
            type, which we do with the proto::extends<>
            class template. Here is our expression wrapper, which imbues an expression
            with calculator-ness. It is described below.
          
// The calculator<> expression wrapper makes expressions // function objects. template< typename Expr > struct calculator : proto::extends< Expr, calculator< Expr >, calculator_domain > { typedef proto::extends< Expr, calculator< Expr >, calculator_domain > base_type; calculator( Expr const &expr = Expr() ) : base_type( expr ) {} // This is usually needed because by default, the compiler- // generated assignment operator hides extends<>::operator= BOOST_PROTO_EXTENDS_USING_ASSIGN(calculator) typedef double result_type; // Hide base_type::operator() by defining our own which // evaluates the calculator expression with a calculator context. result_type operator()( double d1 = 0.0, double d2 = 0.0 ) const { // As defined in the Hello Calculator section. calculator_context ctx; // ctx.args is a vector<double> that holds the values // with which we replace the placeholders (e.g., _1 and _2) // in the expression. ctx.args.push_back( d1 ); // _1 gets the value of d1 ctx.args.push_back( d2 ); // _2 gets the value of d2 return proto::eval(*this, ctx ); // evaluate the expression } };
            We want calculator expressions to be function objects, so we have to
            define an operator()
            that takes and returns doubles. The calculator<> wrapper above does that with
            the help of the proto::extends<>
            template. The first template to proto::extends<>
            parameter is the expression type we are extending. The second is the
            type of the wrapped expression. The third parameter is the domain that
            this wrapper is associated with. A wrapper type like calculator<> that inherits from proto::extends<> behaves just like
            the expression type it has extended, with any additional behaviors you
            choose to give it.
          
| ![[Note]](../../../doc/src/images/note.png) | Note | 
|---|---|
| 
              Why not just inherit from  
              You might be thinking that this expression extension business is unnecessarily
              complicated. After all, isn't this why C++ supports inheritance? Why
              can't  | 
            Although not strictly necessary in this case, we bring extends<>::operator=
            into scope with the BOOST_PROTO_EXTENDS_USING_ASSIGN() macro. This is really only necessary
            if you want expressions like _1
            = 3
            to create a lazily evaluated assignment. proto::extends<>
            defines the appropriate operator= for you, but the compiler-generated
            calculator<>::operator=
            will hide it unless you make it available with the macro.
          
            Note that in the implementation of calculator<>::operator(), we evaluate the expression with the
            calculator_context we
            defined earlier. As we saw before, the context is what gives the operators
            their meaning. In the case of the calculator, the context is also what
            defines the meaning of the placeholder terminals.
          
            Now that we have defined the calculator<> expression wrapper, we need to
            wrap the placeholders to imbue them with calculator-ness:
          
calculator< proto::terminal< placeholder<0> >::type > const _1; calculator< proto::terminal< placeholder<1> >::type > const _2;
BOOST_PROTO_EXTENDS()
          
            To use proto::extends<>, your extension type
            must derive from proto::extends<>.
            Unfortunately, that means that your extension type is no longer POD and
            its instances cannot be statically initialized.
            (See the Static
            Initialization section in the Rationale
            appendix for why this matters.) In particular, as defined above, the
            global placeholder objects _1
            and _2 will need to be
            initialized at runtime, which could lead to subtle order of initialization
            bugs.
          
            There is another way to make an expression extension that doesn't sacrifice
            POD-ness : the BOOST_PROTO_EXTENDS()proto::extends<>.
            We can use BOOST_PROTO_EXTENDS()calculator<>
            a POD and our placeholders statically initialized.
          
// The calculator<> expression wrapper makes expressions // function objects. template< typename Expr > struct calculator { // Use BOOST_PROTO_EXTENDS() instead of proto::extends<> to // make this type a Proto expression extension. BOOST_PROTO_EXTENDS(Expr, calculator<Expr>, calculator_domain) typedef double result_type; result_type operator()( double d1 = 0.0, double d2 = 0.0 ) const { /* ... as before ... */ } };
            With the new calculator<> type, we can redefine our placeholders
            to be statically initialized:
          
calculator< proto::terminal< placeholder<0> >::type > const _1 = {{{}}}; calculator< proto::terminal< placeholder<1> >::type > const _2 = {{{}}};
We need to make one additional small change to accommodate the POD-ness of our expression extension, which we'll describe below in the section on expression generators.
            What does BOOST_PROTO_EXTENDS()operator=, operator[] and operator() overloads for building expression templates;
            and a nested result<>
            template for calculating the return type of operator(). In this case, however, the operator()
            overloads and the result<> template are not needed because
            we are defining our own operator() in the calculator<> type. Proto provides additional
            macros for finer control over which member functions are defined. We
            could improve our calculator<> type as follows:
          
// The calculator<> expression wrapper makes expressions // function objects. template< typename Expr > struct calculator { // Use BOOST_PROTO_BASIC_EXTENDS() instead of proto::extends<> to // make this type a Proto expression extension: BOOST_PROTO_BASIC_EXTENDS(Expr, calculator<Expr>, calculator_domain) // Define operator[] to build expression templates: BOOST_PROTO_EXTENDS_SUBSCRIPT() // Define operator= to build expression templates: BOOST_PROTO_EXTENDS_ASSIGN() typedef double result_type; result_type operator()( double d1 = 0.0, double d2 = 0.0 ) const { /* ... as before ... */ } };
            Notice that we are now using BOOST_PROTO_BASIC_EXTENDS()BOOST_PROTO_EXTENDS()BOOST_PROTO_EXTENDS_ASSIGN()BOOST_PROTO_EXTENDS_SUBSCRIPT()result<>
            template that could have been defined with Proto's BOOST_PROTO_EXTENDS_FUNCTION()
In summary, here are the macros you can use to define expression extensions, and a brief description of each.
Table 32.2. Expression Extension Macros
| Macro | Purpose | 
|---|---|
| 
 
 
 | 
                      Defines a data member of type  | 
| 
                      Defines  | |
| 
                      Defines  | |
| 
                      Defines  | |
| 
 
 
 | Equivalent to: 
 
 | 
| ![[Warning]](../../../doc/src/images/warning.png) | Warning | 
|---|---|
| 
              Argument-Dependent Lookup and  
              Proto's operator overloads are defined in the  
 template<class T> struct my_complex { BOOST_PROTO_EXTENDS( typename proto::terminal<std::complex<T> >::type , my_complex<T> , proto::default_domain ) }; int main() { my_complex<int> c0, c1; c0 + c1; // ERROR: operator+ not found } 
 
              The problem has to do with how argument-dependent lookup works. The
              type  
              So what can we do? By adding an extra dummy template parameter that
              defaults to a type in the  
 template<class T, class Dummy = proto::is_proto_expr> struct my_complex { BOOST_PROTO_EXTENDS( typename proto::terminal<std::complex<T> >::type , my_complex<T> , proto::default_domain ) }; int main() { my_complex<int> c0, c1; c0 + c1; // OK, operator+ found now! } 
 
              The type  | 
            The last thing that remains to be done is to tell Proto that it needs
            to wrap all of our calculator expressions in our calculator<> wrapper. We have already wrapped
            the placeholders, but we want all expressions that
            involve the calculator placeholders to be calculators. We can do that
            by specifying an expression generator when we define our calculator_domain, as follows:
          
// Define the calculator_domain we forward-declared above. // Specify that all expression in this domain should be wrapped // in the calculator<> expression wrapper. struct calculator_domain : proto::domain< proto::generator< calculator > > {};
            The first template parameter to proto::domain<> is the generator. "Generator"
            is just a fancy name for a function object that accepts an expression
            and does something to it. proto::generator<> is a very simple one --- it wraps
            an expression in the wrapper you specify. proto::domain<> inherits from its generator parameter,
            so all domains are themselves function objects.
          
            If we used BOOST_PROTO_EXTENDS()proto::pod_generator<>
            instead of proto::generator<>,
            as follows:
          
// If calculator<> uses BOOST_PROTO_EXTENDS() instead of // use proto::extends<>, use proto::pod_generator<> instead // of proto::generator<>. struct calculator_domain : proto::domain< proto::pod_generator< calculator > > {};
            After Proto has calculated a new expression type, it checks the domains
            of the child expressions. They must match. Assuming they do, Proto creates
            the new expression and passes it to Domain::operator()calculator_domain::operator() returns calculator<> objects.
          
Now we can use calculator expressions as function objects to STL algorithms, as follows:
double data[] = {1., 2., 3., 4.}; // Use the calculator EDSL to square each element ... WORKS! :-) std::transform( data, data + 4, data, _1 * _1 );
            By default, Proto defines every possible operator overload for Protofied
            expressions. This makes it simple to bang together an EDSL. In some cases,
            however, the presence of Proto's promiscuous overloads can lead to confusion
            or worse. When that happens, you'll have to disable some of Proto's overloaded
            operators. That is done by defining the grammar for your domain and specifying
            it as the second parameter of the proto::domain<>
            template.
          
In the Hello Calculator section, we saw an example of a Proto grammar, which is repeated here:
// Define the grammar of calculator expressions struct calculator_grammar : proto::or_< proto::plus< calculator_grammar, calculator_grammar > , proto::minus< calculator_grammar, calculator_grammar > , proto::multiplies< calculator_grammar, calculator_grammar > , proto::divides< calculator_grammar, calculator_grammar > , proto::terminal< proto::_ > > {};
            We'll have much more to say about grammars in subsequent sections, but
            for now, we'll just say that the calculator_grammar
            struct describes a subset of all expression types -- the subset that
            comprise valid calculator expressions. We would like to prohibit Proto
            from creating a calculator expression that does not conform to this grammar.
            We do that by changing the definition of the calculator_domain
            struct.
          
// Define the calculator_domain. Expressions in the calculator // domain are wrapped in the calculator<> wrapper, and they must // conform to the calculator_grammar: struct calculator_domain : proto::domain< proto::generator< calculator >, calculator_grammar > {};
            The only new addition is calculator_grammar
            as the second template parameter to the proto::domain<>
            template. That has the effect of disabling any of Proto's operator overloads
            that would create an invalid calculator expression.
          
            Another common use for this feature would be to disable Proto's unary
            operator&
            overload. It may be surprising for users of your EDSL that they cannot
            take the address of their expressions! You can very easily disable Proto's
            unary operator&
            overload for your domain with a very simple grammar, as below:
          
// For expressions in my_domain, disable Proto's // unary address-of operator. struct my_domain : proto::domain< proto::generator< my_wrapper > // A simple grammar that matches any expression that // is not a unary address-of expression. , proto::not_< proto::address_of< _ > > > {};
            The type proto::not_<
            proto::address_of<
            _ >
            > is a very simple grammar
            that matches all expressions except unary address-of expressions. In
            the section describing Proto's intermediate form, we'll have much more
            to say about grammars.
          
| ![[Note]](../../../doc/src/images/note.png) | Note | 
|---|---|
| This is an advanced topic. Feel free to skip this if you're just getting started with Proto. | 
Proto's operator overloads build expressions from sub-expressions. The sub-expressions become children of the new expression. By default, the children are stored in the parent by reference. This section describes how to change that default.
as_child vs. as_expr
          
            Proto lets you independently customize the behavior of proto::as_child() and proto::as_expr().
            Both accept an object x and return a Proto expression
            by turning x it into a Proto terminal if necessary.
            Although similar, the two functions are used in different situations
            and have subtly different behavior by default. It's important to understand
            the difference so that you know which to customize to achieve the behavior
            you want.
          
            To wit: proto::as_expr() is typically used by
            you to turn an object into a Proto expression that
            is to be held in a local variable, as so:
          
auto l = proto::as_expr(x); // Turn x into a Proto expression, hold the result in a local
            The above works regardless of whether x
            is already a Proto expression or not. The object l
            is guaranteed to be a valid Proto expression. If x
            is a non-Proto object, it is turned into a terminal expression that holds
            x by value.[28] If x is a
            Proto object already, proto::as_expr()
            returns it by value unmodified.
          
            In contrast, proto::as_child()
            is used internally by Proto to pre-process objects before making them
            children of another expression. Since it's internal to Proto, you don't
            see it explicitly, but it's there behind the scenes in expressions like
            this:
          
x + y; // Consider that y is a Proto expression, but x may or may not be.
            In this case, Proto builds a plus node from the two children. Both are
            pre-processed by passing them to proto::as_child()
            before making them children of the new node. If x
            is not a Proto expression, it becomes one by being wrapped in a Proto
            terminal that holds it by reference. If x is already a Proto expression, proto::as_child() returns it by
            reference unmodified. Contrast this with the above description
            for proto::as_expr().
          
The table below summarizes the above description.
Table 32.3. proto::as_expr() vs. proto::as_child()
| Function | 
                      When  | 
                      When  | 
|---|---|---|
| 
                       | 
                      Return (by value) a new Proto terminal holding  | 
                      Return  | 
| 
                       | 
                      Return (by value) a new Proto terminal holding  | 
                      Return  | 
| ![[Note]](../../../doc/src/images/note.png) | Note | 
|---|---|
| 
              There is one important place where Proto uses both  | 
            Now that you know what proto::as_child()
            and proto::as_expr() are, where they are
            used, and what they do by default, you may decide that one or both of
            these functions should have different behavior for your domain. For instance,
            given the above description of proto::as_child(),
            the following code is always wrong:
          
proto::literal<int> i(0); auto l = i + 42; // This is WRONG! Don't do this.
            Why is this wrong? Because proto::as_child()
            will turn the integer literal 42 into a Proto terminal that holds a reference
            to a temporary integer initialized with 42. The lifetime of that temporary
            ends at the semicolon, guaranteeing that the local l
            is left holding a dangling reference to a deceased integer. What to do?
            One answer is to use proto::deep_copy().
            Another is to customize the behavior of proto::as_child()
            for your domain. Read on for the details.
          
as_child
          
            To control how Proto builds expressions out of sub-expressions in your
            domain, define your domain as usual, and then define a nested as_child<>
            class template within it, as follows:
          
class my_domain : proto::domain< my_generator, my_grammar > { // Here is where you define how Proto should handle // sub-expressions that are about to be glommed into // a larger expression. template< typename T > struct as_child { typedefunspecified-Proto-expr-typeresult_type; result_type operator()( T & t ) const { returnunspecified-Proto-expr-object; } }; };
            There's one important thing to note: in the above code, the template
            parameter T may or may not be a Proto expression type,
            but the result must be a Proto expression type,
            or a reference to one. That means that most user-defined as_child<>
            templates will need to check whether T is an expression
            or not (using proto::is_expr<>), and then turn non-expressions
            into Proto terminals by wrapping them as proto::terminal< /* ... */
            >::type
            or equivalent.
          
as_expr
          
            Although less common, Proto also lets you customize the behavior of
            proto::as_expr() on a per-domain basis.
            The technique is identical to that for as_child. See
            below:
          
class my_domain : proto::domain< my_generator, my_grammar > { // Here is where you define how Proto should handle // objects that are to be turned into expressions // fit for storage in local variables. template< typename T > struct as_expr { typedefunspecified-Proto-expr-typeresult_type; result_type operator()( T & t ) const { returnunspecified-Proto-expr-object; } }; };
auto-safe
          
            Let's look again at the problem described above involving the C++11
            auto keyword and the default
            behavior of proto::as_child().
          
proto::literal<int> i(0); auto l = i + 42; // This is WRONG! Don't do this.
            Recall that the problem is the lifetime of the temporary integer created
            to hold the value 42. The local l
            will be left holding a dangling reference to it after its lifetime is
            over. What if we want Proto to make expressions safe to store this way
            in local variables? We can do so very easily by making proto::as_child() behave just like proto::as_expr(). The following code
            achieves this:
          
template< typename E > struct my_expr; struct my_generator : proto::pod_generator< my_expr > {}; struct my_domain : proto::domain< my_generator > { // Make as_child() behave like as_expr() in my_domain. // (proto_base_domain is a typedef for proto::domain< my_generator > // that is defined in proto::domain<>.) template< typename T > struct as_child : proto_base_domain::as_expr< T > {}; }; template< typename E > struct my_expr { BOOST_PROTO_EXTENDS( E, my_expr< E >, my_domain ) }; /* ... */ proto::literal< int, my_domain > i(0); auto l = i + 42; // OK! Everything is stored by value here.
            Notice that my_domain::as_child<> simply defers to the default
            implementation of as_expr<> found in proto::domain<>.
            By simply cross-wiring our domain's as_child<> to as_expr<>, we guarantee that all terminals
            that can be held by value are, and that all child expressions are also
            held by value. This increases copying and may incur a runtime performance
            cost, but it eliminates any spector of lifetime management issues.
          
            For another example, see the definition of lldomain
            in libs/proto/example/lambda.hpp. That example is
            a complete reimplementation of the Boost Lambda Library (BLL) on top
            of Boost.Proto. The function objects the BLL generates are safe to be
            stored in local variables. To emulate this with Proto, the lldomain cross-wires as_child<>
            to as_expr<>
            as above, but with one extra twist: objects with array type are also
            stored by reference. Check it out.
          
| ![[Note]](../../../doc/src/images/note.png) | Note | 
|---|---|
| This is an advanced topic. Feel free to skip this if you're just getting started with Proto. | 
The ability to compose different EDSLs is one of their most exciting features. Consider how you build a parser using yacc. You write your grammar rules in yacc's domain-specific language. Then you embed semantic actions written in C within your grammar. Boost's Spirit parser generator gives you the same ability. You write grammar rules using Spirit.Qi and embed semantic actions using the Phoenix library. Phoenix and Spirit are both Proto-based domain-specific languages with their own distinct syntax and semantics. But you can freely embed Phoenix expressions within Spirit expressions. This section describes Proto's sub-domain feature that lets you define families of interoperable domains.
When you try to create an expression from two sub-expressions in different domains, what is the domain of the resulting expression? This is the fundamental problem that is addressed by sub-domains. Consider the following code:
#include <boost/proto/proto.hpp> namespace proto = boost::proto; // Forward-declare two expression wrappers template<typename E> struct spirit_expr; template<typename E> struct phoenix_expr; // Define two domains struct spirit_domain : proto::domain<proto::generator<spirit_expr> > {}; struct phoenix_domain : proto::domain<proto::generator<phoenix_expr> > {}; // Implement the two expression wrappers template<typename E> struct spirit_expr : proto::extends<E, spirit_expr<E>, spirit_domain> { spirit_expr(E const &e = E()) : spirit_expr::proto_extends(e) {} }; template<typename E> struct phoenix_expr : proto::extends<E, phoenix_expr<E>, phoenix_domain> { phoenix_expr(E const &e = E()) : phoenix_expr::proto_extends(e) {} }; int main() { proto::literal<int, spirit_domain> sp(0); proto::literal<int, phoenix_domain> phx(0); // Whoops! What does it mean to add two expressions in different domains? sp + phx; // ERROR }
            Above, we define two domains called spirit_domain
            and phoenix_domain and
            declare two int literals in each. Then we try to compose them into a
            larger expression using Proto's binary plus operator, and it fails. Proto
            can't figure out whether the resulting expression should be in the Spirit
            domain or the Phoenix domain, and thus whether it should be an instance
            of spirit_expr<>
            or phoenix_expr<>.
            We have to tell Proto how to resolve the conflict. We can do that by
            declaring that Phoenix is a sub-domain of Spirit as in the following
            definition of phoenix_domain:
          
// Declare that phoenix_domain is a sub-domain of spirit_domain struct phoenix_domain : proto::domain<proto::generator<phoenix_expr>, proto::_, spirit_domain> {};
            The third template parameter to proto::domain<>
            is the super-domain. By defining phoenix_domain
            as above, we are saying that Phoenix expressions can be combined with
            Spirit expressions, and that when that happens, the resulting expression
            should be a Spirit expression.
          
| ![[Note]](../../../doc/src/images/note.png) | Note | 
|---|---|
| 
              If you are wondering what the purpose of  | 
            When there are multiple domains in play within a given expression, Proto
            uses some rules to figure out which domain "wins". The rules
            are loosely modeled on the rules for C++ inheritance. Phoenix_domain
            is a sub-domain of spirit_domain.
            You can liken that to a derived/base relationship that gives Phoenix
            expressions a kind of implicit conversion to Spirit expressions. And
            since Phoenix expressions can be "converted" to Spirit expressions,
            they can be freely combined with Spirit expressions and the result is
            a Spirit expression.
          
| ![[Note]](../../../doc/src/images/note.png) | Note | 
|---|---|
| Super- and sub-domains are not actually implemented using inheritance. This is only a helpful mental model. | 
            The analogy with inheritance holds even in the case of three domains
            when two are sub-domains of the third. Imagine another domain called
            foobar_domain that was
            also a sub-domain of spirit_domain.
            Expressions in the foobar_domain
            could be combined with expressions in the phoenix_domain
            and the resulting expression would be in the spirit_domain.
            That's because expressions in the two sub-domains both have "conversions"
            to the super-domain, so the operation is allowed and the super-domain
            wins.
          
            When you don't assign a Proto expression to a particular domain, Proto
            considers it a member of the so-called default domain, proto::default_domain. Even non-Proto objects
            are treated as terminals in the default domain. Consider:
          
int main() { proto::literal<int, spirit_domain> sp(0); // Add 1 to a spirit expression. Result is a spirit expression. sp + 1; }
            Expressions in the default domain (or non-expressions like 1)
            have a kind of implicit conversion to expressions every other domain
            type. What's more, you can define your domain to be a sub-domain of the
            default domain. In so doing, you give expressions in your domain conversions
            to expressions in every other domain. This is like a “free love”
            domain, because it will freely mix with all other domains.
          
            Let's think again about the Phoenix EDSL. Since it provides generally
            useful lambda functionality, it's reasonable to assume that lots of other
            EDSLs besides Spirit might want the ability to embed Phoenix expressions.
            In other words, phoenix_domain
            should be a sub-domain of proto::default_domain,
            not spirit_domain:
          
// Declare that phoenix_domain is a sub-domain of proto::default_domain struct phoenix_domain : proto::domain<proto::generator<phoenix_expr>, proto::_, proto::default_domain> {};
That's much better. Phoenix expressions can now be put anywhere.
            Use Proto sub-domains to make it possible to mix expressions from multiple
            domains. And when you want expressions in your domain to freely combine
            with all expressions, make it a sub-domain of proto::default_domain.
          
          The preceding discussions of defining Proto front ends have all made a
          big assumption: that you have the luxury of defining everything from scratch.
          What happens if you have existing types, say a matrix type and a vector
          type, that you would like to treat as if they were Proto terminals? Proto
          usually trades only in its own expression types, but with BOOST_PROTO_DEFINE_OPERATORS()
Let's say, for instance, that you have the following types and that you can't modify then to make them “native” Proto terminal types.
namespace math { // A matrix type ... struct matrix { /*...*/ }; // A vector type ... struct vector { /*...*/ }; }
          You can non-intrusively make objects of these types Proto terminals by
          defining the proper operator overloads using BOOST_PROTO_DEFINE_OPERATORS()
BOOST_PROTO_DEFINE_OPERATORS()proto::default_domain)
              as the second.
            The following code demonstrates how it works.
namespace math { template<typename T> struct is_terminal : mpl::false_ {}; // OK, "matrix" is a custom terminal type template<> struct is_terminal<matrix> : mpl::true_ {}; // OK, "vector" is a custom terminal type template<> struct is_terminal<vector> : mpl::true_ {}; // Define all the operator overloads to construct Proto // expression templates, treating "matrix" and "vector" // objects as if they were Proto terminals. BOOST_PROTO_DEFINE_OPERATORS(is_terminal, proto::default_domain) }
          The invocation of the BOOST_PROTO_DEFINE_OPERATORS()matrix and vector
          objects as if they were Proto terminals. And since the operators are defined
          in the same namespace as the matrix
          and vector types, the operators
          will be found by argument-dependent lookup. With the code above, we can
          now construct expression templates with matrices and vectors, as shown
          below.
        
math::matrix m1; math::vector v1; proto::literal<int> i(0); m1 * 1; // custom terminal and literals are OK m1 * i; // custom terminal and Proto expressions are OK m1 * v1; // two custom terminals are OK, too.
Sometimes as an EDSL designer, to make the lives of your users easy, you have to make your own life hard. Giving your users natural and flexible syntax often involves writing large numbers of repetitive function overloads. It can be enough to give you repetitive stress injury! Before you hurt yourself, check out the macros Proto provides for automating many repetitive code-generation chores.
Imagine that we are writing a lambda EDSL, and we would like to enable syntax for constructing temporary objects of any type using the following syntax:
// A lambda expression that takes two arguments and // uses them to construct a temporary std::complex<> construct< std::complex<int> >( _1, _2 )
          For the sake of the discussion, imagine that we already have a function
          object template construct_impl<> that accepts arguments and constructs
          new objects from them. We would want the above lambda expression to be
          equivalent to the following:
        
// The above lambda expression should be roughly equivalent // to the following: proto::make_expr<proto::tag::function>( construct_impl<std::complex<int> >() // The function to invoke lazily , boost::ref(_1) // The first argument to the function , boost::ref(_2) // The second argument to the function );
          We can define our construct() function template as follows:
        
template<typename T, typename A0, typename A1> typename proto::result_of::make_expr< proto::tag::function , construct_impl<T> , A0 const & , A1 const & >::type const construct(A0 const &a0, A1 const &a1) { return proto::make_expr<proto::tag::function>( construct_impl<T>() , boost::ref(a0) , boost::ref(a1) ); }
          This works for two arguments, but we would like it to work for any number
          of arguments, up to ( BOOST_PROTO_MAX_ARITYconstruct_impl<T>()
          terminal leaving room for only ( BOOST_PROTO_MAX_ARITY
          For cases like this, Proto provides the BOOST_PROTO_REPEAT()BOOST_PROTO_REPEAT_FROM_TO()
#define M0(N, typename_A, A_const_ref, A_const_ref_a, ref_a) \ template<typename T, typename_A(N)> \ typename proto::result_of::make_expr< \ proto::tag::function \ , construct_impl<T> \ , A_const_ref(N) \ >::type const \ construct(A_const_ref_a(N)) \ { \ return proto::make_expr<proto::tag::function>( \ construct_impl<T>() \ , ref_a(N) \ ); \ }
          Notice that we turned the function into a macro that takes 5 arguments.
          The first is the current iteration number. The rest are the names of other
          macros that generate different sequences. For instance, Proto passes as
          the second parameter the name of a macro that will expand to typename A0, typename A1, ....
        
          Now that we have turned our function into a macro, we can pass the macro
          to BOOST_PROTO_REPEAT_FROM_TO()
// Generate overloads of construct() that accept from // 1 to BOOST_PROTO_MAX_ARITY-1 arguments: BOOST_PROTO_REPEAT_FROM_TO(1, BOOST_PROTO_MAX_ARITY, M0) #undef M0
          As mentioned above, Proto passes as the last 4 arguments to your macro
          the names of other macros that generate various sequences. The macros
          BOOST_PROTO_REPEAT()BOOST_PROTO_REPEAT_FROM_TO()BOOST_PROTO_REPEAT_EX()BOOST_PROTO_REPEAT_FROM_TO_EX()BOOST_PROTO_REPEAT_EX()BOOST_PROTO_REPEAT_FROM_TO_EX()boost/proto/repeat.hpp
          for all the details.
        
          Also, check out BOOST_PROTO_LOCAL_ITERATE()BOOST_PROTO_REPEAT()
By now, you know a bit about how to build a front-end for your EDSL "compiler" -- you can define terminals and functions that generate expression templates. But we haven't said anything about the expression templates themselves. What do they look like? What can you do with them? In this section we'll see.
expr<> Type
      
        All Proto expressions are an instantiation of a template called proto::expr<> (or a wrapper around
        such an instantiation). When we define a terminal as below, we are really
        initializing an instance of the proto::expr<>
        template.
      
// Define a placeholder type template<int I> struct placeholder {}; // Define the Protofied placeholder terminal proto::terminal< placeholder<0> >::type const _1 = {{}};
        The actual type of _1 looks
        like this:
      
proto::expr< proto::tag::terminal, proto::term< placeholder<0> >, 0 >
        The proto::expr<> template is the most
        important type in Proto. Although you will rarely need to deal with it directly,
        it's always there behind the scenes holding your expression trees together.
        In fact, proto::expr<> is
        the expression tree -- branches, leaves and all.
      
        The proto::expr<> template makes up the
        nodes in expression trees. The first template parameter is the node type;
        in this case, proto::tag::terminal.
        That means that _1 is a leaf-node
        in the expression tree. The second template parameter is a list of child
        types, or in the case of terminals, the terminal's value type. Terminals
        will always have only one type in the type list. The last parameter is the
        arity of the expression. Terminals have arity 0, unary expressions have arity
        1, etc.
      
        The proto::expr<> struct is defined as
        follows:
      
template< typename Tag, typename Args, long Arity = Args::arity > struct expr; template< typename Tag, typename Args > struct expr< Tag, Args, 1 > { typedef typename Args::child0 proto_child0; proto_child0 child0; // ... };
        The proto::expr<> struct does not define
        a constructor, or anything else that would prevent static initialization.
        All proto::expr<> objects are initialized
        using aggregate initialization, with curly braces. In
        our example, _1 is initialized
        with the initializer {{}}. The
        outer braces are the initializer for the proto::expr<>
        struct, and the inner braces are for the member _1.child0
        which is of type placeholder<0>.
        Note that we use braces to initialize _1.child0
        because placeholder<0> is also
        an aggregate.
      
        The _1 node is an instantiation
        of proto::expr<>, and expressions containing
        _1 are also instantiations
        of proto::expr<>. To use Proto effectively,
        you won't have to bother yourself with the actual types that Proto generates.
        These are details, but you're likely to encounter these types in compiler
        error messages, so it's helpful to be familiar with them. The types look
        like this:
      
// The type of the expression -_1 typedef proto::expr< proto::tag::negate , proto::list1< proto::expr< proto::tag::terminal , proto::term< placeholder<0> > , 0 > const & > , 1 > negate_placeholder_type; negate_placeholder_type x = -_1; // The type of the expression _1 + 42 typedef proto::expr< proto::tag::plus , proto::list2< proto::expr< proto::tag::terminal , proto::term< placeholder<0> > , 0 > const & , proto::expr< proto::tag::terminal , proto::term< int const & > , 0 > > , 2 > placeholder_plus_int_type; placeholder_plus_int_type y = _1 + 42;
There are a few things to note about these types:
expr<> terminal objects. These new wrappers
            are not themselves held by reference, but the object wrapped is.
            Notice that the type of the Protofied 42
            literal is int const
            & -- held by reference.
          The types make it clear: everything in a Proto expression tree is held by reference. That means that building an expression tree is exceptionally cheap. It involves no copying at all.
| ![[Note]](../../../doc/src/images/note.png) | Note | 
|---|---|
| 
          An astute reader will notice that the object  | 
After assembling an expression into a tree, you'll naturally want to be able to do the reverse, and access a node's children. You may even want to be able to iterate over the children with algorithms from the Boost.Fusion library. This section shows how.
          Every node in an expression tree has both a tag type
          that describes the node, and an arity corresponding
          to the number of child nodes it has. You can use the proto::tag_of<>
          and proto::arity_of<> metafunctions to fetch
          them. Consider the following:
        
template<typename Expr> void check_plus_node(Expr const &) { // Assert that the tag type is proto::tag::plus BOOST_STATIC_ASSERT(( boost::is_same< typename proto::tag_of<Expr>::type , proto::tag::plus >::value )); // Assert that the arity is 2 BOOST_STATIC_ASSERT( proto::arity_of<Expr>::value == 2 ); } // Create a binary plus node and use check_plus_node() // to verify its tag type and arity: check_plus_node( proto::lit(1) + 2 );
          For a given type Expr,
          you could access the tag and arity directly as Expr::proto_tag
          and Expr::proto_arity, where Expr::proto_arity
          is an MPL Integral Constant.
        
          There is no simpler expression than a terminal, and no more basic operation
          than extracting its value. As we've already seen, that is what proto::value() is for.
        
proto::terminal< std::ostream & >::type cout_ = {std::cout}; // Get the value of the cout_ terminal: std::ostream & sout = proto::value( cout_ ); // Assert that we got back what we put in: assert( &sout == &std::cout );
          To compute the return type of the proto::value()
          function, you can use proto::result_of::value<>.
          When the parameter to proto::result_of::value<>
          is a non-reference type, the result type of the metafunction is the type
          of the value as suitable for storage by value; that is, top-level reference
          and qualifiers are stripped from it. But when instantiated with a reference
          type, the result type has a reference added to it,
          yielding a type suitable for storage by reference. If you want to know
          the actual type of the terminal's value including whether it is stored
          by value or reference, you can use fusion::result_of::value_at<Expr, 0>::type.
        
The following table summarizes the above paragraph.
Table 32.4. Accessing Value Types
| Metafunction Invocation | When the Value Type Is ... | The Result Is ... | 
|---|---|---|
| 
                     | 
                     | 
 typename boost::remove_const< typename boost::remove_reference<T>::type >::type [a] 
 | 
| 
                     | 
                     | 
 typename boost::add_reference<T>::type 
 | 
| 
                     | 
                     | 
 typename boost::add_reference< typename boost::add_const<T>::type >::type 
 | 
| 
                     | 
                     | 
                     | 
| [a] If  | ||
          Each non-terminal node in an expression tree corresponds to an operator
          in an expression, and the children correspond to the operands, or arguments
          of the operator. To access them, you can use the proto::child_c()
          function template, as demonstrated below:
        
proto::terminal<int>::type i = {42}; // Get the 0-th operand of an addition operation: proto::terminal<int>::type &ri = proto::child_c<0>( i + 2 ); // Assert that we got back what we put in: assert( &i == &ri );
          You can use the proto::result_of::child_c<>
          metafunction to get the type of the Nth child of an expression node. Usually
          you don't care to know whether a child is stored by value or by reference,
          so when you ask for the type of the Nth child of an expression Expr (where Expr
          is not a reference type), you get the child's type after references and
          cv-qualifiers have been stripped from it.
        
template<typename Expr> void test_result_of_child_c(Expr const &expr) { typedef typename proto::result_of::child_c<Expr, 0>::type type; // Since Expr is not a reference type, // result_of::child_c<Expr, 0>::type is a // non-cv qualified, non-reference type: BOOST_MPL_ASSERT(( boost::is_same< type, proto::terminal<int>::type > )); } // ... proto::terminal<int>::type i = {42}; test_result_of_child_c( i + 2 );
          However, if you ask for the type of the Nth child of Expr
          & or Expr
          const &
          (note the reference), the result type will be a reference, regardless of
          whether the child is actually stored by reference or not. If you need to
          know exactly how the child is stored in the node, whether by reference
          or by value, you can use fusion::result_of::value_at<Expr, N>::type. The following table summarizes
          the behavior of the proto::result_of::child_c<>
          metafunction.
        
Table 32.5. Accessing Child Types
| Metafunction Invocation | When the Child Is ... | The Result Is ... | 
|---|---|---|
| 
                     | 
                     | 
 typename boost::remove_const< typename boost::remove_reference<T>::type >::type 
 | 
| 
                     | 
                     | 
 typename boost::add_reference<T>::type 
 | 
| 
                     | 
                     | 
 typename boost::add_reference< typename boost::add_const<T>::type >::type 
 | 
| 
                     | 
                     | 
                     | 
          Most operators in C++ are unary or binary, so accessing the only operand,
          or the left and right operands, are very common operations. For this reason,
          Proto provides the proto::child(),
          proto::left(), and proto::right()
          functions. proto::child() and proto::left()
          are synonymous with proto::child_c<0>(),
          and proto::right() is synonymous with proto::child_c<1>().
        
          There are also proto::result_of::child<>,
          proto::result_of::left<>, and proto::result_of::right<>
          metafunctions that merely forward to their proto::result_of::child_c<>
          counterparts.
        
          When you build an expression template with Proto, all the intermediate
          child nodes are held by reference. The avoids needless
          copies, which is crucial if you want your EDSL to perform well at runtime.
          Naturally, there is a danger if the temporary objects go out of scope before
          you try to evaluate your expression template. This is especially a problem
          in C++0x with the new decltype
          and auto keywords. Consider:
        
// OOPS: "ex" is left holding dangling references auto ex = proto::lit(1) + 2;
          The problem can happen in today's C++ also if you use BOOST_TYPEOF() or BOOST_AUTO(), or if you try to pass an expression
          template outside the scope of its constituents.
        
          In these cases, you want to deep-copy your expression template so that
          all intermediate nodes and the terminals are held by value.
          That way, you can safely assign the expression template to a local variable
          or return it from a function without worrying about dangling references.
          You can do this with proto::deep_copy()
          as fo llows:
        
// OK, "ex" has no dangling references auto ex = proto::deep_copy( proto::lit(1) + 2 );
If you are using Boost.Typeof, it would look like this:
// OK, use BOOST_AUTO() and proto::deep_copy() to // store an expression template in a local variable BOOST_AUTO( ex, proto::deep_copy( proto::lit(1) + 2 ) );
          For the above code to work, you must include the boost/proto/proto_typeof.hpp
          header, which also defines the BOOST_PROTO_AUTO()BOOST_PROTO_AUTO()
// OK, BOOST_PROTO_AUTO() automatically deep-copies // its argument: BOOST_PROTO_AUTO( ex, proto::lit(1) + 2 );
When deep-copying an expression tree, all intermediate nodes and all terminals are stored by value. The only exception is terminals that are function references, which are left alone.
| ![[Note]](../../../doc/src/images/note.png) | Note | 
|---|---|
| 
             | 
          Proto provides a utility for pretty-printing expression trees that comes
          in very handy when you're trying to debug your EDSL. It's called proto::display_expr(), and you pass it the expression
          to print and optionally, an std::ostream
          to which to send the output. Consider:
        
// Use display_expr() to pretty-print an expression tree proto::display_expr( proto::lit("hello") + 42 );
          The above code writes this to std::cout:
        
plus(
    terminal(hello)
  , terminal(42)
)
          In order to call proto::display_expr(),
          all the terminals in the expression must be Streamable (that is, they can
          be written to a std::ostream). In addition, the tag types
          must all be Streamable as well. Here is an example that includes a custom
          terminal type and a custom tag:
        
// A custom tag type that is Streamable struct MyTag { friend std::ostream &operator<<(std::ostream &s, MyTag) { return s << "MyTag"; } }; // Some other Streamable type struct MyTerminal { friend std::ostream &operator<<(std::ostream &s, MyTerminal) { return s << "MyTerminal"; } }; int main() { // Display an expression tree that contains a custom // tag and a user-defined type in a terminal proto::display_expr( proto::make_expr<MyTag>(MyTerminal()) + 42 ); }
The above code prints the following:
plus(
    MyTag(
        terminal(MyTerminal)
    )
  , terminal(42)
)
The following table lists the overloadable C++ operators, the Proto tag types for each, and the name of the metafunctions for generating the corresponding Proto expression types. And as we'll see later, the metafunctions are also usable as grammars for matching such nodes, as well as pass-through transforms.
Table 32.6. Operators, Tags and Metafunctions
| Operator | Proto Tag | Proto Metafunction | 
|---|---|---|
| 
                    unary  | 
                     | 
                     | 
| 
                    unary  | 
                     | 
                     | 
| 
                    unary  | 
                     | 
                     | 
| 
                    unary  | 
                     | 
                     | 
| 
                    unary  | 
                     | 
                     | 
| 
                    unary  | 
                     | 
                     | 
| 
                    unary prefix  | 
                     | 
                     | 
| 
                    unary prefix  | 
                     | 
                     | 
| 
                    unary postfix  | 
                     | 
                     | 
| 
                    unary postfix  | 
                     | 
                     | 
| 
                    binary  | 
                     | 
                     | 
| 
                    binary  | 
                     | 
                     | 
| 
                    binary  | 
                     | 
                     | 
| 
                    binary  | 
                     | 
                     | 
| 
                    binary  | 
                     | 
                     | 
| 
                    binary  | 
                     | 
                     | 
| 
                    binary  | 
                     | 
                     | 
| 
                    binary  | 
                     | 
                     | 
| 
                    binary  | 
                     | 
                     | 
| 
                    binary  | 
                     | 
                     | 
| 
                    binary  | 
                     | 
                     | 
| 
                    binary  | 
                     | 
                     | 
| 
                    binary  | 
                     | 
                     | 
| 
                    binary  | 
                     | 
                     | 
| 
                    binary  | 
                     | 
                     | 
| 
                    binary  | 
                     | 
                     | 
| 
                    binary  | 
                     | 
                     | 
| 
                    binary  | 
                     | 
                     | 
| 
                    binary  | 
                     | 
                     | 
| 
                    binary  | 
                     | 
                     | 
| 
                    binary  | 
                     | 
                     | 
| 
                    binary  | 
                     | 
                     | 
| 
                    binary  | 
                     | 
                     | 
| 
                    binary  | 
                     | 
                     | 
| 
                    binary  | 
                     | 
                     | 
| 
                    binary  | 
                     | 
                     | 
| 
                    binary  | 
                     | 
                     | 
| 
                    binary  | 
                     | 
                     | 
| 
                    binary  | 
                     | 
                     | 
| 
                    binary  | 
                     | 
                     | 
| 
                    binary  | 
                     | 
                     | 
| binary subscript | 
                     | 
                     | 
| 
                    ternary  | 
                     | 
                     | 
| n-ary function call | 
                     | 
                     | 
          Boost.Fusion is a library of iterators, algorithms, containers and adaptors
          for manipulating heterogeneous sequences. In essence, a Proto expression
          is just a heterogeneous sequence of its child expressions, and so Proto
          expressions are valid Fusion random-access sequences. That means you can
          apply Fusion algorithms to them, transform them, apply Fusion filters and
          views to them, and access their elements using fusion::at(). The things Fusion can do to heterogeneous
          sequences are beyond the scope of this users' guide, but below is a simple
          example. It takes a lazy function invocation like fun(1,2,3,4)
          and uses Fusion to print the function arguments in order.
        
struct display { template<typename T> void operator()(T const &t) const { std::cout << t << std::endl; } }; struct fun_t {}; proto::terminal<fun_t>::type const fun = {{}}; // ... fusion::for_each( fusion::transform( // pop_front() removes the "fun" child fusion::pop_front(fun(1,2,3,4)) // Extract the ints from the terminal nodes , proto::functional::value() ) , display() );
          Recall from the Introduction that types in the proto::functional
          namespace define function objects that correspond to Proto's free functions.
          So proto::functional::value()
          creates a function object that is equivalent to the proto::value() function. The above invocation of fusion::for_each()
          displays the following:
        
1 2 3 4
Terminals are also valid Fusion sequences. They contain exactly one element: their value.
Imagine a slight variation of the above example where, instead of iterating over the arguments of a lazy function invocation, we would like to iterate over the terminals in an addition expression:
proto::terminal<int>::type const _1 = {1}; // ERROR: this doesn't work! Why? fusion::for_each( fusion::transform( _1 + 2 + 3 + 4 , proto::functional::value() ) , display() );
          The reason this doesn't work is because the expression _1
          + 2 + 3 +
          4 does not describe a flat sequence
          of terminals --- it describes a binary tree. We can treat it as a flat
          sequence of terminals, however, using Proto's proto::flatten()
          function. proto::flatten() returns a view which makes
          a tree appear as a flat Fusion sequence. If the top-most node has a tag
          type T, then the elements
          of the flattened sequence are the child nodes that do not
          have tag type T. This process
          is evaluated recursively. So the above can correctly be written as:
        
proto::terminal<int>::type const _1 = {1}; // OK, iterate over a flattened view fusion::for_each( fusion::transform( proto::flatten(_1 + 2 + 3 + 4) , proto::functional::value() ) , display() );
          The above invocation of fusion::for_each() displays the following:
        
1 2 3 4
Expression trees can have a very rich and complicated structure. Often, you need to know some things about an expression's structure before you can process it. This section describes the tools Proto provides for peering inside an expression tree and discovering its structure. And as you'll see in later sections, all the really interesting things you can do with Proto begin right here.
Imagine your EDSL is a miniature I/O facility, with iostream operations that execute lazily. You might want expressions representing input operations to be processed by one function, and output operations to be processed by a different function. How would you do that?
            The answer is to write patterns (a.k.a, grammars)
            that match the structure of input and output expressions. Proto provides
            utilities for defining the grammars, and the proto::matches<>
            template for checking whether a given expression type matches the grammar.
          
First, let's define some terminals we can use in our lazy I/O expressions:
proto::terminal< std::istream & >::type cin_ = { std::cin }; proto::terminal< std::ostream & >::type cout_ = { std::cout };
            Now, we can use cout_
            instead of std::cout, and get I/O expression trees
            that we can execute later. To define grammars that match input and output
            expressions of the form cin_
            >> i
            and cout_ <<
            1 we do this:
          
struct Input : proto::shift_right< proto::terminal< std::istream & >, proto::_ > {}; struct Output : proto::shift_left< proto::terminal< std::ostream & >, proto::_ > {};
            We've seen the template proto::terminal<> before, but here we're using
            it without accessing the nested ::type. When used like this, it is a
            very simple grammar, as are proto::shift_right<> and proto::shift_left<>. The newcomer here is _ in the proto
            namespace. It is a wildcard that matches anything. The Input struct is a grammar that matches
            any right-shift expression that has a std::istream
            terminal as its left operand.
          
            We can use these grammars together with the proto::matches<>
            template to query at compile time whether a given I/O expression type
            is an input or output operation. Consider the following:
          
template< typename Expr > void input_output( Expr const & expr ) { if( proto::matches< Expr, Input >::value ) { std::cout << "Input!\n"; } if( proto::matches< Expr, Output >::value ) { std::cout << "Output!\n"; } } int main() { int i = 0; input_output( cout_ << 1 ); input_output( cin_ >> i ); return 0; }
This program prints the following:
Output! Input!
            If we wanted to break the input_output() function into two functions, one that
            handles input expressions and one for output expressions, we can use
            boost::enable_if<>,
            as follows:
          
template< typename Expr > typename boost::enable_if< proto::matches< Expr, Input > >::type input_output( Expr const & expr ) { std::cout << "Input!\n"; } template< typename Expr > typename boost::enable_if< proto::matches< Expr, Output > >::type input_output( Expr const & expr ) { std::cout << "Output!\n"; }
This works as the previous version did. However, the following does not compile at all:
input_output( cout_ << 1 << 2 ); // oops!
            What's wrong? The problem is that this expression does not match our
            grammar. The expression groups as if it were written like (cout_ << 1) << 2. It will not match the Output grammar, which expects the left
            operand to be a terminal, not another left-shift operation. We need to
            fix the grammar.
          
            We notice that in order to verify an expression as input or output, we'll
            need to recurse down to the bottom-left-most leaf and check that it is
            a std::istream or std::ostream.
            When we get to the terminal, we must stop recursing. We can express this
            in our grammar using proto::or_<>.
            Here are the correct Input
            and Output grammars:
          
struct Input : proto::or_< proto::shift_right< proto::terminal< std::istream & >, proto::_ > , proto::shift_right< Input, proto::_ > > {}; struct Output : proto::or_< proto::shift_left< proto::terminal< std::ostream & >, proto::_ > , proto::shift_left< Output, proto::_ > > {};
            This may look a little odd at first. We seem to be defining the Input and Output
            types in terms of themselves. This is perfectly OK, actually. At the
            point in the grammar that the Input
            and Output types are
            being used, they are incomplete, but by the time
            we actually evaluate the grammar with proto::matches<>,
            the types will be complete. These are recursive grammars, and rightly
            so because they must match a recursive data structure!
          
            Matching an expression such as cout_
            << 1
            << 2
            against the Output grammar
            procedes as follows:
          
proto::or_<>
                is tried first. It will fail, because the expression cout_ <<
                1 <<
                2 does not match the grammar
                proto::shift_left<
                proto::terminal<
                std::ostream &
                >, proto::_ >.
              proto::shift_left<
                Output,
                proto::_ >.
                The expression is a left-shift, so we next try to match the operands.
              2 matches
                proto::_ trivially.
              cout_
                << 1
                matches Output, we
                must recursively evaluate the Output
                grammar. This time we succeed, because cout_
                << 1
                will match the first alternate of the proto::or_<>.
              We're done -- the grammar matches successfully.
            The terminals in an expression tree could be const or non-const references,
            or they might not be references at all. When writing grammars, you usually
            don't have to worry about it because proto::matches<>
            gives you a little wiggle room when matching terminals. A grammar such
            as proto::terminal<int>
            will match a terminal of type int,
            int &,
            or int const
            &.
          
            You can explicitly specify that you want to match a reference type. If
            you do, the type must match exactly. For instance, a grammar such as
            proto::terminal<int &>
            will only match an int &. It will not match an int or an int
            const &.
          
The table below shows how Proto matches terminals. The simple rule is: if you want to match only reference types, you must specify the reference in your grammar. Otherwise, leave it off and Proto will ignore const and references.
Table 32.7. proto::matches<> and Reference / CV-Qualification of Terminals
| Terminal | Grammar | Matches? | 
|---|---|---|
| T | T | yes | 
| T & | T | yes | 
| T const & | T | yes | 
| T | T & | no | 
| T & | T & | yes | 
| T const & | T & | no | 
| T | T const & | no | 
| T & | T const & | no | 
| T const & | T const & | yes | 
            This begs the question: What if you want to match an int,
            but not an int &
            or an int const
            &? For forcing exact matches,
            Proto provides the proto::exact<>
            template. For instance, proto::terminal< proto::exact<int> >
            would only match an int
            held by value.
          
            Proto gives you extra wiggle room when matching array types. Array types
            match themselves or the pointer types they decay to. This is especially
            useful with character arrays. The type returned by proto::as_expr("hello") is proto::terminal<char const[6]>::type. That's a terminal containing
            a 6-element character array. Naturally, you can match this terminal with
            the grammar proto::terminal<char const[6]>,
            but the grammar proto::terminal<char const *>
            will match it as well, as the following code fragment illustrates.
          
struct CharString : proto::terminal< char const * > {}; typedef proto::terminal< char const[6] >::type char_array; BOOST_MPL_ASSERT(( proto::matches< char_array, CharString > ));
            What if we only wanted CharString
            to match terminals of exactly the type char
            const *?
            You can use proto::exact<> here to turn off
            the fuzzy matching of terminals, as follows:
          
struct CharString : proto::terminal< proto::exact< char const * > > {}; typedef proto::terminal<char const[6]>::type char_array; typedef proto::terminal<char const *>::type char_string; BOOST_MPL_ASSERT(( proto::matches< char_string, CharString > )); BOOST_MPL_ASSERT_NOT(( proto::matches< char_array, CharString > ));
            Now, CharString does
            not match array types, only character string pointers.
          
            The inverse problem is a little trickier: what if you wanted to match
            all character arrays, but not character pointers? As mentioned above,
            the expression as_expr("hello") has the type proto::terminal< char const[ 6 ] >::type. If you wanted to match character
            arrays of arbitrary size, you could use proto::N,
            which is an array-size wildcard. The following grammar would match any
            string literal: proto::terminal< char const[ proto::N ] >.
          
            Sometimes you need even more wiggle room when matching terminals. For
            example, maybe you're building a calculator EDSL and you want to allow
            any terminals that are convertible to double.
            For that, Proto provides the proto::convertible_to<>
            template. You can use it as: proto::terminal< proto::convertible_to< double
            > >.
          
            There is one more way you can perform a fuzzy match on terminals. Consider
            the problem of trying to match a std::complex<> terminal. You can easily match
            a std::complex<float>
            or a std::complex<double>,
            but how would you match any instantiation of std::complex<>? You can use proto::_
            here to solve this problem. Here is the grammar to match any std::complex<>
            instantiation:
          
struct StdComplex : proto::terminal< std::complex< proto::_ > > {};
When given a grammar like this, Proto will deconstruct the grammar and the terminal it is being matched against and see if it can match all the constituents.
            We've already seen how to use expression generators like proto::terminal<>
            and proto::shift_right<>
            as grammars. We've also seen proto::or_<>,
            which we can use to express a set of alternate grammars. There are a
            few others of interest; in particular, proto::if_<>,
            proto::and_<> and proto::not_<>.
          
            The proto::not_<> template is the simplest.
            It takes a grammar as a template parameter and logically negates it;
            not_<Grammar>
            will match any expression that Grammar
            does not match.
          
            The proto::if_<> template is used
            together with a Proto transform that is evaluated against expression
            types to find matches. (Proto transforms will be described later.)
          
            The proto::and_<> template is like
            proto::or_<>, except that each
            argument of the proto::and_<> must match in order
            for the proto::and_<> to match. As an example,
            consider the definition of CharString
            above that uses proto::exact<>. It could have been
            written without proto::exact<> as follows:
          
struct CharString : proto::and_< proto::terminal< proto::_ > , proto::if_< boost::is_same< proto::_value, char const * >() > > {};
            This says that a CharString
            must be a terminal, and its value type must be the
            same as char const
            *. Notice the template argument
            of proto::if_<>: boost::is_same< proto::_value, char const * >(). This is Proto transform that compares
            the value type of a terminal to char
            const *.
          
            The proto::if_<> template has a couple
            of variants. In addition to if_<Condition> you can also say if_<Condition, ThenGrammar> and if_<Condition, ThenGrammar, ElseGrammar>. These let you select one sub-grammar
            or another based on the Condition.
          
            When your Proto grammar gets large, you'll start to run into some scalability
            problems with proto::or_<>, the construct you
            use to specify alternate sub-grammars. First, due to limitations in C++,
            proto::or_<> can only accept up
            to a certain number of sub-grammars, controlled by the BOOST_PROTO_MAX_LOGICAL_ARITY macro.
            This macro defaults to eight, and you can set it higher, but doing so
            will aggravate another scalability problem: long compile times. With
            proto::or_<>, alternate sub-grammars
            are tried in order -- like a series of cascading if's
            -- leading to lots of unnecessary template instantiations. What you would
            prefer instead is something like switch
            that avoids the expense of cascading if's.
            That's the purpose of proto::switch_<>;
            although less convenient than proto::or_<>,
            it improves compile times for larger grammars and does not have an arbitrary
            fixed limit on the number of sub-grammars.
          
            Let's illustrate how to use proto::switch_<>
            by first writing a big grammar with proto::or_<>
            and then translating it to an equivalent grammar using proto::switch_<>:
          
// Here is a big, inefficient grammar struct ABigGrammar : proto::or_< proto::terminal<int> , proto::terminal<double> , proto::unary_plus<ABigGrammar> , proto::negate<ABigGrammar> , proto::complement<ABigGrammar> , proto::plus<ABigGrammar, ABigGrammar> , proto::minus<ABigGrammar, ABigGrammar> , proto::or_< proto::multiplies<ABigGrammar, ABigGrammar> , proto::divides<ABigGrammar, ABigGrammar> , proto::modulus<ABigGrammar, ABigGrammar> > > {};
            The above might be the grammar to a more elaborate calculator EDSL. Notice
            that since there are more than eight sub-grammars, we had to chain the
            sub-grammars with a nested proto::or_<>
            -- not very nice.
          
            The idea behind proto::switch_<>
            is to dispatch based on an expression's tag type to a sub-grammar that
            handles expressions of that type. To use proto::switch_<>,
            you define a struct with a nested case_<> template, specialized on tag
            types. The above grammar can be expressed using proto::switch_<>
            as follows. It is described below.
          
// Redefine ABigGrammar more efficiently using proto::switch_<> struct ABigGrammar; struct ABigGrammarCases { // The primary template matches nothing: template<typename Tag> struct case_ : proto::not_<_> {}; }; // Terminal expressions are handled here template<> struct ABigGrammarCases::case_<proto::tag::terminal> : proto::or_< proto::terminal<int> , proto::terminal<double> > {}; // Non-terminals are handled similarly template<> struct ABigGrammarCases::case_<proto::tag::unary_plus> : proto::unary_plus<ABigGrammar> {}; template<> struct ABigGrammarCases::case_<proto::tag::negate> : proto::negate<ABigGrammar> {}; template<> struct ABigGrammarCases::case_<proto::tag::complement> : proto::complement<ABigGrammar> {}; template<> struct ABigGrammarCases::case_<proto::tag::plus> : proto::plus<ABigGrammar, ABigGrammar> {}; template<> struct ABigGrammarCases::case_<proto::tag::minus> : proto::minus<ABigGrammar, ABigGrammar> {}; template<> struct ABigGrammarCases::case_<proto::tag::multiplies> : proto::multiplies<ABigGrammar, ABigGrammar> {}; template<> struct ABigGrammarCases::case_<proto::tag::divides> : proto::divides<ABigGrammar, ABigGrammar> {}; template<> struct ABigGrammarCases::case_<proto::tag::modulus> : proto::modulus<ABigGrammar, ABigGrammar> {}; // Define ABigGrammar in terms of ABigGrammarCases // using proto::switch_<> struct ABigGrammar : proto::switch_<ABigGrammarCases> {};
            Matching an expression type E
            against proto::switch_<C>
            is equivalent to matching it against C::case_<E::proto_tag>. By dispatching on the expression's
            tag type, we can jump to the sub-grammar that handles expressions of
            that type, skipping over all the other sub-grammars that couldn't possibly
            match. If there is no specialization of case_<> for a particular tag type, we
            select the primary template. In this case, the primary template inherits
            from proto::not_<_>
            which matches no expressions.
          
Notice the specialization that handles terminals:
// Terminal expressions are handled here template<> struct ABigGrammarCases::case_<proto::tag::terminal> : proto::or_< proto::terminal<int> , proto::terminal<double> > {};
            The proto::tag::terminal type by itself isn't enough
            to select an appropriate sub-grammar, so we use proto::or_<>
            to list the alternate sub-grammars that match terminals.
          
| ![[Note]](../../../doc/src/images/note.png) | Note | 
|---|---|
| 
              You might be tempted to define your  
 struct ABigGrammarCases { template<typename Tag> struct case_ : proto::not_<_> {}; // ERROR: not legal C++ template<> struct case_<proto::tag::terminal> /* ... */ }; 
 Unfortunately, for arcane reasons, it is not legal to define an explicit nested specialization in situ like this. It is, however, perfectly legal to define partial specializations in situ, so you can add a extra dummy template parameter that has a default, as follows: 
 struct ABigGrammarCases { // Note extra "Dummy" template parameter here: template<typename Tag, int Dummy = 0> struct case_ : proto::not_<_> {}; // OK: "Dummy" makes this a partial specialization // instead of an explicit specialization. template<int Dummy> struct case_<proto::tag::terminal, Dummy> /* ... */ }; 
 
              You might find this cleaner than defining explicit  | 
            Not all of C++'s overloadable operators are unary or binary. There is
            the oddball operator()
            -- the function call operator -- which can have any number of arguments.
            Likewise, with Proto you may define your own "operators" that
            could also take more that two arguments. As a result, there may be nodes
            in your Proto expression tree that have an arbitrary number of children
            (up to BOOST_PROTO_MAX_ARITY
            For such cases, Proto provides the proto::vararg<>
            class template. Its template argument is a grammar, and the proto::vararg<> will match the grammar
            zero or more times. Consider a Proto lazy function called fun()
            that can take zero or more characters as arguments, as follows:
          
struct fun_tag {}; struct FunTag : proto::terminal< fun_tag > {}; FunTag::type const fun = {{}}; // example usage: fun(); fun('a'); fun('a', 'b'); ...
            Below is the grammar that matches all the allowable invocations of fun():
          
struct FunCall : proto::function< FunTag, proto::vararg< proto::terminal< char > > > {};
            The FunCall grammar uses
            proto::vararg<> to match zero or
            more character literals as arguments of the fun() function.
          
As another example, can you guess what the following grammar matches?
struct Foo : proto::or_< proto::terminal< proto::_ > , proto::nary_expr< proto::_, proto::vararg< Foo > > > {};
            Here's a hint: the first template parameter to proto::nary_expr<> represents the node type, and
            any additional template parameters represent child nodes. The answer
            is that this is a degenerate grammar that matches every possible expression
            tree, from root to leaves.
          
In this section we'll see how to use Proto to define a grammar for your EDSL and use it to validate expression templates, giving short, readable compile-time errors for invalid expressions.
| ![[Tip]](../../../doc/src/images/tip.png) | Tip | 
|---|---|
| You might think that this is a backwards way of doing things. “If Proto let me select which operators to overload, my users wouldn't be able to create invalid expressions in the first place, and I wouldn't need a grammar at all!” That may be true, but there are reasons for preferring to do things this way. First, it lets you develop your EDSL rapidly -- all the operators are there for you already! -- and worry about invalid syntax later. Second, it might be the case that some operators are only allowed in certain contexts within your EDSL. This is easy to express with a grammar, and hard to do with straight operator overloading. Third, using an EDSL grammar to flag invalid expressions can often yield better errors than manually selecting the overloaded operators. Fourth, the grammar can be used for more than just validation. You can use your grammar to define tree transformations that convert expression templates into other more useful objects. If none of the above convinces you, you actually can use Proto to control which operators are overloaded within your domain. And to do it, you need to define a grammar! | 
In a previous section, we used Proto to define an EDSL for a lazily evaluated calculator that allowed any combination of placeholders, floating-point literals, addition, subtraction, multiplication, division and grouping. If we were to write the grammar for this EDSL in EBNF, it might look like this:
group       ::= '(' expression ')'
factor      ::= double | '_1' | '_2' | group
term        ::= factor (('*' factor) | ('/' factor))*
expression  ::= term (('+' term) | ('-' term))*
This captures the syntax, associativity and precedence rules of a calculator. Writing the grammar for our calculator EDSL using Proto is even simpler. Since we are using C++ as the host language, we are bound to the associativity and precedence rules for the C++ operators. Our grammar can assume them. Also, in C++ grouping is already handled for us with the use of parenthesis, so we don't have to code that into our grammar.
Let's begin our grammar for forward-declaring it:
struct CalculatorGrammar;
It's an incomplete type at this point, but we'll still be able to use it to define the rules of our grammar. Let's define grammar rules for the terminals:
struct Double : proto::terminal< proto::convertible_to< double > > {}; struct Placeholder1 : proto::terminal< placeholder<0> > {}; struct Placeholder2 : proto::terminal< placeholder<1> > {}; struct Terminal : proto::or_< Double, Placeholder1, Placeholder2 > {};
            Now let's define the rules for addition, subtraction, multiplication
            and division. Here, we can ignore issues of associativity and precedence
            -- the C++ compiler will enforce that for us. We only must enforce that
            the arguments to the operators must themselves conform to the CalculatorGrammar that we forward-declared
            above.
          
struct Plus : proto::plus< CalculatorGrammar, CalculatorGrammar > {}; struct Minus : proto::minus< CalculatorGrammar, CalculatorGrammar > {}; struct Multiplies : proto::multiplies< CalculatorGrammar, CalculatorGrammar > {}; struct Divides : proto::divides< CalculatorGrammar, CalculatorGrammar > {};
            Now that we've defined all the parts of the grammar, we can define CalculatorGrammar:
          
struct CalculatorGrammar : proto::or_< Terminal , Plus , Minus , Multiplies , Divides > {};
            That's it! Now we can use CalculatorGrammar
            to enforce that an expression template conforms to our grammar. We can
            use proto::matches<> and BOOST_MPL_ASSERT()
            to issue readable compile-time errors for invalid expressions, as below:
          
template< typename Expr > void evaluate( Expr const & expr ) { BOOST_MPL_ASSERT(( proto::matches< Expr, CalculatorGrammar > )); // ... }
Now that you've written the front end for your EDSL compiler, and you've learned a bit about the intermediate form it produces, it's time to think about what to do with the intermediate form. This is where you put your domain-specific algorithms and optimizations. Proto gives you two ways to evaluate and manipulate expression templates: contexts and transforms.
proto::eval()
            function. It associates behaviors with node types. proto::eval()
            walks the expression and invokes your context at each node.
          Two ways to evaluate expressions! How to choose? Since contexts are largely procedural, they are a bit simpler to understand and debug so they are a good place to start. But although transforms are more advanced, they are also more powerful; since they are associated with rules in your grammar, you can select the proper transform based on the entire structure of a sub-expression rather than simply on the type of its top-most node.
Also, transforms have a concise and declarative syntax that can be confusing at first, but highly expressive and fungible once you become accustomed to it. And -- this is admittedly very subjective -- the author finds programming with Proto transforms to be an inordinate amount of fun! Your mileage may vary.
          Once you have constructed a Proto expression tree, either by using Proto's
          operator overloads or with proto::make_expr()
          and friends, you probably want to actually do something
          with it. The simplest option is to use proto::eval(), a generic expression evaluator. To use
          proto::eval(), you'll need to define
          a context that tells proto::eval()
          how each node should be evaluated. This section goes through the nuts and
          bolts of using proto::eval(), defining evaluation contexts,
          and using the contexts that Proto provides.
        
| ![[Note]](../../../doc/src/images/note.png) | Note | 
|---|---|
| 
             | 
Synopsis:
namespace proto { namespace result_of { // A metafunction for calculating the return // type of proto::eval() given certain Expr // and Context types. template<typename Expr, typename Context> struct eval { typedef typename Context::template eval<Expr>::result_type type; }; } namespace functional { // A callable function object type for evaluating // a Proto expression with a certain context. struct eval : callable { template<typename Sig> struct result; template<typename Expr, typename Context> typename proto::result_of::eval<Expr, Context>::type operator ()(Expr &expr, Context &context) const; template<typename Expr, typename Context> typename proto::result_of::eval<Expr, Context>::type operator ()(Expr &expr, Context const &context) const; }; } template<typename Expr, typename Context> typename proto::result_of::eval<Expr, Context>::type eval(Expr &expr, Context &context); template<typename Expr, typename Context> typename proto::result_of::eval<Expr, Context>::type eval(Expr &expr, Context const &context); }
            Given an expression and an evaluation context, using proto::eval()
            is quite simple. Simply pass the expression and the context to proto::eval() and it does the rest
            and returns the result. You can use the eval<> metafunction in the proto::result_of namespace to compute the
            return type of proto::eval(). The following demonstrates
            a use of proto::eval():
          
template<typename Expr> typename proto::result_of::eval<Expr const, MyContext>::type MyEvaluate(Expr const &expr) { // Some user-defined context type MyContext ctx; // Evaluate an expression with the context return proto::eval(expr, ctx); }
            What proto::eval() does is also very simple.
            It defers most of the work to the context itself. Here essentially is
            the implementation of proto::eval():
          
// eval() dispatches to a nested "eval<>" function // object within the Context: template<typename Expr, typename Context> typename Context::template eval<Expr>::result_type eval(Expr &expr, Context &ctx) { typename Context::template eval<Expr> eval_fun; return eval_fun(expr, ctx); }
            Really, proto::eval() is nothing more than
            a thin wrapper that dispatches to the appropriate handler within the
            context class. In the next section, we'll see how to implement a context
            class from scratch.
          
            As we saw in the previous section, there is really not much to the proto::eval() function. Rather, all
            the interesting expression evaluation goes on within a context class.
            This section shows how to implement one from scratch.
          
All context classes have roughly the following form:
// A prototypical user-defined context. struct MyContext { // A nested eval<> class template template< typename Expr , typename Tag = typename proto::tag_of<Expr>::type > struct eval; // Handle terminal nodes here... template<typename Expr> struct eval<Expr, proto::tag::terminal> { // Must have a nested result_type typedef. typedef ... result_type; // Must have a function call operator that takes // an expression and the context. result_type operator()(Expr &expr, MyContext &ctx) const { return ...; } }; // ... other specializations of struct eval<> ... };
            Context classes are nothing more than a collection of specializations
            of a nested eval<>
            class template. Each specialization handles a different expression type.
          
            In the Hello
            Calculator section, we saw an example of a user-defined context
            class for evaluating calculator expressions. That context class was implemented
            with the help of Proto's proto::callable_context<>.
            If we were to implement it from scratch, it would look something like
            this:
          
// The calculator_context from the "Hello Calculator" section, // implemented from scratch. struct calculator_context { // The values with which we'll replace the placeholders std::vector<double> args; template< typename Expr // defaulted template parameters, so we can // specialize on the expressions that need // special handling. , typename Tag = typename proto::tag_of<Expr>::type , typename Arg0 = typename proto::child_c<Expr, 0>::type > struct eval; // Handle placeholder terminals here... template<typename Expr, int I> struct eval<Expr, proto::tag::terminal, placeholder<I> > { typedef double result_type; result_type operator()(Expr &, MyContext &ctx) const { return ctx.args[I]; } }; // Handle other terminals here... template<typename Expr, typename Arg0> struct eval<Expr, proto::tag::terminal, Arg0> { typedef double result_type; result_type operator()(Expr &expr, MyContext &) const { return proto::child(expr); } }; // Handle addition here... template<typename Expr, typename Arg0> struct eval<Expr, proto::tag::plus, Arg0> { typedef double result_type; result_type operator()(Expr &expr, MyContext &ctx) const { return proto::eval(proto::left(expr), ctx) + proto::eval(proto::right(expr), ctx); } }; // ... other eval<> specializations for other node types ... };
            Now we can use proto::eval() with the context class
            above to evaluate calculator expressions as follows:
          
// Evaluate an expression with a calculator_context calculator_context ctx; ctx.args.push_back(5); ctx.args.push_back(6); double d = proto::eval(_1 + _2, ctx); assert(11 == d);
Defining a context from scratch this way is tedious and verbose, but it gives you complete control over how the expression is evaluated. The context class in the Hello Calculator example was much simpler. In the next section we'll see the helper class Proto provides to ease the job of implementing context classes.
Proto provides some ready-made context classes that you can use as-is, or that you can use to help while implementing your own contexts. They are:
default_context
                  An evaluation context that assigns the usual C++ meanings to all
                  the operators. For example, addition nodes are handled by evaluating
                  the left and right children and then adding the results. The proto::default_context
                  uses Boost.Typeof to deduce the types of the expressions it evaluates.
                
null_contextA simple context that recursively evaluates children but does not combine the results in any way and returns void.
callable_context<>
                  A helper that simplifies the job of writing context classes. Rather
                  than writing template specializations, with proto::callable_context<>
                  you write a function object with an overloaded function call operator.
                  Any expressions not handled by an overload are automatically dispatched
                  to a default evaluation context that you can specify.
                
              The proto::default_context is an
              evaluation context that assigns the usual C++ meanings to all the operators.
              For example, addition nodes are handled by evaluating the left and
              right children and then adding the results. The proto::default_context uses
              Boost.Typeof to deduce the types of the expressions it evaluates.
            
For example, consider the following "Hello World" example:
#include <iostream> #include <boost/proto/proto.hpp> #include <boost/proto/context.hpp> #include <boost/typeof/std/ostream.hpp> using namespace boost; proto::terminal< std::ostream & >::type cout_ = { std::cout }; template< typename Expr > void evaluate( Expr const & expr ) { // Evaluate the expression with default_context, // to give the operators their C++ meanings: proto::default_context ctx; proto::eval(expr, ctx); } int main() { evaluate( cout_ << "hello" << ',' << " world" ); return 0; }
This program outputs the following:
hello, world
              proto::default_context is trivially
              defined in terms of a default_eval<> template, as follows:
            
// Definition of default_context struct default_context { template<typename Expr> struct eval : default_eval< Expr , default_context const , typename tag_of<Expr>::type > {}; };
              There are a bunch of default_eval<> specializations, each of which
              handles a different C++ operator. Here, for instance, is the specialization
              for binary addition:
            
// A default expression evaluator for binary addition template<typename Expr, typename Context> struct default_eval<Expr, Context, proto::tag::plus> { private: static Expr & s_expr; static Context & s_ctx; public: typedef decltype( proto::eval(proto::child_c<0>(s_expr), s_ctx) + proto::eval(proto::child_c<1>(s_expr), s_ctx) ) result_type; result_type operator ()(Expr &expr, Context &ctx) const { return proto::eval(proto::child_c<0>(expr), ctx) + proto::eval(proto::child_c<1>(expr), ctx); } };
              The above code uses decltype
              to calculate the return type of the function call operator. decltype is a new keyword in the next
              version of C++ that gets the type of any expression. Most compilers
              do not yet support decltype
              directly, so default_eval<> uses the Boost.Typeof library
              to emulate it. On some compilers, that may mean that default_context either doesn't work
              or that it requires you to register your types with the Boost.Typeof
              library. Check the documentation for Boost.Typeof to see.
            
              The proto::null_context<>
              is a simple context that recursively evaluates children but does not
              combine the results in any way and returns void. It is useful in conjunction
              with callable_context<>, or when defining your own
              contexts which mutate an expression tree in-place rather than accumulate
              a result, as we'll see below.
            
              proto::null_context<>
              is trivially implemented in terms of null_eval<> as follows:
            
// Definition of null_context struct null_context { template<typename Expr> struct eval : null_eval<Expr, null_context const, Expr::proto_arity::value> {}; };
              And null_eval<>
              is also trivially implemented. Here, for instance is a binary null_eval<>:
            
// Binary null_eval<> template<typename Expr, typename Context> struct null_eval<Expr, Context, 2> { typedef void result_type; void operator()(Expr &expr, Context &ctx) const { proto::eval(proto::child_c<0>(expr), ctx); proto::eval(proto::child_c<1>(expr), ctx); } };
When would such classes be useful? Imagine you have an expression tree with integer terminals, and you would like to increment each integer in-place. You might define an evaluation context as follows:
struct increment_ints { // By default, just evaluate all children by delegating // to the null_eval<> template<typename Expr, typename Arg = proto::result_of::child<Expr>::type> struct eval : null_eval<Expr, increment_ints const> {}; // Increment integer terminals template<typename Expr> struct eval<Expr, int> { typedef void result_type; void operator()(Expr &expr, increment_ints const &) const { ++proto::child(expr); } }; };
              In the next section on proto::callable_context<>,
              we'll see an even simpler way to achieve the same thing.
            
              The proto::callable_context<>
              is a helper that simplifies the job of writing context classes. Rather
              than writing template specializations, with proto::callable_context<>
              you write a function object with an overloaded function call operator.
              Any expressions not handled by an overload are automatically dispatched
              to a default evaluation context that you can specify.
            
              Rather than an evaluation context in its own right, proto::callable_context<>
              is more properly thought of as a context adaptor. To use it, you must
              define your own context that inherits from proto::callable_context<>.
            
              In the null_context
              section, we saw how to implement an evaluation context that increments
              all the integers within an expression tree. Here is how to do the same
              thing with the proto::callable_context<>:
            
// An evaluation context that increments all // integer terminals in-place. struct increment_ints : callable_context< increment_ints const // derived context , null_context const // fall-back context > { typedef void result_type; // Handle int terminals here: void operator()(proto::tag::terminal, int &i) const { ++i; } };
With such a context, we can do the following:
literal<int> i = 0, j = 10; proto::eval( i - j * 3.14, increment_ints() ); std::cout << "i = " << i.get() << std::endl; std::cout << "j = " << j.get() << std::endl;
              This program outputs the following, which shows that the integers
              i and j have been incremented by 1:
            
i = 1 j = 11
              In the increment_ints
              context, we didn't have to define any nested eval<> templates. That's because
              proto::callable_context<>
              implements them for us. proto::callable_context<>
              takes two template parameters: the derived context and a fall-back
              context. For each node in the expression tree being evaluated, proto::callable_context<> checks to see if
              there is an overloaded operator() in the derived context that accepts
              it. Given some expression expr
              of type Expr, and a
              context ctx, it attempts
              to call:
            
ctx( typename Expr::proto_tag() , proto::child_c<0>(expr) , proto::child_c<1>(expr) ... );
              Using function overloading and metaprogramming tricks, proto::callable_context<>
              can detect at compile-time whether such a function exists or not. If
              so, that function is called. If not, the current expression is passed
              to the fall-back evaluation context to be processed.
            
              We saw another example of the proto::callable_context<>
              when we looked at the simple calculator expression evaluator. There,
              we wanted to customize the evaluation of placeholder terminals, and
              delegate the handling of all other nodes to the proto::default_context. We did
              that as follows:
            
// An evaluation context for calculator expressions that // explicitly handles placeholder terminals, but defers the // processing of all other nodes to the default_context. struct calculator_context : proto::callable_context< calculator_context const > { std::vector<double> args; // Define the result type of the calculator. typedef double result_type; // Handle the placeholders: template<int I> double operator()(proto::tag::terminal, placeholder<I>) const { return this->args[I]; } };
              In this case, we didn't specify a fall-back context. In that case,
              proto::callable_context<>
              uses the proto::default_context. With
              the above calculator_context
              and a couple of appropriately defined placeholder terminals, we can
              evaluate calculator expressions, as demonstrated below:
            
template<int I> struct placeholder {}; terminal<placeholder<0> >::type const _1 = {{}}; terminal<placeholder<1> >::type const _2 = {{}}; // ... calculator_context ctx; ctx.args.push_back(4); ctx.args.push_back(5); double j = proto::eval( (_2 - _1) / _2 * 100, ctx ); std::cout << "j = " << j << std::endl;
The above code displays the following:
j = 20
If you have ever built a parser with the help of a tool like Antlr, yacc or Boost.Spirit, you might be familiar with semantic actions. In addition to allowing you to define the grammar of the language recognized by the parser, these tools let you embed code within your grammar that executes when parts of the grammar participate in a parse. Proto has the equivalent of semantic actions. They are called transforms. This section describes how to embed transforms within your Proto grammars, turning your grammars into function objects that can manipulate or evaluate expressions in powerful ways.
Proto transforms are an advanced topic. We'll take it slow, using examples to illustrate the key concepts, starting simple.
The Proto grammars we've seen so far are static. You can check at compile-time to see if an expression type matches a grammar, but that's it. Things get more interesting when you give them runtime behaviors. A grammar with embedded transforms is more than just a static grammar. It is a function object that accepts expressions that match the grammar and does something with them.
Below is a very simple grammar. It matches terminal expressions.
// A simple Proto grammar that matches all terminals proto::terminal< _ >
Here is the same grammar with a transform that extracts the value from the terminal:
// A simple Proto grammar that matches all terminals // *and* a function object that extracts the value from // the terminal proto::when< proto::terminal< _ > , proto::_value // <-- Look, a transform! >
            You can read this as follows: when you match a terminal expression, extract
            the value. The type proto::_value
            is a so-called transform. Later we'll see what makes it a transform,
            but for now just think of it as a kind of function object. Note the use
            of proto::when<>: the first template
            parameter is the grammar to match and the second is the transform to
            execute. The result is both a grammar that matches terminal expressions
            and a function object that accepts terminal expressions and extracts
            their values.
          
As with ordinary grammars, we can define an empty struct that inherits from a grammar+transform to give us an easy way to refer back to the thing we're defining, as follows:
// A grammar and a function object, as before struct Value : proto::when< proto::terminal< _ > , proto::_value > {}; // "Value" is a grammar that matches terminal expressions BOOST_MPL_ASSERT(( proto::matches< proto::terminal<int>::type, Value > )); // "Value" also defines a function object that accepts terminals // and extracts their value. proto::terminal<int>::type answer = {42}; Value get_value; int i = get_value( answer );
            As already mentioned, Value
            is a grammar that matches terminal expressions and a function object
            that operates on terminal expressions. It would be an error to pass a
            non-terminal expression to the Value
            function object. This is a general property of grammars with transforms;
            when using them as function objects, expressions passed to them must
            match the grammar.
          
            Proto grammars are valid TR1-style function objects. That means you can
            use boost::result_of<>
            to ask a grammar what its return type will be, given a particular expression
            type. For instance, we can access the Value
            grammar's return type as follows:
          
// We can use boost::result_of<> to get the return type // of a Proto grammar. typedef typename boost::result_of<Value(proto::terminal<int>::type)>::type result_type; // Check that we got the type we expected BOOST_MPL_ASSERT(( boost::is_same<result_type, int> ));
| ![[Note]](../../../doc/src/images/note.png) | Note | 
|---|---|
| 
              A grammar with embedded transforms is both a grammar and a function
              object. Calling these things "grammars with transforms" would
              get tedious. We could call them something like "active grammars",
              but as we'll see every grammar that you can define
              with Proto is "active"; that is, every grammar has some behavior
              when used as a function object. So we'll continue calling these things
              plain "grammars". The term "transform" is reserved
              for the thing that is used as the second parameter to the  | 
Most grammars are a little more complicated than the one in the preceding section. For the sake of illustration, let's define a rather nonsensical grammar that matches any expression and recurses to the leftmost terminal and returns its value. It will demonstrate how two key concepts of Proto grammars -- alternation and recursion -- interact with transforms. The grammar is described below.
// A grammar that matches any expression, and a function object // that returns the value of the leftmost terminal. struct LeftmostLeaf : proto::or_< // If the expression is a terminal, return its value proto::when< proto::terminal< _ > , proto::_value > // Otherwise, it is a non-terminal. Return the result // of invoking LeftmostLeaf on the 0th (leftmost) child. , proto::when< _ , LeftmostLeaf( proto::_child0 ) > > {}; // A Proto terminal wrapping std::cout proto::terminal< std::ostream & >::type cout_ = { std::cout }; // Create an expression and use LeftmostLeaf to extract the // value of the leftmost terminal, which will be std::cout. std::ostream & sout = LeftmostLeaf()( cout_ << "the answer: " << 42 << '\n' );
            We've seen proto::or_<>
            before. Here it is serving two roles. First, it is a grammar that matches
            any of its alternate sub-grammars; in this case, either a terminal or
            a non-terminal. Second, it is also a function object that accepts an
            expression, finds the alternate sub-grammar that matches the expression,
            and applies its transform. And since LeftmostLeaf
            inherits from proto::or_<>,
            LeftmostLeaf is also
            both a grammar and a function object.
          
| ![[Note]](../../../doc/src/images/note.png) | Note | 
|---|---|
| 
              The second alternate uses  | 
The next section describes this grammar further.
In the grammar defined in the preceding section, the transform associated with non-terminals is a little strange-looking:
proto::when< _ , LeftmostLeaf( proto::_child0 ) // <-- a "callable" transform >
            It has the effect of accepting non-terminal expressions, taking the 0th
            (leftmost) child and recursively invoking the LeftmostLeaf
            function on it. But LeftmostLeaf( proto::_child0
            ) is actually a function
            type. Literally, it is the type of a function that accepts
            an object of type proto::_child0
            and returns an object of type LeftmostLeaf.
            So how do we make sense of this transform? Clearly, there is no function
            that actually has this signature, nor would such a function be useful.
            The key is in understanding how proto::when<> interprets
            its second template parameter.
          
            When the second template parameter to proto::when<>
            is a function type, proto::when<>
            interprets the function type as a transform. In this case, LeftmostLeaf is treated as the type
            of a function object to invoke, and proto::_child0
            is treated as a transform. First, proto::_child0
            is applied to the current expression (the non-terminal that matched this
            alternate sub-grammar), and the result (the 0th child) is passed as an
            argument to LeftmostLeaf.
          
| ![[Note]](../../../doc/src/images/note.png) | Note | 
|---|---|
| Transforms are a Domain-Specific Language 
               | 
            The type LeftmostLeaf( proto::_child0
            ) is an example of a callable
            transform. It is a function type that represents a function
            object to call and its arguments. The types proto::_child0
            and proto::_value are primitive transforms.
            They are plain structs, not unlike function objects, from which callable
            transforms can be composed. There is one other type of transform, object
            transforms, that we'll encounter next.
          
The very first transform we looked at simply extracted the value of terminals. Let's do the same thing, but this time we'll promote all ints to longs first. (Please forgive the contrived-ness of the examples so far; they get more interesting later.) Here's the grammar:
// A simple Proto grammar that matches all terminals, // and a function object that extracts the value from // the terminal, promoting ints to longs: struct ValueWithPomote : proto::or_< proto::when< proto::terminal< int > , long(proto::_value) // <-- an "object" transform > , proto::when< proto::terminal< _ > , proto::_value > > {};
            You can read the above grammar as follows: when you match an int terminal,
            extract the value from the terminal and use it to initialize a long;
            otherwise, when you match another kind of terminal, just extract the
            value. The type long(proto::_value)
            is a so-called object transform. It looks like the
            creation of a temporary long, but it's really a function type. Just as
            a callable transform is a function type that represents a function to
            call and its arguments, an object transforms is a function type that
            represents an object to construct and the arguments to its constructor.
          
| ![[Note]](../../../doc/src/images/note.png) | Note | 
|---|---|
| Object Transforms vs. Callable Transforms 
              When using function types as Proto transforms, they can either represent
              an object to construct or a function to call. It is similar to "normal"
              C++ where the syntax  
 LeftmostLeaf(proto::_child0) // <-- a callable transform long(proto::_value) // <-- an object transform 
 
              Proto can't know in general which is which, so it uses a trait,  | 
            Now that we have the basics of Proto transforms down, let's consider
            a slightly more realistic example. We can use transforms to improve the
            type-safety of the calculator
            EDSL. If you recall, it lets you write infix arithmetic expressions
            involving argument placeholders like _1
            and _2 and pass them
            to STL algorithms as function objects, as follows:
          
double a1[4] = { 56, 84, 37, 69 }; double a2[4] = { 65, 120, 60, 70 }; double a3[4] = { 0 }; // Use std::transform() and a calculator expression // to calculate percentages given two input sequences: std::transform(a1, a1+4, a2, a3, (_2 - _1) / _2 * 100);
            This works because we gave calculator expressions an operator() that evaluates the expression, replacing
            the placeholders with the arguments to operator(). The overloaded calculator<>::operator() looked like this:
          
// Overload operator() to invoke proto::eval() with // our calculator_context. template<typename Expr> double calculator<Expr>::operator()(double a1 = 0, double a2 = 0) const { calculator_context ctx; ctx.args.push_back(a1); ctx.args.push_back(a2); return proto::eval(*this, ctx); }
Although this works, it's not ideal because it doesn't warn users if they supply too many or too few arguments to a calculator expression. Consider the following mistakes:
(_1 * _1)(4, 2); // Oops, too many arguments! (_2 * _2)(42); // Oops, too few arguments!
            The expression _1 *
            _1 defines a unary calculator
            expression; it takes one argument and squares it. If we pass more than
            one argument, the extra arguments will be silently ignored, which might
            be surprising to users. The next expression, _2
            * _2
            defines a binary calculator expression; it takes two arguments, ignores
            the first and squares the second. If we only pass one argument, the code
            silently fills in 0.0 for
            the second argument, which is also probably not what users expect. What
            can be done?
          
            We can say that the arity of a calculator expression
            is the number of arguments it expects, and it is equal to the largest
            placeholder in the expression. So, the arity of _1
            * _1
            is one, and the arity of _2
            * _2
            is two. We can increase the type-safety of our calculator EDSL by making
            sure the arity of an expression equals the actual number of arguments
            supplied. Computing the arity of an expression is simple with the help
            of Proto transforms.
          
            It's straightforward to describe in words how the arity of an expression
            should be calculated. Consider that calculator expressions can be made
            of _1, _2, literals, unary expressions and
            binary expressions. The following table shows the arities for each of
            these 5 constituents.
          
Table 32.8. Calculator Sub-Expression Arities
| Sub-Expression | Arity | 
|---|---|
| Placeholder 1 | 
                       | 
| Placeholder 2 | 
                       | 
| Literal | 
                       | 
| Unary Expression | arity of the operand | 
| Binary Expression | max arity of the two operands | 
Using this information, we can write the grammar for calculator expressions and attach transforms for computing the arity of each constituent. The code below computes the expression arity as a compile-time integer, using integral wrappers and metafunctions from the Boost MPL Library. The grammar is described below.
struct CalcArity : proto::or_< proto::when< proto::terminal< placeholder<0> >, mpl::int_<1>() > , proto::when< proto::terminal< placeholder<1> >, mpl::int_<2>() > , proto::when< proto::terminal<_>, mpl::int_<0>() > , proto::when< proto::unary_expr<_, CalcArity>, CalcArity(proto::_child) > , proto::when< proto::binary_expr<_, CalcArity, CalcArity>, mpl::max<CalcArity(proto::_left), CalcArity(proto::_right)>() > > {};
            When we find a placeholder terminal or a literal, we use an object
            transform such as mpl::int_<1>()
            to create a (default-constructed) compile-time integer representing the
            arity of that terminal.
          
            For unary expressions, we use CalcArity(proto::_child) which is a callable transform
            that computes the arity of the expression's child.
          
The transform for binary expressions has a few new tricks. Let's look more closely:
// Compute the left and right arities and // take the larger of the two. mpl::max<CalcArity(proto::_left), CalcArity(proto::_right)>()
            This is an object transform; it default-constructs ... what exactly?
            The mpl::max<>
            template is an MPL metafunction that accepts two compile-time integers.
            It has a nested ::type
            typedef (not shown) that is the maximum of the two. But here, we appear
            to be passing it two things that are not compile-time
            integers; they're Proto callable transforms. Proto is smart enough to
            recognize that fact. It first evaluates the two nested callable transforms,
            computing the arities of the left and right child expressions. Then it
            puts the resulting integers into mpl::max<> and evaluates the metafunction
            by asking for the nested ::type. That is the type of the object
            that gets default-constructed and returned.
          
            More generally, when evaluating object transforms, Proto looks at the
            object type and checks whether it is a template specialization, like
            mpl::max<>.
            If it is, Proto looks for nested transforms that it can evaluate. After
            any nested transforms have been evaluated and substituted back into the
            template, the new template specialization is the result type, unless
            that type has a nested ::type, in which case that becomes the
            result.
          
            Now that we can calculate the arity of a calculator expression, let's
            redefine the calculator<> expression wrapper we wrote in
            the Getting Started guide to use the CalcArity
            grammar and some macros from Boost.MPL to issue compile-time errors when
            users specify too many or too few arguments.
          
// The calculator expression wrapper, as defined in the Hello // Calculator example in the Getting Started guide. It behaves // just like the expression it wraps, but with extra operator() // member functions that evaluate the expression. // NEW: Use the CalcArity grammar to ensure that the correct // number of arguments are supplied. template<typename Expr> struct calculator : proto::extends<Expr, calculator<Expr>, calculator_domain> { typedef proto::extends<Expr, calculator<Expr>, calculator_domain> base_type; calculator(Expr const &expr = Expr()) : base_type(expr) {} typedef double result_type; // Use CalcArity to compute the arity of Expr: static int const arity = boost::result_of<CalcArity(Expr)>::type::value; double operator()() const { BOOST_MPL_ASSERT_RELATION(0, ==, arity); calculator_context ctx; return proto::eval(*this, ctx); } double operator()(double a1) const { BOOST_MPL_ASSERT_RELATION(1, ==, arity); calculator_context ctx; ctx.args.push_back(a1); return proto::eval(*this, ctx); } double operator()(double a1, double a2) const { BOOST_MPL_ASSERT_RELATION(2, ==, arity); calculator_context ctx; ctx.args.push_back(a1); ctx.args.push_back(a2); return proto::eval(*this, ctx); } };
            Note the use of boost::result_of<> to access the return type of
            the CalcArity function
            object. Since we used compile-time integers in our transforms, the arity
            of the expression is encoded in the return type of the CalcArity function object. Proto grammars
            are valid TR1-style function objects, so you can use boost::result_of<> to figure out their return types.
          
With our compile-time assertions in place, when users provide too many or too few arguments to a calculator expression, as in:
(_2 * _2)(42); // Oops, too few arguments!
... they will get a compile-time error message on the line with the assertion that reads something like this[29]:
c:\boost\org\trunk\libs\proto\scratch\main.cpp(97) : error C2664: 'boost::mpl::asse
rtion_failed' : cannot convert parameter 1 from 'boost::mpl::failed ************boo
st::mpl::assert_relation<x,y,__formal>::************' to 'boost::mpl::assert<false>
::type'
   with
   [
       x=1,
       y=2,
       __formal=bool boost::mpl::operator==(boost::mpl::failed,boost::mpl::failed)
   ]
The point of this exercise was to show that we can write a fairly simple Proto grammar with embedded transforms that is declarative and readable and can compute interesting properties of arbitrarily complicated expressions. But transforms can do more than that. Boost.Xpressive uses transforms to turn expressions into finite state automata for matching regular expressions, and Boost.Spirit uses transforms to build recursive descent parser generators. Proto comes with a collection of built-in transforms that you can use to perform very sophisticated expression manipulations like these. In the next few sections we'll see some of them in action.
            So far, we've only seen examples of grammars with transforms that accept
            one argument: the expression to transform. But consider for a moment
            how, in ordinary procedural code, you would turn a binary tree into a
            linked list. You would start with an empty list. Then, you would recursively
            convert the right branch to a list, and use the result as the initial
            state while converting the left branch to a list. That is, you would
            need a function that takes two parameters: the current node and the list
            so far. These sorts of accumulation problems are
            quite common when processing trees. The linked list is an example of
            an accumulation variable or state. Each iteration
            of the algorithm takes the current element and state, applies some binary
            function to the two and creates a new state. In the STL, this algorithm
            is called std::accumulate().
            In many other languages, it is called fold. Let's
            see how to implement a fold algorithm with Proto transforms.
          
            All Proto grammars can optionally accept a state parameter in addition
            to the expression to transform. If you want to fold a tree to a list,
            you'll need to make use of the state parameter to pass around the list
            you've built so far. As for the list, the Boost.Fusion library provides
            a fusion::cons<>
            type from which you can build heterogeneous lists. The type fusion::nil represents an empty list.
          
            Below is a grammar that recognizes output expressions like cout_ <<
            42 <<
            '\n' and puts the arguments into
            a Fusion list. It is explained below.
          
// Fold the terminals in output statements like // "cout_ << 42 << '\n'" into a Fusion cons-list. struct FoldToList : proto::or_< // Don't add the ostream terminal to the list proto::when< proto::terminal< std::ostream & > , proto::_state > // Put all other terminals at the head of the // list that we're building in the "state" parameter , proto::when< proto::terminal<_> , fusion::cons<proto::_value, proto::_state>( proto::_value, proto::_state ) > // For left-shift operations, first fold the right // child to a list using the current state. Use // the result as the state parameter when folding // the left child to a list. , proto::when< proto::shift_left<FoldToList, FoldToList> , FoldToList( proto::_left , FoldToList(proto::_right, proto::_state) ) > > {};
Before reading on, see if you can apply what you know already about object, callable and primitive transforms to figure out how this grammar works.
            When you use the FoldToList
            function, you'll need to pass two arguments: the expression to fold,
            and the initial state: an empty list. Those two arguments get passed
            around to each transform. We learned previously that proto::_value
            is a primitive transform that accepts a terminal expression and extracts
            its value. What we didn't know until now was that it also accepts the
            current state and ignores it. proto::_state
            is also a primitive transform. It accepts the current expression, which
            it ignores, and the current state, which it returns.
          
            When we find a terminal, we stick it at the head of the cons list, using
            the current state as the tail of the list. (The first alternate causes
            the ostream to be skipped.
            We don't want cout in
            the list.) When we find a shift-left node, we apply the following transform:
          
// Fold the right child and use the result as // state while folding the right. FoldToList( proto::_left , FoldToList(proto::_right, proto::_state) )
You can read this transform as follows: using the current state, fold the right child to a list. Use the new list as the state while folding the left child to a list.
| ![[Tip]](../../../doc/src/images/tip.png) | Tip | 
|---|---|
| 
              If your compiler is Microsoft Visual C++, you'll find that the above
              transform does not compile. The compiler has bugs with its handling
              of nested function types. You can work around the bug by wrapping the
              inner transform in  
 FoldToList( proto::_left , proto::call<FoldToList(proto::_right, proto::_state)> ) 
 
               | 
            Now that we have defined the FoldToList
            function object, we can use it to turn output expressions into lists
            as follows:
          
proto::terminal<std::ostream &>::type const cout_ = {std::cout}; // This is the type of the list we build below typedef fusion::cons< int , fusion::cons< double , fusion::cons< char , fusion::nil > > > result_type; // Fold an output expression into a Fusion list, using // fusion::nil as the initial state of the transformation. FoldToList to_list; result_type args = to_list(cout_ << 1 << 3.14 << '\n', fusion::nil()); // Now "args" is the list: {1, 3.14, '\n'}
When writing transforms, "fold" is such a basic operation that Proto provides a number of built-in fold transforms. We'll get to them later. For now, rest assured that you won't always have to stretch your brain so far to do such basic things.
In the last section, we saw that we can pass a second parameter to grammars with transforms: an accumulation variable or state that gets updated as your transform executes. There are times when your transforms will need to access auxiliary data that does not accumulate, so bundling it with the state parameter is impractical. Instead, you can pass auxiliary data as a third parameter, known as the data parameter.
            Let's modify our previous example so that it writes each terminal to
            std::cout before it puts it into a list.
            This could be handy for debugging your transforms, for instance. We can
            make it general by passing a std::ostream
            into the transform in the data parameter. Within the transform itself,
            we can retrieve the ostream
            with the proto::_data
            transform. The strategy is as follows: use the proto::and_<>
            transform to chain two actions. The second action will create the fusion::cons<>
            node as before. The first action, however, will display the current expression.
            For that, we first construct an instance of proto::functional::display_expr and then call
            it.
          
// Fold the terminals in output statements like // "cout_ << 42 << '\n'" into a Fusion cons-list. struct FoldToList : proto::or_< // Don't add the ostream terminal to the list proto::when< proto::terminal< std::ostream & > , proto::_state > // Put all other terminals at the head of the // list that we're building in the "state" parameter , proto::when< proto::terminal<_> , proto::and_< // First, write the terminal to an ostream passed // in the data parameter proto::lazy< proto::make<proto::functional::display_expr(proto::_data)>(_) > // Then, constuct the new cons list. , fusion::cons<proto::_value, proto::_state>( proto::_value, proto::_state ) > > // For left-shift operations, first fold the right // child to a list using the current state. Use // the result as the state parameter when folding // the left child to a list. , proto::when< proto::shift_left<FoldToList, FoldToList> , FoldToList( proto::_left , FoldToList(proto::_right, proto::_state, proto::_data) , proto::_data ) > > {};
            This is a lot to take in, no doubt. But focus on the second when clause above. It says: when you
            find a terminal, first display the terminal using the ostream
            you find in the data parameter, then take the value of the terminal and
            the current state to build a new cons
            list. The function object display_expr
            does the job of printing the terminal, and proto::and_<> chains the actions together and
            executes them in sequence, returning the result of the last one.
          
| ![[Note]](../../../doc/src/images/note.png) | Note | 
|---|---|
| 
              Also new is  | 
            We can use the above transform as before, but now we can pass an ostream as the third parameter and
            get to watch the transform in action. Here's a sample usage:
          
proto::terminal<std::ostream &>::type const cout_ = {std::cout}; // This is the type of the list we build below typedef fusion::cons< int , fusion::cons< double , fusion::cons< char , fusion::nil > > > result_type; // Fold an output expression into a Fusion list, using // fusion::nil as the initial state of the transformation. // Pass std::cout as the data parameter so that we can track // the progress of the transform on the console. FoldToList to_list; result_type args = to_list(cout_ << 1 << 3.14 << '\n', fusion::nil(), std::cout); // Now "args" is the list: {1, 3.14, '\n'}
This code displays the following:
terminal( ) terminal(3.14) terminal(1)
This is a rather round-about way of demonstrating that you can pass extra data to a transform as a third parameter. There are no restrictions on what this parameter can be, and, unlike the state parameter, Proto will never mess with it.
| ![[Note]](../../../doc/src/images/note.png) | Note | 
|---|---|
| This is an advanced topic. Feel free to skip if you are new to Proto. | 
            The example above uses the data parameter as a transport mechanism for
            an unstructured blob of data; in this case, a reference to an ostream. As your Proto algorithms become
            more sophisticated, you may find that an unstructured blob of data isn't
            terribly convenient to work with. Different parts of your algorithm may
            be interested in different bits of data. What you want, instead, is a
            way to pass in a collection of environment variables
            to a transform, like a collection of key/value pairs. Then, you can easily
            get at the piece of data you want by asking the data parameter for the
            value associated with a particular key. Proto's transform environments
            give you just that.
          
Let's start by defining a key.
BOOST_PROTO_DEFINE_ENV_VAR(mykey_type, mykey);
            This defines a global constant mykey
            with the type mykey_type.
            We can use mykey to store
            a piece of assiciated data in a transform environment, as so:
          
// Call the MyEval algorithm with a transform environment containing // two key/value pairs: one for proto::data and one for mykey MyEval()( expr, state, (proto::data = 42, mykey = "hello world") );
            The above means to invoke the MyEval
            algorithm with three parameters: an expression, an initial state, and
            a transform environment containing two key/value pairs.
          
            From within a Proto algorithm, you can access the values associated with
            different keys using the proto::_env_var<>
            transform. For instance, proto::_env_var<mykey_type> would fetch the value "hello world" from the transform
            environment created above.
          
            The proto::_data transform has some additional
            smarts. Rather than always returning the third parameter regarless of
            whether it is a blob or a transform environment, it checks first to see
            if it's a blob or not. If so, that's what gets returned. If not, it returns
            the value associated with the proto::data
            key. In the above example, that would be the value 42.
          
            There's a small host of functions, metafunction, and classes that you
            can use to create and manipulate transform environments, some for testing
            whether an object is a transform environment, some for coercing an object
            to be a transform environment, and some for querying a transform environment
            whether or not is has a value for a particular key. For an exhaustive
            treatment of the topic, check out the reference for the boost/proto/transform/env.hpp
            header.
          
            Let's use FoldToList
            example from the previous two sections to illustrate some other niceties
            of Proto transforms. We've seen that grammars, when used as function
            objects, can accept up to 3 parameters, and that when using these grammars
            in callable transforms, you can also specify up to 3 parameters. Let's
            take another look at the transform associated with non-terminals from
            the last section:
          
FoldToList( proto::_left , FoldToList(proto::_right, proto::_state, proto::_data) , proto::_data )
            Here we specify all three parameters to both invocations of the FoldToList grammar. But we don't have
            to specify all three. If we don't specify a third parameter, proto::_data is assumed. Likewise for the
            second parameter and proto::_state.
            So the above transform could have been written more simply as:
          
FoldToList( proto::_left , StringCopy(proto::_right) )
The same is true for any primitive transform. The following are all equivalent:
Table 32.9. Implicit Parameters to Primitive Transforms
| Equivalent Transforms | 
|---|
| 
                       | 
| 
                       | 
| 
                       | 
| 
                       | 
| 
                       | 
| ![[Note]](../../../doc/src/images/note.png) | Note | 
|---|---|
| Grammars Are Primitive Transforms Are Function Objects So far, we've said that all Proto grammars are function objects. But it's more accurate to say that Proto grammars are primitive transforms -- a special kind of function object that takes between 1 and 3 arguments, and that Proto knows to treat specially when used in a callable transform, as in the table above. | 
| ![[Note]](../../../doc/src/images/note.png) | Note | 
|---|---|
| Not All Function Objects Are Primitive Transforms 
              You might be tempted now to drop the  | 
Once you know that primitive transforms will always receive all three parameters -- expression, state, and data -- it makes things possible that wouldn't be otherwise. For instance, consider that for binary expressions, these two transforms are equivalent. Can you see why?
Table 32.10. Two Equivalent Transforms
| 
                      Without  | 
                      With  | 
|---|---|
| 
 FoldToList( proto::_left , FoldToList(proto::_right, proto::_state, proto::_data) , proto::_data ) 
 | 
 proto::reverse_fold<_, proto::_state, FoldToList> 
 | 
Processing expressions with an arbitrary number of children can be a pain. What if you want to do something to each child, then pass the results as arguments to some other function? Can you do it just once without worrying about how many children an expression has? Yes. This is where Proto's unpacking expressions come in handy. Unpacking expressions give you a way to write callable and object transforms that handle n-ary expressions.
| ![[Note]](../../../doc/src/images/note.png) | Note | 
|---|---|
| Inspired by C++11 Variadic Templates Proto's unpacking expressions take inspiration from the C++11 feature of the same name. If you are familiar with variadic functions, and in particular how to expand a function parameter pack, this discussion should seem very familiar. However, this feature doesn't actually use any C++11 features, so the code describe here will work with any compliant C++98 compiler. | 
            Proto has the built-in proto::_default<>
            transform for evaluating Proto expressions in a C++-ish way. But if it
            didn't, it wouldn't be too hard to implement one from scratch using Proto's
            unpacking patterns. The transform eval
            below does just that.
          
// A callable polymorphic function object that takes an unpacked expression // and a tag, and evaluates the expression. A plus tag and two operands adds // them with operator +, for instance. struct do_eval : proto::callable { typedef double result_type; #define UNARY_OP(TAG, OP) \ template<typename Arg> \ double operator()(proto::tag::TAG, Arg arg) const \ { \ return OP arg; \ } \ /**/ #define BINARY_OP(TAG, OP) \ template<typename Left, typename Right> \ double operator()(proto::tag::TAG, Left left, Right right) const \ { \ return left OP right; \ } \ /**/ UNARY_OP(negate, -) BINARY_OP(plus, +) BINARY_OP(minus, -) BINARY_OP(multiplies, *) BINARY_OP(divides, /) /*... others ...*/ }; struct eval : proto::or_< // Evaluate terminals by simply returning their value proto::when<proto::terminal<_>, proto::_value> // Non-terminals are handled by unpacking the expression, // recursively calling eval on each child, and passing // the results along with the expression's tag to do_eval // defined above. , proto::otherwise<do_eval(proto::tag_of<_>(), eval(proto::pack(_))...)> // UNPACKING PATTERN HERE -------------------^^^^^^^^^^^^^^^^^^^^^^^^ > {};
            The bulk of the above code is devoted to the do_eval
            function object that maps tag types to behaviors, but the interesting
            bit is the definition of the eval
            algorithm at the bottom. Terminals are handled quite simply, but non-terminals
            could be unary, binary, ternary, even n-ary if we
            consider function call expressions. The eval
            algorithm handles this uniformly with the help of an unpacking pattern.
          
Non-terminals are evaluated with this callable transform:
do_eval(proto::tag_of<_>(), eval(proto::pack(_))...)
            You can read this as: call the do_eval
            function object with the tag of the current expression and all its children
            after they have each been evaluated with eval.
            The unpacking pattern is the bit just before the ellipsis: eval(proto::pack(_)).
          
            What's going on here is this. The unpacking expression gets repeated
            once for each child in the expression currently being evaluated. In each
            repetition, the type proto::pack(_) gets replaced with proto::_child_c<N>.
            So, if a unary expression is passed to eval,
            it actually gets evaluated like this:
          
// After the unpacking pattern is expanded for a unary expression do_eval(proto::tag_of<_>(), eval(proto::_child_c<0>))
And when passed a binary expression, the unpacking pattern expands like this:
// After the unpacking pattern is expanded for a binary expression do_eval(proto::tag_of<_>(), eval(proto::_child_c<0>), eval(proto::_child_c<1>))
Although it can't happen in our example, when passed a terminal, the unpacking pattern expands such that it extracts the value from the terminal instead of the children. So it gets handled like this:
// If a terminal were passed to this transform, Proto would try // to evaluate it like this, which would fail: do_eval(proto::tag_of<_>(), eval(proto::_value))
            That doesn't make sense. proto::_value
            would return something that isn't a Proto expression, and eval wouldn't be able to evaluate it.
            Proto algorithms don't work unless you pass them Proto expressions.
          
| ![[Note]](../../../doc/src/images/note.png) | Note | 
|---|---|
| Kickin' It Old School 
              You may be thinking, my compiler doesn't support C++11 variadic templates!
              How can this possibly work? The answer is simple: The  | 
            Unpacking patterns are very expressive. Any callable or object transform
            can be used as an unpacking pattern, so long as proto::pack(_) appears exactly once somewhere within
            it. This gives you a lot of flexibility in how you want to process the
            children of an expression before passing them on to some function object
            or object constructor.
          
| ![[Note]](../../../doc/src/images/note.png) | Note | 
|---|---|
| This is an advanced topic that is only necessary for people defining large EDSLs. Feel free to skip this if you're just getting started with Proto. | 
            So far, we've seen examples of grammars with embedded transforms. In
            practice, grammars can get pretty large, and you may want to use them
            to drive several different computations. For instance, you may have a
            grammar for a linear algebra domain, and you may want to use it to compute
            the shape of the result (vector or matrix?) and also to compute the result
            optimally. You don't want to have to copy and paste the whole shebang
            just to tweak one of the embedded transforms. What you want instead is
            to define the grammar once, and specify the transforms later when you're
            ready to evaluate an expression. For that, you use external
            transforms. The pattern you'll use is this: replace one or
            more of the transforms in your grammar with the special placeholder
            proto::external_transform.
            Then, you'll create a bundle of transforms that you will pass to the
            grammar in the data parameter (the 3rd parameter after the expression
            and state) when evaluating it.
          
To illustrate external transforms, we'll build a calculator evaluator that can be configured to throw an exception on division by zero. Here is a bare-bones front end that defines a domain, a grammar, an expression wrapper, and some placeholder terminals.
#include <boost/assert.hpp> #include <boost/mpl/int.hpp> #include <boost/fusion/container/vector.hpp> #include <boost/fusion/container/generation/make_vector.hpp> #include <boost/proto/proto.hpp> namespace mpl = boost::mpl; namespace proto = boost::proto; namespace fusion = boost::fusion; // The argument placeholder type template<typename I> struct placeholder : I {}; // The grammar for valid calculator expressions struct calc_grammar : proto::or_< proto::terminal<placeholder<proto::_> > , proto::terminal<int> , proto::plus<calc_grammar, calc_grammar> , proto::minus<calc_grammar, calc_grammar> , proto::multiplies<calc_grammar, calc_grammar> , proto::divides<calc_grammar, calc_grammar> > {}; template<typename E> struct calc_expr; struct calc_domain : proto::domain<proto::generator<calc_expr> > {}; template<typename E> struct calc_expr : proto::extends<E, calc_expr<E>, calc_domain> { calc_expr(E const &e = E()) : calc_expr::proto_extends(e) {} }; calc_expr<proto::terminal<placeholder<mpl::int_<0> > >::type> _1; calc_expr<proto::terminal<placeholder<mpl::int_<1> > >::type> _2; int main() { // Build a calculator expression, and do nothing with it. (_1 + _2); }
            Now, let's embed transforms into calc_grammar
            so that we can use it to evaluate calculator expressions:
          
// The calculator grammar with embedded transforms for evaluating expression. struct calc_grammar : proto::or_< proto::when< proto::terminal<placeholder<proto::_> > , proto::functional::at(proto::_state, proto::_value) > , proto::when< proto::terminal<int> , proto::_value > , proto::when< proto::plus<calc_grammar, calc_grammar> , proto::_default<calc_grammar> > , proto::when< proto::minus<calc_grammar, calc_grammar> , proto::_default<calc_grammar> > , proto::when< proto::multiplies<calc_grammar, calc_grammar> , proto::_default<calc_grammar> > , proto::when< proto::divides<calc_grammar, calc_grammar> , proto::_default<calc_grammar> > > {};
            With this definition of calc_grammar
            we can evaluate expressions by passing along a Fusion vector containing
            the values to use for the _1
            and _2 placeholders:
          
int result = calc_grammar()(_1 + _2, fusion::make_vector(3, 4)); BOOST_ASSERT(result == 7);
            We also want an alternative evaluation strategy that checks for division
            by zero and throws an exception. Just how ridiculous would it be to copy
            the entire calc_grammar
            just to change the one line that transforms division expressions?! External
            transforms are ideally suited to this problem.
          
First, we give the division rule in our grammar a "name"; that is, we make it a struct. We'll use this unique type later to dispatch to the right transforms.
struct calc_grammar; struct divides_rule : proto::divides<calc_grammar, calc_grammar> {};
            Next, we change calc_grammar
            to make the handling of division expressions external.
          
// The calculator grammar with an external transform for evaluating // division expressions. struct calc_grammar : proto::or_< /* ... as before ... */ , proto::when< divides_rule , proto::external_transform > > {};
            The use of proto::external_transform above
            makes the handling of division expressions externally parameterizeable.
          
            Next, we use proto::external_transforms<>
            (note the trailing 's') to capture our evaluation strategy in a bundle
            that we can pass along to the transform in the data parameter. Read on
            for the explanation.
          
// Evaluate division nodes as before struct non_checked_division : proto::external_transforms< proto::when< divides_rule, proto::_default<calc_grammar> > > {}; /* ... */ non_checked_division non_checked; int result2 = calc_grammar()(_1 / _2, fusion::make_vector(6, 2), non_checked);
            The struct non_cecked_division
            associates the transform proto::_default<calc_grammar> with the divides_rule
            grammar rule. An instance of that struct is passed along as the third
            parameter when invoking calc_grammar.
          
Now, let's implement checked division. The rest should be unsurprising.
struct division_by_zero : std::exception {}; struct do_checked_divide : proto::callable { typedef int result_type; int operator()(int left, int right) const { if (right == 0) throw division_by_zero(); return left / right; } }; struct checked_division : proto::external_transforms< proto::when< divides_rule , do_checked_divide(calc_grammar(proto::_left), calc_grammar(proto::_right)) > > {}; /* ... */ try { checked_division checked; int result3 = calc_grammar_extern()(_1 / _2, fusion::make_vector(6, 0), checked); } catch(division_by_zero) { std::cout << "caught division by zero!\n"; }
The above code demonstrates how a single grammar can be used with different transforms specified externally. This makes it possible to reuse a grammar to drive several different computations.
            As described above, the external transforms feature usurps the data parameter,
            which is intended to be a place where you can pass arbitrary data, and
            gives it a specific meaning. But what if you are already using the data
            parameter for something else? The answer is to use a transform environment.
            By associating your external transforms with the proto::transforms
            key, you are free to pass arbitrary data in other slots.
          
To continue the above example, what if we also needed to pass a piece of data into our transform along with the external transforms? It would look like this:
int result3 = calc_grammar_extern()( _1 / _2 , fusion::make_vector(6, 0) , (proto::data = 42, proto::transforms = checked) );
            In the above invocation of the calc_grammar_extern
            algorithm, the map of external transforms is associated with the proto::transforms key and passed to the algorithm
            in a transform environment. Also in the transform environment is a key/value
            pair that associates the value 42
            with the proto::data key.
          
Primitive transforms are the building blocks for more interesting composite transforms. Proto defines a bunch of generally useful primitive transforms. They are summarized below.
proto::_valueGiven a terminal expression, return the value of the terminal.
proto::_child_c<>
                  Given a non-terminal expression, proto::_child_c< returns the N>N-th
                  child.
                
proto::_child
                  A synonym for proto::_child_c<0>.
                
proto::_left
                  A synonym for proto::_child_c<0>.
                
proto::_right
                  A synonym for proto::_child_c<1>.
                
proto::_exprReturns the current expression unmodified.
proto::_stateReturns the current state unmodified.
proto::_dataReturns the current data unmodified.
proto::call<>
                  For a given callable transform CTproto::call< turns the callable transform
                  into a primitive transform. This is useful for disambiguating callable
                  transforms from object transforms, and also for working around
                  compiler bugs with nested function types.
                CT>
proto::make<>
                  For a given object transform OTproto::make< turns the object transform
                  into a primitive transform. This is useful for disambiguating object
                  transforms from callable transforms, and also for working around
                  compiler bugs with nested function types.
                OT>
proto::_default<>
                  Given a grammar G, proto::_default< evaluates the current node
                  according to the standard C++ meaning of the operation the node
                  represents. For instance, if the current node is a binary plus
                  node, the two children will both be evaluated according to G>G
proto::fold<>
                  Given three transforms ETSTFTproto::fold< first evaluates ET, ST, FT>ETSTFT
proto::reverse_fold<>
                  Like proto::fold<>, except the
                  elements in the Fusion sequence are iterated in reverse order.
                
proto::fold_tree<>
                  Like proto::fold<, except that the result of
                  the ET, ST, FT>ETa
                  >> b
                  >> c
                  would be flattened to the sequence [a,
                  b, c], and this is the sequence
                  that would be folded.
                
proto::reverse_fold_tree<>
                  Like proto::fold_tree<>, except that
                  the flattened sequence is iterated in reverse order.
                
proto::lazy<>
                  A combination of proto::make<>
                  and proto::call<> that is useful
                  when the nature of the transform depends on the expression, state
                  and/or data parameters. proto::lazy<R(A0,A1...An)> first evaluates proto::make<R()>
                  to compute a callable type R2.
                  Then, it evaluates proto::call<R2(A0,A1...An)>.
                
In addition to the above primitive transforms, all of Proto's grammar elements are also primitive transforms. Their behaviors are described below.
proto::_Return the current expression unmodified.
proto::or_<>For the specified set of alternate sub-grammars, find the one that matches the given expression and apply its associated transform.
proto::and_<>For the given set of sub-grammars, apply all the associated transforms and return the result of the last.
proto::not_<>Return the current expression unmodified.
proto::if_<>Given three transforms, evaluate the first and treat the result as a compile-time Boolean value. If it is true, evaluate the second transform. Otherwise, evaluate the third.
proto::switch_<>
                  As with proto::or_<>, find the sub-grammar
                  that matches the given expression and apply its associated transform.
                
proto::terminal<>Return the current terminal expression unmodified.
proto::plus<>, proto::nary_expr<>,
              et. al.
                  A Proto grammar that matches a non-terminal such as proto::plus<, when used as a primitive transform,
                  creates a new plus node where the left child is transformed according
                  to G0, G1>G0G1
            Note the primitive transform associated with grammar elements such as
            proto::plus<> described above.
            They possess a so-called pass-through transform.
            The pass-through transform accepts an expression of a certain tag type
            (say, proto::tag::plus) and creates a new expression
            of the same tag type, where each child expression is transformed according
            to the corresponding child grammar of the pass-through transform. So
            for instance this grammar ...
          
proto::function< X, proto::vararg<Y> >
            ... matches function expressions where the first child matches the X grammar and the rest match the Y grammar. When used as a transform,
            the above grammar will create a new function expression where the first
            child is transformed according to X
            and the rest are transformed according to Y.
          
The following class templates in Proto can be used as grammars with pass-through transforms:
Table 32.11. Class Templates With Pass-Through Transforms
| Templates with Pass-Through Transforms | 
|---|
| 
                       | 
| 
                       | 
| 
                       | 
| 
                       | 
| 
                       | 
| 
                       | 
| 
                       | 
| 
                       | 
| 
                       | 
| 
                       | 
| 
                       | 
| 
                       | 
| 
                       | 
| 
                       | 
| 
                       | 
| 
                       | 
| 
                       | 
| 
                       | 
| 
                       | 
| 
                       | 
| 
                       | 
| 
                       | 
| 
                       | 
| 
                       | 
| 
                       | 
| 
                       | 
| 
                       | 
| 
                       | 
| 
                       | 
| 
                       | 
| 
                       | 
| 
                       | 
| 
                       | 
| 
                       | 
| 
                       | 
| 
                       | 
| 
                       | 
| 
                       | 
| 
                       | 
| 
                       | 
| 
                       | 
| 
                       | 
| 
                       | 
| 
                       | 
| 
                       | 
| 
                       | 
| 
                       | 
            We've seen templates such as proto::terminal<>,
            proto::plus<> and proto::nary_expr<>
            fill many roles. They are metafunction that generate expression types.
            They are grammars that match expression types. And they are primitive
            transforms. The following code samples show examples of each.
          
As Metafunctions ...
// proto::terminal<> and proto::plus<> are metafunctions // that generate expression types: typedef proto::terminal<int>::type int_; typedef proto::plus<int_, int_>::type plus_; int_ i = {42}, j = {24}; plus_ p = {i, j};
As Grammars ...
// proto::terminal<> and proto::plus<> are grammars that // match expression types struct Int : proto::terminal<int> {}; struct Plus : proto::plus<Int, Int> {}; BOOST_MPL_ASSERT(( proto::matches< int_, Int > )); BOOST_MPL_ASSERT(( proto::matches< plus_, Plus > ));
As Primitive Transforms ...
// A transform that removes all unary_plus nodes in an expression struct RemoveUnaryPlus : proto::or_< proto::when< proto::unary_plus<RemoveUnaryPlus> , RemoveUnaryPlus(proto::_child) > // Use proto::terminal<> and proto::nary_expr<> // both as grammars and as primitive transforms. , proto::terminal<_> , proto::nary_expr<_, proto::vararg<RemoveUnaryPlus> > > {}; int main() { proto::literal<int> i(0); proto::display_expr( +i - +(i - +i) ); proto::display_expr( RemoveUnaryPlus()( +i - +(i - +i) ) ); }
The above code displays the following, which shows that unary plus nodes have been stripped from the expression:
minus(
    unary_plus(
        terminal(0)
    )
  , unary_plus(
        minus(
            terminal(0)
          , unary_plus(
                terminal(0)
            )
        )
    )
)
minus(
    terminal(0)
  , minus(
        terminal(0)
      , terminal(0)
    )
)
            In previous sections, we've seen how to compose larger transforms out
            of smaller transforms using function types. The smaller transforms from
            which larger transforms are composed are primitive transforms,
            and Proto provides a bunch of common ones such as _child0
            and _value. In this section
            we'll see how to author your own primitive transforms.
          
| ![[Note]](../../../doc/src/images/note.png) | Note | 
|---|---|
| There are a few reasons why you might want to write your own primitive transforms. For instance, your transform may be complicated, and composing it out of primitives becomes unwieldy. You might also need to work around compiler bugs on legacy compilers that make composing transforms using function types problematic. Finally, you might also decide to define your own primitive transforms to improve compile times. Since Proto can simply invoke a primitive transform directly without having to process arguments or differentiate callable transforms from object transforms, primitive transforms are more efficient. | 
            Primitive transforms inherit from proto::transform<> and have a nested impl<>
            template that inherits from proto::transform_impl<>. For example, this is how Proto
            defines the _child_c<
            transform, which returns the N>N-th child of
            the current expression:
          
namespace boost { namespace proto { // A primitive transform that returns N-th child // of the current expression. template<int N> struct _child_c : transform<_child_c<N> > { template<typename Expr, typename State, typename Data> struct impl : transform_impl<Expr, State, Data> { typedef typename result_of::child_c<Expr, N>::type result_type; result_type operator ()( typename impl::expr_param expr , typename impl::state_param state , typename impl::data_param data ) const { return proto::child_c<N>(expr); } }; }; // Note that _child_c<N> is callable, so that // it can be used in callable transforms, as: // _child_c<0>(_child_c<1>) template<int N> struct is_callable<_child_c<N> > : mpl::true_ {}; }}
            The proto::transform<>
            base class provides the operator() overloads and the nested result<>
            template that make your transform a valid function object. These are
            implemented in terms of the nested impl<> template you define.
          
            The proto::transform_impl<>
            base class is a convenience. It provides some nested typedefs that are
            generally useful. They are specified in the table below:
          
Table 32.12. proto::transform_impl<Expr, State, Data> typedefs
| typedef | Equivalent To | 
|---|---|
| 
                       | 
                       | 
| 
                       | 
                       | 
| 
                       | 
                       | 
| 
                       | 
                       | 
| 
                       | 
                       | 
| 
                       | 
                       | 
            You'll notice that _child_c::impl::operator() takes arguments of types expr_param, state_param,
            and data_param. The typedefs
            make it easy to accept arguments by reference or const reference accordingly.
          
            The only other interesting bit is the is_callable<> specialization, which will be
            described in the next
            section.
          
            Transforms are typically of the form proto::when< Something, R(A0,A1,...) >.
            The question is whether R
            represents a function to call or an object to construct, and the answer
            determines how proto::when<> evaluates the transform.
            proto::when<> uses the proto::is_callable<>
            trait to disambiguate between the two. Proto does its best to guess whether
            a type is callable or not, but it doesn't always get it right. It's best
            to know the rules Proto uses, so that you know when you need to be more
            explicit.
          
            For most types R, proto::is_callable<R>
            checks for inheritance from proto::callable.
            However, if the type R
            is a template specialization, Proto assumes that it is not
            callable even if the template inherits from proto::callable.
            We'll see why in a minute. Consider the following erroneous callable
            object:
          
// Proto can't tell this defines something callable! template<typename T> struct times2 : proto::callable { typedef T result_type; T operator()(T i) const { return i * 2; } }; // ERROR! This is not going to multiply the int by 2: struct IntTimes2 : proto::when< proto::terminal<int> , times2<int>(proto::_value) > {};
            The problem is that Proto doesn't know that times2<int> is callable, so rather that invoking
            the times2<int>
            function object, Proto will try to construct a times2<int> object and initialize it will an
            int. That will not compile.
          
| ![[Note]](../../../doc/src/images/note.png) | Note | 
|---|---|
| 
              Why can't Proto tell that  | 
            There are a couple of solutions to the times2<int> problem. One solution is to wrap
            the transform in proto::call<>. This forces Proto to treat
            times2<int>
            as callable:
          
// OK, calls times2<int> struct IntTimes2 : proto::when< proto::terminal<int> , proto::call<times2<int>(proto::_value)> > {};
            This can be a bit of a pain, because we need to wrap every use of times2<int>,
            which can be tedious and error prone, and makes our grammar cluttered
            and harder to read.
          
            Another solution is to specialize proto::is_callable<> on our times2<> template:
          
namespace boost { namespace proto { // Tell Proto that times2<> is callable template<typename T> struct is_callable<times2<T> > : mpl::true_ {}; }} // OK, times2<> is callable struct IntTimes2 : proto::when< proto::terminal<int> , times2<int>(proto::_value) > {};
This is better, but still a pain because of the need to open Proto's namespace.
You could simply make sure that the callable type is not a template specialization. Consider the following:
// No longer a template specialization! struct times2int : times2<int> {}; // OK, times2int is callable struct IntTimes2 : proto::when< proto::terminal<int> , times2int(proto::_value) > {};
            This works because now Proto can tell that times2int
            inherits (indirectly) from proto::callable.
            Any non-template types can be safely checked for inheritance because,
            as they are not templates, there is no worry about instantiation errors.
          
            There is one last way to tell Proto that times2<> is callable. You could add an
            extra dummy template parameter that defaults to proto::callable:
          
// Proto will recognize this as callable template<typename T, typename Callable = proto::callable> struct times2 : proto::callable { typedef T result_type; T operator()(T i) const { return i * 2; } }; // OK, this works! struct IntTimes2 : proto::when< proto::terminal<int> , times2<int>(proto::_value) > {};
            Note that in addition to the extra template parameter, times2<>
            still inherits from proto::callable.
            That's not necessary in this example but it is good style because any
            types derived from times2<> (as times2int
            defined above) will still be considered callable.
          
proto::extends<>A code example is worth a thousand words ...
A trivial example which builds and expression template and evaluates it.
//////////////////////////////////////////////////////////////////// // Copyright 2008 Eric Niebler. Distributed under the Boost // Software License, Version 1.0. (See accompanying file // LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) #include <iostream> #include <boost/proto/core.hpp> #include <boost/proto/context.hpp> // This #include is only needed for compilers that use typeof emulation: #include <boost/typeof/std/ostream.hpp> namespace proto = boost::proto; proto::terminal< std::ostream & >::type cout_ = {std::cout}; template< typename Expr > void evaluate( Expr const & expr ) { proto::default_context ctx; proto::eval(expr, ctx); } int main() { evaluate( cout_ << "hello" << ',' << " world" ); return 0; }
A simple example that builds a miniature embedded domain-specific language for lazy arithmetic expressions, with TR1 bind-style argument placeholders.
// Copyright 2008 Eric Niebler. Distributed under the Boost // Software License, Version 1.0. (See accompanying file // LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) // // This is a simple example of how to build an arithmetic expression // evaluator with placeholders. #include <iostream> #include <boost/proto/core.hpp> #include <boost/proto/context.hpp> namespace proto = boost::proto; using proto::_; template<int I> struct placeholder {}; // Define some placeholders proto::terminal< placeholder< 1 > >::type const _1 = {{}}; proto::terminal< placeholder< 2 > >::type const _2 = {{}}; // Define a calculator context, for evaluating arithmetic expressions struct calculator_context : proto::callable_context< calculator_context const > { // The values bound to the placeholders double d[2]; // The result of evaluating arithmetic expressions typedef double result_type; explicit calculator_context(double d1 = 0., double d2 = 0.) { d[0] = d1; d[1] = d2; } // Handle the evaluation of the placeholder terminals template<int I> double operator ()(proto::tag::terminal, placeholder<I>) const { return d[ I - 1 ]; } }; template<typename Expr> double evaluate( Expr const &expr, double d1 = 0., double d2 = 0. ) { // Create a calculator context with d1 and d2 substituted for _1 and _2 calculator_context const ctx(d1, d2); // Evaluate the calculator expression with the calculator_context return proto::eval(expr, ctx); } int main() { // Displays "5" std::cout << evaluate( _1 + 2.0, 3.0 ) << std::endl; // Displays "6" std::cout << evaluate( _1 * _2, 3.0, 2.0 ) << std::endl; // Displays "0.5" std::cout << evaluate( (_1 - _2) / _2, 3.0, 2.0 ) << std::endl; return 0; }
          An extension of the Calc1 example that uses proto::extends<>
          to make calculator expressions valid function objects that can be used
          with STL algorithms.
        
// Copyright 2008 Eric Niebler. Distributed under the Boost // Software License, Version 1.0. (See accompanying file // LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) // // This example enhances the simple arithmetic expression evaluator // in calc1.cpp by using proto::extends to make arithmetic // expressions immediately evaluable with operator (), a-la a // function object #include <iostream> #include <boost/proto/core.hpp> #include <boost/proto/context.hpp> namespace proto = boost::proto; using proto::_; template<typename Expr> struct calculator_expression; // Tell proto how to generate expressions in the calculator_domain struct calculator_domain : proto::domain<proto::generator<calculator_expression> > {}; // Will be used to define the placeholders _1 and _2 template<int I> struct placeholder {}; // Define a calculator context, for evaluating arithmetic expressions // (This is as before, in calc1.cpp) struct calculator_context : proto::callable_context< calculator_context const > { // The values bound to the placeholders double d[2]; // The result of evaluating arithmetic expressions typedef double result_type; explicit calculator_context(double d1 = 0., double d2 = 0.) { d[0] = d1; d[1] = d2; } // Handle the evaluation of the placeholder terminals template<int I> double operator ()(proto::tag::terminal, placeholder<I>) const { return d[ I - 1 ]; } }; // Wrap all calculator expressions in this type, which defines // operator () to evaluate the expression. template<typename Expr> struct calculator_expression : proto::extends<Expr, calculator_expression<Expr>, calculator_domain> { explicit calculator_expression(Expr const &expr = Expr()) : calculator_expression::proto_extends(expr) {} BOOST_PROTO_EXTENDS_USING_ASSIGN(calculator_expression<Expr>) // Override operator () to evaluate the expression double operator ()() const { calculator_context const ctx; return proto::eval(*this, ctx); } double operator ()(double d1) const { calculator_context const ctx(d1); return proto::eval(*this, ctx); } double operator ()(double d1, double d2) const { calculator_context const ctx(d1, d2); return proto::eval(*this, ctx); } }; // Define some placeholders (notice they're wrapped in calculator_expression<>) calculator_expression<proto::terminal< placeholder< 1 > >::type> const _1; calculator_expression<proto::terminal< placeholder< 2 > >::type> const _2; // Now, our arithmetic expressions are immediately executable function objects: int main() { // Displays "5" std::cout << (_1 + 2.0)( 3.0 ) << std::endl; // Displays "6" std::cout << ( _1 * _2 )( 3.0, 2.0 ) << std::endl; // Displays "0.5" std::cout << ( (_1 - _2) / _2 )( 3.0, 2.0 ) << std::endl; return 0; }
An extension of the Calc2 example that uses a Proto transform to calculate the arity of a calculator expression and statically assert that the correct number of arguments are passed.
// Copyright 2008 Eric Niebler. Distributed under the Boost // Software License, Version 1.0. (See accompanying file // LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) // // This example enhances the arithmetic expression evaluator // in calc2.cpp by using a proto transform to calculate the // number of arguments an expression requires and using a // compile-time assert to guarantee that the right number of // arguments are actually specified. #include <iostream> #include <boost/mpl/int.hpp> #include <boost/mpl/assert.hpp> #include <boost/mpl/min_max.hpp> #include <boost/proto/core.hpp> #include <boost/proto/context.hpp> #include <boost/proto/transform.hpp> namespace mpl = boost::mpl; namespace proto = boost::proto; using proto::_; // Will be used to define the placeholders _1 and _2 template<typename I> struct placeholder : I {}; // This grammar basically says that a calculator expression is one of: // - A placeholder terminal // - Some other terminal // - Some non-terminal whose children are calculator expressions // In addition, it has transforms that say how to calculate the // expression arity for each of the three cases. struct CalculatorGrammar : proto::or_< // placeholders have a non-zero arity ... proto::when< proto::terminal< placeholder<_> >, proto::_value > // Any other terminals have arity 0 ... , proto::when< proto::terminal<_>, mpl::int_<0>() > // For any non-terminals, find the arity of the children and // take the maximum. This is recursive. , proto::when< proto::nary_expr<_, proto::vararg<_> > , proto::fold<_, mpl::int_<0>(), mpl::max<CalculatorGrammar, proto::_state>() > > > {}; // Simple wrapper for calculating a calculator expression's arity. // It specifies mpl::int_<0> as the initial state. The data, which // is not used, is mpl::void_. template<typename Expr> struct calculator_arity : boost::result_of<CalculatorGrammar(Expr)> {}; template<typename Expr> struct calculator_expression; // Tell proto how to generate expressions in the calculator_domain struct calculator_domain : proto::domain<proto::generator<calculator_expression> > {}; // Define a calculator context, for evaluating arithmetic expressions // (This is as before, in calc1.cpp and calc2.cpp) struct calculator_context : proto::callable_context< calculator_context const > { // The values bound to the placeholders double d[2]; // The result of evaluating arithmetic expressions typedef double result_type; explicit calculator_context(double d1 = 0., double d2 = 0.) { d[0] = d1; d[1] = d2; } // Handle the evaluation of the placeholder terminals template<typename I> double operator ()(proto::tag::terminal, placeholder<I>) const { return d[ I() - 1 ]; } }; // Wrap all calculator expressions in this type, which defines // operator () to evaluate the expression. template<typename Expr> struct calculator_expression : proto::extends<Expr, calculator_expression<Expr>, calculator_domain> { typedef proto::extends<Expr, calculator_expression<Expr>, calculator_domain> base_type; explicit calculator_expression(Expr const &expr = Expr()) : base_type(expr) {} BOOST_PROTO_EXTENDS_USING_ASSIGN(calculator_expression<Expr>) // Override operator () to evaluate the expression double operator ()() const { // Assert that the expression has arity 0 BOOST_MPL_ASSERT_RELATION(0, ==, calculator_arity<Expr>::type::value); calculator_context const ctx; return proto::eval(*this, ctx); } double operator ()(double d1) const { // Assert that the expression has arity 1 BOOST_MPL_ASSERT_RELATION(1, ==, calculator_arity<Expr>::type::value); calculator_context const ctx(d1); return proto::eval(*this, ctx); } double operator ()(double d1, double d2) const { // Assert that the expression has arity 2 BOOST_MPL_ASSERT_RELATION(2, ==, calculator_arity<Expr>::type::value); calculator_context const ctx(d1, d2); return proto::eval(*this, ctx); } }; // Define some placeholders (notice they're wrapped in calculator_expression<>) calculator_expression<proto::terminal< placeholder< mpl::int_<1> > >::type> const _1; calculator_expression<proto::terminal< placeholder< mpl::int_<2> > >::type> const _2; // Now, our arithmetic expressions are immediately executable function objects: int main() { // Displays "5" std::cout << (_1 + 2.0)( 3.0 ) << std::endl; // Displays "6" std::cout << ( _1 * _2 )( 3.0, 2.0 ) << std::endl; // Displays "0.5" std::cout << ( (_1 - _2) / _2 )( 3.0, 2.0 ) << std::endl; // This won't compile because the arity of the // expression doesn't match the number of arguments // ( (_1 - _2) / _2 )( 3.0 ); return 0; }
This example constructs a mini-library for linear algebra, using expression templates to eliminate the need for temporaries when adding vectors of numbers.
This example uses a domain with a grammar to prune the set of overloaded operators. Only those operators that produce valid lazy vector expressions are allowed.
/////////////////////////////////////////////////////////////////////////////// // Copyright 2008 Eric Niebler. Distributed under the Boost // Software License, Version 1.0. (See accompanying file // LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) // // This example constructs a mini-library for linear algebra, using // expression templates to eliminate the need for temporaries when // adding vectors of numbers. // // This example uses a domain with a grammar to prune the set // of overloaded operators. Only those operators that produce // valid lazy vector expressions are allowed. #include <vector> #include <iostream> #include <boost/mpl/int.hpp> #include <boost/proto/core.hpp> #include <boost/proto/context.hpp> namespace mpl = boost::mpl; namespace proto = boost::proto; using proto::_; template<typename Expr> struct lazy_vector_expr; // This grammar describes which lazy vector expressions // are allowed; namely, vector terminals and addition // and subtraction of lazy vector expressions. struct LazyVectorGrammar : proto::or_< proto::terminal< std::vector<_> > , proto::plus< LazyVectorGrammar, LazyVectorGrammar > , proto::minus< LazyVectorGrammar, LazyVectorGrammar > > {}; // Tell proto that in the lazy_vector_domain, all // expressions should be wrapped in laxy_vector_expr<> // and must conform to the lazy vector grammar. struct lazy_vector_domain : proto::domain<proto::generator<lazy_vector_expr>, LazyVectorGrammar> {}; // Here is an evaluation context that indexes into a lazy vector // expression, and combines the result. template<typename Size = std::size_t> struct lazy_subscript_context { lazy_subscript_context(Size subscript) : subscript_(subscript) {} // Use default_eval for all the operations ... template<typename Expr, typename Tag = typename Expr::proto_tag> struct eval : proto::default_eval<Expr, lazy_subscript_context> {}; // ... except for terminals, which we index with our subscript template<typename Expr> struct eval<Expr, proto::tag::terminal> { typedef typename proto::result_of::value<Expr>::type::value_type result_type; result_type operator ()( Expr const & expr, lazy_subscript_context & ctx ) const { return proto::value( expr )[ ctx.subscript_ ]; } }; Size subscript_; }; // Here is the domain-specific expression wrapper, which overrides // operator [] to evaluate the expression using the lazy_subscript_context. template<typename Expr> struct lazy_vector_expr : proto::extends<Expr, lazy_vector_expr<Expr>, lazy_vector_domain> { lazy_vector_expr( Expr const & expr = Expr() ) : lazy_vector_expr::proto_extends( expr ) {} // Use the lazy_subscript_context<> to implement subscripting // of a lazy vector expression tree. template< typename Size > typename proto::result_of::eval< Expr, lazy_subscript_context<Size> >::type operator []( Size subscript ) const { lazy_subscript_context<Size> ctx(subscript); return proto::eval(*this, ctx); } }; // Here is our lazy_vector terminal, implemented in terms of lazy_vector_expr template< typename T > struct lazy_vector : lazy_vector_expr< typename proto::terminal< std::vector<T> >::type > { typedef typename proto::terminal< std::vector<T> >::type expr_type; lazy_vector( std::size_t size = 0, T const & value = T() ) : lazy_vector_expr<expr_type>( expr_type::make( std::vector<T>( size, value ) ) ) {} // Here we define a += operator for lazy vector terminals that // takes a lazy vector expression and indexes it. expr[i] here // uses lazy_subscript_context<> under the covers. template< typename Expr > lazy_vector &operator += (Expr const & expr) { std::size_t size = proto::value(*this).size(); for(std::size_t i = 0; i < size; ++i) { proto::value(*this)[i] += expr[i]; } return *this; } }; int main() { // lazy_vectors with 4 elements each. lazy_vector< double > v1( 4, 1.0 ), v2( 4, 2.0 ), v3( 4, 3.0 ); // Add two vectors lazily and get the 2nd element. double d1 = ( v2 + v3 )[ 2 ]; // Look ma, no temporaries! std::cout << d1 << std::endl; // Subtract two vectors and add the result to a third vector. v1 += v2 - v3; // Still no temporaries! std::cout << '{' << v1[0] << ',' << v1[1] << ',' << v1[2] << ',' << v1[3] << '}' << std::endl; // This expression is disallowed because it does not conform // to the LazyVectorGrammar //(v2 + v3) += v1; return 0; }
This is a simple example of doing arbitrary type manipulations with Proto transforms. It takes some expression involving primary colors and combines the colors according to arbitrary rules. It is a port of the RGB example from PETE.
/////////////////////////////////////////////////////////////////////////////// // Copyright 2008 Eric Niebler. Distributed under the Boost // Software License, Version 1.0. (See accompanying file // LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) // // This is a simple example of doing arbitrary type manipulations with proto // transforms. It takes some expression involving primary colors and combines // the colors according to arbitrary rules. It is a port of the RGB example // from PETE (http://www.codesourcery.com/pooma/download.html). #include <iostream> #include <boost/proto/core.hpp> #include <boost/proto/transform.hpp> namespace proto = boost::proto; struct RedTag { friend std::ostream &operator <<(std::ostream &sout, RedTag) { return sout << "This expression is red."; } }; struct BlueTag { friend std::ostream &operator <<(std::ostream &sout, BlueTag) { return sout << "This expression is blue."; } }; struct GreenTag { friend std::ostream &operator <<(std::ostream &sout, GreenTag) { return sout << "This expression is green."; } }; typedef proto::terminal<RedTag>::type RedT; typedef proto::terminal<BlueTag>::type BlueT; typedef proto::terminal<GreenTag>::type GreenT; struct Red; struct Blue; struct Green; /////////////////////////////////////////////////////////////////////////////// // A transform that produces new colors according to some arbitrary rules: // red & green give blue, red & blue give green, blue and green give red. struct Red : proto::or_< proto::plus<Green, Blue> , proto::plus<Blue, Green> , proto::plus<Red, Red> , proto::terminal<RedTag> > {}; struct Green : proto::or_< proto::plus<Red, Blue> , proto::plus<Blue, Red> , proto::plus<Green, Green> , proto::terminal<GreenTag> > {}; struct Blue : proto::or_< proto::plus<Red, Green> , proto::plus<Green, Red> , proto::plus<Blue, Blue> , proto::terminal<BlueTag> > {}; struct RGB : proto::or_< proto::when< Red, RedTag() > , proto::when< Blue, BlueTag() > , proto::when< Green, GreenTag() > > {}; template<typename Expr> void printColor(Expr const & expr) { int i = 0; // dummy state and data parameter, not used std::cout << RGB()(expr, i, i) << std::endl; } int main() { printColor(RedT() + GreenT()); printColor(RedT() + GreenT() + BlueT()); printColor(RedT() + (GreenT() + BlueT())); return 0; }
This example constructs a mini-library for linear algebra, using expression templates to eliminate the need for temporaries when adding arrays of numbers. It duplicates the TArray example from PETE.
/////////////////////////////////////////////////////////////////////////////// // Copyright 2008 Eric Niebler. Distributed under the Boost // Software License, Version 1.0. (See accompanying file // LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) // // This example constructs a mini-library for linear algebra, using // expression templates to eliminate the need for temporaries when // adding arrays of numbers. It duplicates the TArray example from // PETE (http://www.codesourcery.com/pooma/download.html) #include <iostream> #include <boost/mpl/int.hpp> #include <boost/proto/core.hpp> #include <boost/proto/context.hpp> namespace mpl = boost::mpl; namespace proto = boost::proto; using proto::_; // This grammar describes which TArray expressions // are allowed; namely, int and array terminals // plus, minus, multiplies and divides of TArray expressions. struct TArrayGrammar : proto::or_< proto::terminal< int > , proto::terminal< int[3] > , proto::plus< TArrayGrammar, TArrayGrammar > , proto::minus< TArrayGrammar, TArrayGrammar > , proto::multiplies< TArrayGrammar, TArrayGrammar > , proto::divides< TArrayGrammar, TArrayGrammar > > {}; template<typename Expr> struct TArrayExpr; // Tell proto that in the TArrayDomain, all // expressions should be wrapped in TArrayExpr<> and // must conform to the TArrayGrammar struct TArrayDomain : proto::domain<proto::generator<TArrayExpr>, TArrayGrammar> {}; // Here is an evaluation context that indexes into a TArray // expression, and combines the result. struct TArraySubscriptCtx : proto::callable_context< TArraySubscriptCtx const > { typedef int result_type; TArraySubscriptCtx(std::ptrdiff_t i) : i_(i) {} // Index array terminals with our subscript. Everything // else will be handled by the default evaluation context. int operator ()(proto::tag::terminal, int const (&data)[3]) const { return data[this->i_]; } std::ptrdiff_t i_; }; // Here is an evaluation context that prints a TArray expression. struct TArrayPrintCtx : proto::callable_context< TArrayPrintCtx const > { typedef std::ostream &result_type; TArrayPrintCtx() {} std::ostream &operator ()(proto::tag::terminal, int i) const { return std::cout << i; } std::ostream &operator ()(proto::tag::terminal, int const (&arr)[3]) const { return std::cout << '{' << arr[0] << ", " << arr[1] << ", " << arr[2] << '}'; } template<typename L, typename R> std::ostream &operator ()(proto::tag::plus, L const &l, R const &r) const { return std::cout << '(' << l << " + " << r << ')'; } template<typename L, typename R> std::ostream &operator ()(proto::tag::minus, L const &l, R const &r) const { return std::cout << '(' << l << " - " << r << ')'; } template<typename L, typename R> std::ostream &operator ()(proto::tag::multiplies, L const &l, R const &r) const { return std::cout << l << " * " << r; } template<typename L, typename R> std::ostream &operator ()(proto::tag::divides, L const &l, R const &r) const { return std::cout << l << " / " << r; } }; // Here is the domain-specific expression wrapper, which overrides // operator [] to evaluate the expression using the TArraySubscriptCtx. template<typename Expr> struct TArrayExpr : proto::extends<Expr, TArrayExpr<Expr>, TArrayDomain> { typedef proto::extends<Expr, TArrayExpr<Expr>, TArrayDomain> base_type; TArrayExpr( Expr const & expr = Expr() ) : base_type( expr ) {} // Use the TArraySubscriptCtx to implement subscripting // of a TArray expression tree. int operator []( std::ptrdiff_t i ) const { TArraySubscriptCtx const ctx(i); return proto::eval(*this, ctx); } // Use the TArrayPrintCtx to display a TArray expression tree. friend std::ostream &operator <<(std::ostream &sout, TArrayExpr<Expr> const &expr) { TArrayPrintCtx const ctx; return proto::eval(expr, ctx); } }; // Here is our TArray terminal, implemented in terms of TArrayExpr // It is basically just an array of 3 integers. struct TArray : TArrayExpr< proto::terminal< int[3] >::type > { explicit TArray( int i = 0, int j = 0, int k = 0 ) { (*this)[0] = i; (*this)[1] = j; (*this)[2] = k; } // Here we override operator [] to give read/write access to // the elements of the array. (We could use the TArrayExpr // operator [] if we made the subscript context smarter about // returning non-const reference when appropriate.) int &operator [](std::ptrdiff_t i) { return proto::value(*this)[i]; } int const &operator [](std::ptrdiff_t i) const { return proto::value(*this)[i]; } // Here we define a operator = for TArray terminals that // takes a TArray expression. template< typename Expr > TArray &operator =(Expr const & expr) { // proto::as_expr<TArrayDomain>(expr) is the same as // expr unless expr is an integer, in which case it // is made into a TArrayExpr terminal first. return this->assign(proto::as_expr<TArrayDomain>(expr)); } template< typename Expr > TArray &printAssign(Expr const & expr) { *this = expr; std::cout << *this << " = " << expr << std::endl; return *this; } private: template< typename Expr > TArray &assign(Expr const & expr) { // expr[i] here uses TArraySubscriptCtx under the covers. (*this)[0] = expr[0]; (*this)[1] = expr[1]; (*this)[2] = expr[2]; return *this; } }; int main() { TArray a(3,1,2); TArray b; std::cout << a << std::endl; std::cout << b << std::endl; b[0] = 7; b[1] = 33; b[2] = -99; TArray c(a); std::cout << c << std::endl; a = 0; std::cout << a << std::endl; std::cout << b << std::endl; std::cout << c << std::endl; a = b + c; std::cout << a << std::endl; a.printAssign(b+c*(b + 3*c)); return 0; }
          This is a simple example using proto::extends<> to extend a terminal type with
          additional behaviors, and using custom contexts and proto::eval() for evaluating expressions. It is a port
          of the Vec3 example from PETE.
        
/////////////////////////////////////////////////////////////////////////////// // Copyright 2008 Eric Niebler. Distributed under the Boost // Software License, Version 1.0. (See accompanying file // LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) // // This is a simple example using proto::extends to extend a terminal type with // additional behaviors, and using custom contexts and proto::eval for // evaluating expressions. It is a port of the Vec3 example // from PETE (http://www.codesourcery.com/pooma/download.html). #include <iostream> #include <functional> #include <boost/assert.hpp> #include <boost/mpl/int.hpp> #include <boost/proto/core.hpp> #include <boost/proto/context.hpp> #include <boost/proto/proto_typeof.hpp> #include <boost/proto/transform.hpp> namespace mpl = boost::mpl; namespace proto = boost::proto; using proto::_; // Here is an evaluation context that indexes into a Vec3 // expression, and combines the result. struct Vec3SubscriptCtx : proto::callable_context< Vec3SubscriptCtx const > { typedef int result_type; Vec3SubscriptCtx(int i) : i_(i) {} // Index array terminals with our subscript. Everything // else will be handled by the default evaluation context. int operator ()(proto::tag::terminal, int const (&arr)[3]) const { return arr[this->i_]; } int i_; }; // Here is an evaluation context that counts the number // of Vec3 terminals in an expression. struct CountLeavesCtx : proto::callable_context< CountLeavesCtx, proto::null_context > { CountLeavesCtx() : count(0) {} typedef void result_type; void operator ()(proto::tag::terminal, int const(&)[3]) { ++this->count; } int count; }; struct iplus : std::plus<int>, proto::callable {}; // Here is a transform that does the same thing as the above context. // It demonstrates the use of the std::plus<> function object // with the fold transform. With minor modifications, this // transform could be used to calculate the leaf count at compile // time, rather than at runtime. struct CountLeaves : proto::or_< // match a Vec3 terminal, return 1 proto::when<proto::terminal<int[3]>, mpl::int_<1>() > // match a terminal, return int() (which is 0) , proto::when<proto::terminal<_>, int() > // fold everything else, using std::plus<> to add // the leaf count of each child to the accumulated state. , proto::otherwise< proto::fold<_, int(), iplus(CountLeaves, proto::_state) > > > {}; // Here is the Vec3 struct, which is a vector of 3 integers. struct Vec3 : proto::extends<proto::terminal<int[3]>::type, Vec3> { explicit Vec3(int i=0, int j=0, int k=0) { (*this)[0] = i; (*this)[1] = j; (*this)[2] = k; } int &operator [](int i) { return proto::value(*this)[i]; } int const &operator [](int i) const { return proto::value(*this)[i]; } // Here we define a operator = for Vec3 terminals that // takes a Vec3 expression. template< typename Expr > Vec3 &operator =(Expr const & expr) { typedef Vec3SubscriptCtx const CVec3SubscriptCtx; (*this)[0] = proto::eval(proto::as_expr(expr), CVec3SubscriptCtx(0)); (*this)[1] = proto::eval(proto::as_expr(expr), CVec3SubscriptCtx(1)); (*this)[2] = proto::eval(proto::as_expr(expr), CVec3SubscriptCtx(2)); return *this; } // This copy-assign is needed because a template is never // considered for copy assignment. Vec3 &operator=(Vec3 const &that) { (*this)[0] = that[0]; (*this)[1] = that[1]; (*this)[2] = that[2]; return *this; } void print() const { std::cout << '{' << (*this)[0] << ", " << (*this)[1] << ", " << (*this)[2] << '}' << std::endl; } }; // The count_leaves() function uses the CountLeaves transform and // to count the number of leaves in an expression. template<typename Expr> int count_leaves(Expr const &expr) { // Count the number of Vec3 terminals using the // CountLeavesCtx evaluation context. CountLeavesCtx ctx; proto::eval(expr, ctx); // This is another way to count the leaves using a transform. int i = 0; BOOST_ASSERT( CountLeaves()(expr, i, i) == ctx.count ); return ctx.count; } int main() { Vec3 a, b, c; c = 4; b[0] = -1; b[1] = -2; b[2] = -3; a = b + c; a.print(); Vec3 d; BOOST_PROTO_AUTO(expr1, b + c); d = expr1; d.print(); int num = count_leaves(expr1); std::cout << num << std::endl; BOOST_PROTO_AUTO(expr2, b + 3 * c); num = count_leaves(expr2); std::cout << num << std::endl; BOOST_PROTO_AUTO(expr3, b + c * d); num = count_leaves(expr3); std::cout << num << std::endl; return 0; }
          This is an example of using BOOST_PROTO_DEFINE_OPERATORS() to Protofy expressions using std::vector<>,
          a non-Proto type. It is a port of the Vector example from PETE.
        
/////////////////////////////////////////////////////////////////////////////// // Copyright 2008 Eric Niebler. Distributed under the Boost // Software License, Version 1.0. (See accompanying file // LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) // // This is an example of using BOOST_PROTO_DEFINE_OPERATORS to Protofy // expressions using std::vector<>, a non-proto type. It is a port of the // Vector example from PETE (http://www.codesourcery.com/pooma/download.html). #include <vector> #include <iostream> #include <stdexcept> #include <boost/mpl/bool.hpp> #include <boost/proto/core.hpp> #include <boost/proto/debug.hpp> #include <boost/proto/context.hpp> #include <boost/utility/enable_if.hpp> namespace mpl = boost::mpl; namespace proto = boost::proto; using proto::_; template<typename Expr> struct VectorExpr; // Here is an evaluation context that indexes into a std::vector // expression and combines the result. struct VectorSubscriptCtx { VectorSubscriptCtx(std::size_t i) : i_(i) {} // Unless this is a vector terminal, use the // default evaluation context template<typename Expr, typename EnableIf = void> struct eval : proto::default_eval<Expr, VectorSubscriptCtx const> {}; // Index vector terminals with our subscript. template<typename Expr> struct eval< Expr , typename boost::enable_if< proto::matches<Expr, proto::terminal<std::vector<_, _> > > >::type > { typedef typename proto::result_of::value<Expr>::type::value_type result_type; result_type operator ()(Expr &expr, VectorSubscriptCtx const &ctx) const { return proto::value(expr)[ctx.i_]; } }; std::size_t i_; }; // Here is an evaluation context that verifies that all the // vectors in an expression have the same size. struct VectorSizeCtx { VectorSizeCtx(std::size_t size) : size_(size) {} // Unless this is a vector terminal, use the // null evaluation context template<typename Expr, typename EnableIf = void> struct eval : proto::null_eval<Expr, VectorSizeCtx const> {}; // Index array terminals with our subscript. Everything // else will be handled by the default evaluation context. template<typename Expr> struct eval< Expr , typename boost::enable_if< proto::matches<Expr, proto::terminal<std::vector<_, _> > > >::type > { typedef void result_type; result_type operator ()(Expr &expr, VectorSizeCtx const &ctx) const { if(ctx.size_ != proto::value(expr).size()) { throw std::runtime_error("LHS and RHS are not compatible"); } } }; std::size_t size_; }; // A grammar which matches all the assignment operators, // so we can easily disable them. struct AssignOps : proto::switch_<struct AssignOpsCases> {}; // Here are the cases used by the switch_ above. struct AssignOpsCases { template<typename Tag, int D = 0> struct case_ : proto::not_<_> {}; template<int D> struct case_< proto::tag::plus_assign, D > : _ {}; template<int D> struct case_< proto::tag::minus_assign, D > : _ {}; template<int D> struct case_< proto::tag::multiplies_assign, D > : _ {}; template<int D> struct case_< proto::tag::divides_assign, D > : _ {}; template<int D> struct case_< proto::tag::modulus_assign, D > : _ {}; template<int D> struct case_< proto::tag::shift_left_assign, D > : _ {}; template<int D> struct case_< proto::tag::shift_right_assign, D > : _ {}; template<int D> struct case_< proto::tag::bitwise_and_assign, D > : _ {}; template<int D> struct case_< proto::tag::bitwise_or_assign, D > : _ {}; template<int D> struct case_< proto::tag::bitwise_xor_assign, D > : _ {}; }; // A vector grammar is a terminal or some op that is not an // assignment op. (Assignment will be handled specially.) struct VectorGrammar : proto::or_< proto::terminal<_> , proto::and_<proto::nary_expr<_, proto::vararg<VectorGrammar> >, proto::not_<AssignOps> > > {}; // Expressions in the vector domain will be wrapped in VectorExpr<> // and must conform to the VectorGrammar struct VectorDomain : proto::domain<proto::generator<VectorExpr>, VectorGrammar> {}; // Here is VectorExpr, which extends a proto expr type by // giving it an operator [] which uses the VectorSubscriptCtx // to evaluate an expression with a given index. template<typename Expr> struct VectorExpr : proto::extends<Expr, VectorExpr<Expr>, VectorDomain> { explicit VectorExpr(Expr const &expr) : proto::extends<Expr, VectorExpr<Expr>, VectorDomain>(expr) {} // Use the VectorSubscriptCtx to implement subscripting // of a Vector expression tree. typename proto::result_of::eval<Expr const, VectorSubscriptCtx const>::type operator []( std::size_t i ) const { VectorSubscriptCtx const ctx(i); return proto::eval(*this, ctx); } }; // Define a trait type for detecting vector terminals, to // be used by the BOOST_PROTO_DEFINE_OPERATORS macro below. template<typename T> struct IsVector : mpl::false_ {}; template<typename T, typename A> struct IsVector<std::vector<T, A> > : mpl::true_ {}; namespace VectorOps { // This defines all the overloads to make expressions involving // std::vector to build expression templates. BOOST_PROTO_DEFINE_OPERATORS(IsVector, VectorDomain) typedef VectorSubscriptCtx const CVectorSubscriptCtx; // Assign to a vector from some expression. template<typename T, typename A, typename Expr> std::vector<T, A> &assign(std::vector<T, A> &arr, Expr const &expr) { VectorSizeCtx const size(arr.size()); proto::eval(proto::as_expr<VectorDomain>(expr), size); // will throw if the sizes don't match for(std::size_t i = 0; i < arr.size(); ++i) { arr[i] = proto::as_expr<VectorDomain>(expr)[i]; } return arr; } // Add-assign to a vector from some expression. template<typename T, typename A, typename Expr> std::vector<T, A> &operator +=(std::vector<T, A> &arr, Expr const &expr) { VectorSizeCtx const size(arr.size()); proto::eval(proto::as_expr<VectorDomain>(expr), size); // will throw if the sizes don't match for(std::size_t i = 0; i < arr.size(); ++i) { arr[i] += proto::as_expr<VectorDomain>(expr)[i]; } return arr; } } int main() { using namespace VectorOps; int i; const int n = 10; std::vector<int> a,b,c,d; std::vector<double> e(n); for (i = 0; i < n; ++i) { a.push_back(i); b.push_back(2*i); c.push_back(3*i); d.push_back(i); } VectorOps::assign(b, 2); VectorOps::assign(d, a + b * c); a += if_else(d < 30, b, c); VectorOps::assign(e, c); e += e - 4 / (c + 1); for (i = 0; i < n; ++i) { std::cout << " a(" << i << ") = " << a[i] << " b(" << i << ") = " << b[i] << " c(" << i << ") = " << c[i] << " d(" << i << ") = " << d[i] << " e(" << i << ") = " << e[i] << std::endl; } }
          This is an example of using BOOST_PROTO_DEFINE_OPERATORS() to Protofy expressions using std::vector<>
          and std::list<>,
          non-Proto types. It is a port of the Mixed example from PETE.
        
/////////////////////////////////////////////////////////////////////////////// // Copyright 2008 Eric Niebler. Distributed under the Boost // Software License, Version 1.0. (See accompanying file // LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) // // This is an example of using BOOST_PROTO_DEFINE_OPERATORS to Protofy // expressions using std::vector<> and std::list, non-proto types. It is a port // of the Mixed example from PETE. // (http://www.codesourcery.com/pooma/download.html). #include <list> #include <cmath> #include <vector> #include <complex> #include <iostream> #include <stdexcept> #include <boost/proto/core.hpp> #include <boost/proto/debug.hpp> #include <boost/proto/context.hpp> #include <boost/proto/transform.hpp> #include <boost/utility/enable_if.hpp> #include <boost/typeof/std/list.hpp> #include <boost/typeof/std/vector.hpp> #include <boost/typeof/std/complex.hpp> #include <boost/type_traits/remove_reference.hpp> namespace proto = boost::proto; namespace mpl = boost::mpl; using proto::_; template<typename Expr> struct MixedExpr; template<typename Iter> struct iterator_wrapper { typedef Iter iterator; explicit iterator_wrapper(Iter iter) : it(iter) {} mutable Iter it; }; struct begin : proto::callable { template<class Sig> struct result; template<class This, class Cont> struct result<This(Cont)> : proto::result_of::as_expr< iterator_wrapper<typename boost::remove_reference<Cont>::type::const_iterator> > {}; template<typename Cont> typename result<begin(Cont const &)>::type operator ()(Cont const &cont) const { iterator_wrapper<typename Cont::const_iterator> it(cont.begin()); return proto::as_expr(it); } }; // Here is a grammar that replaces vector and list terminals with their // begin iterators struct Begin : proto::or_< proto::when< proto::terminal< std::vector<_, _> >, begin(proto::_value) > , proto::when< proto::terminal< std::list<_, _> >, begin(proto::_value) > , proto::when< proto::terminal<_> > , proto::when< proto::nary_expr<_, proto::vararg<Begin> > > > {}; // Here is an evaluation context that dereferences iterator // terminals. struct DereferenceCtx { // Unless this is an iterator terminal, use the // default evaluation context template<typename Expr, typename EnableIf = void> struct eval : proto::default_eval<Expr, DereferenceCtx const> {}; // Dereference iterator terminals. template<typename Expr> struct eval< Expr , typename boost::enable_if< proto::matches<Expr, proto::terminal<iterator_wrapper<_> > > >::type > { typedef typename proto::result_of::value<Expr>::type IteratorWrapper; typedef typename IteratorWrapper::iterator iterator; typedef typename std::iterator_traits<iterator>::reference result_type; result_type operator ()(Expr &expr, DereferenceCtx const &) const { return *proto::value(expr).it; } }; }; // Here is an evaluation context that increments iterator // terminals. struct IncrementCtx { // Unless this is an iterator terminal, use the // default evaluation context template<typename Expr, typename EnableIf = void> struct eval : proto::null_eval<Expr, IncrementCtx const> {}; // advance iterator terminals. template<typename Expr> struct eval< Expr , typename boost::enable_if< proto::matches<Expr, proto::terminal<iterator_wrapper<_> > > >::type > { typedef void result_type; result_type operator ()(Expr &expr, IncrementCtx const &) const { ++proto::value(expr).it; } }; }; // A grammar which matches all the assignment operators, // so we can easily disable them. struct AssignOps : proto::switch_<struct AssignOpsCases> {}; // Here are the cases used by the switch_ above. struct AssignOpsCases { template<typename Tag, int D = 0> struct case_ : proto::not_<_> {}; template<int D> struct case_< proto::tag::plus_assign, D > : _ {}; template<int D> struct case_< proto::tag::minus_assign, D > : _ {}; template<int D> struct case_< proto::tag::multiplies_assign, D > : _ {}; template<int D> struct case_< proto::tag::divides_assign, D > : _ {}; template<int D> struct case_< proto::tag::modulus_assign, D > : _ {}; template<int D> struct case_< proto::tag::shift_left_assign, D > : _ {}; template<int D> struct case_< proto::tag::shift_right_assign, D > : _ {}; template<int D> struct case_< proto::tag::bitwise_and_assign, D > : _ {}; template<int D> struct case_< proto::tag::bitwise_or_assign, D > : _ {}; template<int D> struct case_< proto::tag::bitwise_xor_assign, D > : _ {}; }; // An expression conforms to the MixedGrammar if it is a terminal or some // op that is not an assignment op. (Assignment will be handled specially.) struct MixedGrammar : proto::or_< proto::terminal<_> , proto::and_< proto::nary_expr<_, proto::vararg<MixedGrammar> > , proto::not_<AssignOps> > > {}; // Expressions in the MixedDomain will be wrapped in MixedExpr<> // and must conform to the MixedGrammar struct MixedDomain : proto::domain<proto::generator<MixedExpr>, MixedGrammar> {}; // Here is MixedExpr, a wrapper for expression types in the MixedDomain. template<typename Expr> struct MixedExpr : proto::extends<Expr, MixedExpr<Expr>, MixedDomain> { explicit MixedExpr(Expr const &expr) : MixedExpr::proto_extends(expr) {} private: // hide this: using proto::extends<Expr, MixedExpr<Expr>, MixedDomain>::operator []; }; // Define a trait type for detecting vector and list terminals, to // be used by the BOOST_PROTO_DEFINE_OPERATORS macro below. template<typename T> struct IsMixed : mpl::false_ {}; template<typename T, typename A> struct IsMixed<std::list<T, A> > : mpl::true_ {}; template<typename T, typename A> struct IsMixed<std::vector<T, A> > : mpl::true_ {}; namespace MixedOps { // This defines all the overloads to make expressions involving // std::vector to build expression templates. BOOST_PROTO_DEFINE_OPERATORS(IsMixed, MixedDomain) struct assign_op { template<typename T, typename U> void operator ()(T &t, U const &u) const { t = u; } }; struct plus_assign_op { template<typename T, typename U> void operator ()(T &t, U const &u) const { t += u; } }; struct minus_assign_op { template<typename T, typename U> void operator ()(T &t, U const &u) const { t -= u; } }; struct sin_ { template<typename Sig> struct result; template<typename This, typename Arg> struct result<This(Arg)> : boost::remove_const<typename boost::remove_reference<Arg>::type> {}; template<typename Arg> Arg operator ()(Arg const &a) const { return std::sin(a); } }; template<typename A> typename proto::result_of::make_expr< proto::tag::function , MixedDomain , sin_ const , A const & >::type sin(A const &a) { return proto::make_expr<proto::tag::function, MixedDomain>(sin_(), boost::ref(a)); } template<typename FwdIter, typename Expr, typename Op> void evaluate(FwdIter begin, FwdIter end, Expr const &expr, Op op) { IncrementCtx const inc = {}; DereferenceCtx const deref = {}; typename boost::result_of<Begin(Expr const &)>::type expr2 = Begin()(expr); for(; begin != end; ++begin) { op(*begin, proto::eval(expr2, deref)); proto::eval(expr2, inc); } } // Add-assign to a vector from some expression. template<typename T, typename A, typename Expr> std::vector<T, A> &assign(std::vector<T, A> &arr, Expr const &expr) { evaluate(arr.begin(), arr.end(), proto::as_expr<MixedDomain>(expr), assign_op()); return arr; } // Add-assign to a list from some expression. template<typename T, typename A, typename Expr> std::list<T, A> &assign(std::list<T, A> &arr, Expr const &expr) { evaluate(arr.begin(), arr.end(), proto::as_expr<MixedDomain>(expr), assign_op()); return arr; } // Add-assign to a vector from some expression. template<typename T, typename A, typename Expr> std::vector<T, A> &operator +=(std::vector<T, A> &arr, Expr const &expr) { evaluate(arr.begin(), arr.end(), proto::as_expr<MixedDomain>(expr), plus_assign_op()); return arr; } // Add-assign to a list from some expression. template<typename T, typename A, typename Expr> std::list<T, A> &operator +=(std::list<T, A> &arr, Expr const &expr) { evaluate(arr.begin(), arr.end(), proto::as_expr<MixedDomain>(expr), plus_assign_op()); return arr; } // Minus-assign to a vector from some expression. template<typename T, typename A, typename Expr> std::vector<T, A> &operator -=(std::vector<T, A> &arr, Expr const &expr) { evaluate(arr.begin(), arr.end(), proto::as_expr<MixedDomain>(expr), minus_assign_op()); return arr; } // Minus-assign to a list from some expression. template<typename T, typename A, typename Expr> std::list<T, A> &operator -=(std::list<T, A> &arr, Expr const &expr) { evaluate(arr.begin(), arr.end(), proto::as_expr<MixedDomain>(expr), minus_assign_op()); return arr; } } int main() { using namespace MixedOps; int n = 10; std::vector<int> a,b,c,d; std::list<double> e; std::list<std::complex<double> > f; int i; for(i = 0;i < n; ++i) { a.push_back(i); b.push_back(2*i); c.push_back(3*i); d.push_back(i); e.push_back(0.0); f.push_back(std::complex<double>(1.0, 1.0)); } MixedOps::assign(b, 2); MixedOps::assign(d, a + b * c); a += if_else(d < 30, b, c); MixedOps::assign(e, c); e += e - 4 / (c + 1); f -= sin(0.1 * e * std::complex<double>(0.2, 1.2)); std::list<double>::const_iterator ei = e.begin(); std::list<std::complex<double> >::const_iterator fi = f.begin(); for (i = 0; i < n; ++i) { std::cout << "a(" << i << ") = " << a[i] << " b(" << i << ") = " << b[i] << " c(" << i << ") = " << c[i] << " d(" << i << ") = " << d[i] << " e(" << i << ") = " << *ei++ << " f(" << i << ") = " << *fi++ << std::endl; } }
          A demonstration of how to implement map_list_of() from the Boost.Assign library using Proto.
          map_list_assign()
          is used to conveniently initialize a std::map<>. By using Proto, we can avoid any
          dynamic allocation while building the intermediate representation.
        
// Copyright 2008 Eric Niebler. Distributed under the Boost // Software License, Version 1.0. (See accompanying file // LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) // // This is a port of map_list_of() from the Boost.Assign library. // It has the advantage of being more efficient at runtime by not // building any temporary container that requires dynamic allocation. #include <map> #include <string> #include <iostream> #include <boost/proto/core.hpp> #include <boost/proto/transform.hpp> #include <boost/type_traits/add_reference.hpp> namespace proto = boost::proto; using proto::_; struct map_list_of_tag {}; // A simple callable function object that inserts a // (key,value) pair into a map. struct insert : proto::callable { template<typename Sig> struct result; template<typename This, typename Map, typename Key, typename Value> struct result<This(Map, Key, Value)> : boost::add_reference<Map> {}; template<typename Map, typename Key, typename Value> Map &operator()(Map &map, Key const &key, Value const &value) const { map.insert(typename Map::value_type(key, value)); return map; } }; // Work-arounds for Microsoft Visual C++ 7.1 #if BOOST_WORKAROUND(BOOST_MSVC, == 1310) #define MapListOf(x) proto::call<MapListOf(x)> #define _value(x) call<proto::_value(x)> #endif // The grammar for valid map-list expressions, and a // transform that populates the map. struct MapListOf : proto::or_< proto::when< // map_list_of(a,b) proto::function< proto::terminal<map_list_of_tag> , proto::terminal<_> , proto::terminal<_> > , insert( proto::_data , proto::_value(proto::_child1) , proto::_value(proto::_child2) ) > , proto::when< // map_list_of(a,b)(c,d)... proto::function< MapListOf , proto::terminal<_> , proto::terminal<_> > , insert( MapListOf(proto::_child0) , proto::_value(proto::_child1) , proto::_value(proto::_child2) ) > > {}; #if BOOST_WORKAROUND(BOOST_MSVC, == 1310) #undef MapListOf #undef _value #endif template<typename Expr> struct map_list_of_expr; struct map_list_of_dom : proto::domain<proto::pod_generator<map_list_of_expr>, MapListOf> {}; // An expression wrapper that provides a conversion to a // map that uses the MapListOf template<typename Expr> struct map_list_of_expr { BOOST_PROTO_BASIC_EXTENDS(Expr, map_list_of_expr, map_list_of_dom) BOOST_PROTO_EXTENDS_FUNCTION() template<typename Key, typename Value, typename Cmp, typename Al> operator std::map<Key, Value, Cmp, Al> () const { BOOST_MPL_ASSERT((proto::matches<Expr, MapListOf>)); std::map<Key, Value, Cmp, Al> map; return MapListOf()(*this, 0, map); } }; map_list_of_expr<proto::terminal<map_list_of_tag>::type> const map_list_of = {{{}}}; int main() { // Initialize a map: std::map<std::string, int> op = map_list_of ("<", 1) ("<=",2) (">", 3) (">=",4) ("=", 5) ("<>",6) ; std::cout << "\"<\" --> " << op["<"] << std::endl; std::cout << "\"<=\" --> " << op["<="] << std::endl; std::cout << "\">\" --> " << op[">"] << std::endl; std::cout << "\">=\" --> " << op[">="] << std::endl; std::cout << "\"=\" --> " << op["="] << std::endl; std::cout << "\"<>\" --> " << op["<>"] << std::endl; return 0; }
An advanced example of a Proto transform that implements Howard Hinnant's design for future groups that block for all or some asynchronous operations to complete and returns their results in a tuple of the appropriate type.
// Copyright 2008 Eric Niebler. Distributed under the Boost // Software License, Version 1.0. (See accompanying file // LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) // // This is an example of using Proto transforms to implement // Howard Hinnant's future group proposal. #include <boost/fusion/include/vector.hpp> #include <boost/fusion/include/as_vector.hpp> #include <boost/fusion/include/joint_view.hpp> #include <boost/fusion/include/single_view.hpp> #include <boost/proto/core.hpp> #include <boost/proto/transform.hpp> namespace mpl = boost::mpl; namespace proto = boost::proto; namespace fusion = boost::fusion; using proto::_; template<class L,class R> struct pick_left { BOOST_MPL_ASSERT((boost::is_same<L, R>)); typedef L type; }; // Work-arounds for Microsoft Visual C++ 7.1 #if BOOST_WORKAROUND(BOOST_MSVC, == 1310) #define FutureGroup(x) proto::call<FutureGroup(x)> #endif // Define the grammar of future group expression, as well as a // transform to turn them into a Fusion sequence of the correct // type. struct FutureGroup : proto::or_< // terminals become a single-element Fusion sequence proto::when< proto::terminal<_> , fusion::single_view<proto::_value>(proto::_value) > // (a && b) becomes a concatenation of the sequence // from 'a' and the one from 'b': , proto::when< proto::logical_and<FutureGroup, FutureGroup> , fusion::joint_view< boost::add_const<FutureGroup(proto::_left) > , boost::add_const<FutureGroup(proto::_right) > >(FutureGroup(proto::_left), FutureGroup(proto::_right)) > // (a || b) becomes the sequence for 'a', so long // as it is the same as the sequence for 'b'. , proto::when< proto::logical_or<FutureGroup, FutureGroup> , pick_left< FutureGroup(proto::_left) , FutureGroup(proto::_right) >(FutureGroup(proto::_left)) > > {}; #if BOOST_WORKAROUND(BOOST_MSVC, == 1310) #undef FutureGroup #endif template<class E> struct future_expr; struct future_dom : proto::domain<proto::generator<future_expr>, FutureGroup> {}; // Expressions in the future group domain have a .get() // member function that (ostensibly) blocks for the futures // to complete and returns the results in an appropriate // tuple. template<class E> struct future_expr : proto::extends<E, future_expr<E>, future_dom> { explicit future_expr(E const &e) : future_expr::proto_extends(e) {} typename fusion::result_of::as_vector< typename boost::result_of<FutureGroup(E)>::type >::type get() const { return fusion::as_vector(FutureGroup()(*this)); } }; // The future<> type has an even simpler .get() // member function. template<class T> struct future : future_expr<typename proto::terminal<T>::type> { future(T const &t = T()) : future::proto_derived_expr(future::proto_base_expr::make(t)) {} T get() const { return proto::value(*this); } }; // TEST CASES struct A {}; struct B {}; struct C {}; int main() { using fusion::vector; future<A> a; future<B> b; future<C> c; future<vector<A,B> > ab; // Verify that various future groups have the // correct return types. A t0 = a.get(); vector<A, B, C> t1 = (a && b && c).get(); vector<A, C> t2 = ((a || a) && c).get(); vector<A, B, C> t3 = ((a && b || a && b) && c).get(); vector<vector<A, B>, C> t4 = ((ab || ab) && c).get(); return 0; }
This is an advanced example that shows how to implement a simple lambda EDSL with Proto, like the Boost.Lambda_library. It uses contexts, transforms and expression extension.
/////////////////////////////////////////////////////////////////////////////// // Copyright 2008 Eric Niebler. Distributed under the Boost // Software License, Version 1.0. (See accompanying file // LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) // // This example builds a simple but functional lambda library using Proto. #include <iostream> #include <algorithm> #include <boost/mpl/int.hpp> #include <boost/mpl/min_max.hpp> #include <boost/mpl/eval_if.hpp> #include <boost/mpl/identity.hpp> #include <boost/mpl/next_prior.hpp> #include <boost/fusion/tuple.hpp> #include <boost/typeof/typeof.hpp> #include <boost/typeof/std/ostream.hpp> #include <boost/typeof/std/iostream.hpp> #include <boost/proto/core.hpp> #include <boost/proto/context.hpp> #include <boost/proto/transform.hpp> namespace mpl = boost::mpl; namespace proto = boost::proto; namespace fusion = boost::fusion; using proto::_; // Forward declaration of the lambda expression wrapper template<typename T> struct lambda; struct lambda_domain : proto::domain<proto::pod_generator<lambda> > {}; template<typename I> struct placeholder { typedef I arity; }; template<typename T> struct placeholder_arity { typedef typename T::arity type; }; // The lambda grammar, with the transforms for calculating the max arity struct lambda_arity : proto::or_< proto::when< proto::terminal< placeholder<_> > , mpl::next<placeholder_arity<proto::_value> >() > , proto::when< proto::terminal<_> , mpl::int_<0>() > , proto::when< proto::nary_expr<_, proto::vararg<_> > , proto::fold<_, mpl::int_<0>(), mpl::max<lambda_arity, proto::_state>()> > > {}; // The lambda context is the same as the default context // with the addition of special handling for lambda placeholders template<typename Tuple> struct lambda_context : proto::callable_context<lambda_context<Tuple> const> { lambda_context(Tuple const &args) : args_(args) {} template<typename Sig> struct result; template<typename This, typename I> struct result<This(proto::tag::terminal, placeholder<I> const &)> : fusion::result_of::at<Tuple, I> {}; template<typename I> typename fusion::result_of::at<Tuple, I>::type operator ()(proto::tag::terminal, placeholder<I> const &) const { return fusion::at<I>(this->args_); } Tuple args_; }; // The lambda<> expression wrapper makes expressions polymorphic // function objects template<typename T> struct lambda { BOOST_PROTO_BASIC_EXTENDS(T, lambda<T>, lambda_domain) BOOST_PROTO_EXTENDS_ASSIGN() BOOST_PROTO_EXTENDS_SUBSCRIPT() // Calculate the arity of this lambda expression static int const arity = boost::result_of<lambda_arity(T)>::type::value; template<typename Sig> struct result; // Define nested result<> specializations to calculate the return // type of this lambda expression. But be careful not to evaluate // the return type of the nullary function unless we have a nullary // lambda! template<typename This> struct result<This()> : mpl::eval_if_c< 0 == arity , proto::result_of::eval<T const, lambda_context<fusion::tuple<> > > , mpl::identity<void> > {}; template<typename This, typename A0> struct result<This(A0)> : proto::result_of::eval<T const, lambda_context<fusion::tuple<A0> > > {}; template<typename This, typename A0, typename A1> struct result<This(A0, A1)> : proto::result_of::eval<T const, lambda_context<fusion::tuple<A0, A1> > > {}; // Define our operator () that evaluates the lambda expression. typename result<lambda()>::type operator ()() const { fusion::tuple<> args; lambda_context<fusion::tuple<> > ctx(args); return proto::eval(*this, ctx); } template<typename A0> typename result<lambda(A0 const &)>::type operator ()(A0 const &a0) const { fusion::tuple<A0 const &> args(a0); lambda_context<fusion::tuple<A0 const &> > ctx(args); return proto::eval(*this, ctx); } template<typename A0, typename A1> typename result<lambda(A0 const &, A1 const &)>::type operator ()(A0 const &a0, A1 const &a1) const { fusion::tuple<A0 const &, A1 const &> args(a0, a1); lambda_context<fusion::tuple<A0 const &, A1 const &> > ctx(args); return proto::eval(*this, ctx); } }; // Define some lambda placeholders lambda<proto::terminal<placeholder<mpl::int_<0> > >::type> const _1 = {{}}; lambda<proto::terminal<placeholder<mpl::int_<1> > >::type> const _2 = {{}}; template<typename T> lambda<typename proto::terminal<T>::type> const val(T const &t) { lambda<typename proto::terminal<T>::type> that = {{t}}; return that; } template<typename T> lambda<typename proto::terminal<T &>::type> const var(T &t) { lambda<typename proto::terminal<T &>::type> that = {{t}}; return that; } template<typename T> struct construct_helper { typedef T result_type; // for TR1 result_of T operator()() const { return T(); } // Generate BOOST_PROTO_MAX_ARITY overloads of the // following function call operator. #define BOOST_PROTO_LOCAL_MACRO(N, typename_A, A_const_ref, A_const_ref_a, a)\ template<typename_A(N)> \ T operator()(A_const_ref_a(N)) const \ { return T(a(N)); } #define BOOST_PROTO_LOCAL_a BOOST_PROTO_a #include BOOST_PROTO_LOCAL_ITERATE() }; // Generate BOOST_PROTO_MAX_ARITY-1 overloads of the // following construct() function template. #define M0(N, typename_A, A_const_ref, A_const_ref_a, ref_a) \ template<typename T, typename_A(N)> \ typename proto::result_of::make_expr< \ proto::tag::function \ , lambda_domain \ , construct_helper<T> \ , A_const_ref(N) \ >::type const \ construct(A_const_ref_a(N)) \ { \ return proto::make_expr< \ proto::tag::function \ , lambda_domain \ >( \ construct_helper<T>() \ , ref_a(N) \ ); \ } BOOST_PROTO_REPEAT_FROM_TO(1, BOOST_PROTO_MAX_ARITY, M0) #undef M0 struct S { S() {} S(int i, char c) { std::cout << "S(" << i << "," << c << ")\n"; } }; int main() { // Create some lambda objects and immediately // invoke them by applying their operator(): int i = ( (_1 + 2) / 4 )(42); std::cout << i << std::endl; // prints 11 int j = ( (-(_1 + 2)) / 4 )(42); std::cout << j << std::endl; // prints -11 double d = ( (4 - _2) * 3 )(42, 3.14); std::cout << d << std::endl; // prints 2.58 // check non-const ref terminals (std::cout << _1 << " -- " << _2 << '\n')(42, "Life, the Universe and Everything!"); // prints "42 -- Life, the Universe and Everything!" // "Nullary" lambdas work too int k = (val(1) + val(2))(); std::cout << k << std::endl; // prints 3 // check array indexing for kicks int integers[5] = {0}; (var(integers)[2] = 2)(); (var(integers)[_1] = _1)(3); std::cout << integers[2] << std::endl; // prints 2 std::cout << integers[3] << std::endl; // prints 3 // Now use a lambda with an STL algorithm! int rgi[4] = {1,2,3,4}; char rgc[4] = {'a','b','c','d'}; S rgs[4]; std::transform(rgi, rgi+4, rgc, rgs, construct<S>(_1, _2)); return 0; }
This is an advanced example that shows how to externally parameterize a grammar's transforms. It defines a calculator EDSL with a grammar that can perform either checked or unchecked arithmetic.
// Copyright 2011 Eric Niebler. Distributed under the Boost // Software License, Version 1.0. (See accompanying file // LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) // // This is an example of how to specify a transform externally so // that a single grammar can be used to drive multiple differnt // calculations. In particular, it defines a calculator grammar // that computes the result of an expression with either checked // or non-checked division. #include <iostream> #include <boost/assert.hpp> #include <boost/mpl/int.hpp> #include <boost/mpl/next.hpp> #include <boost/mpl/min_max.hpp> #include <boost/fusion/container/vector.hpp> #include <boost/fusion/container/generation/make_vector.hpp> #include <boost/proto/proto.hpp> namespace mpl = boost::mpl; namespace proto = boost::proto; namespace fusion = boost::fusion; // The argument placeholder type template<typename I> struct placeholder : I {}; // Give each rule in the grammar a "name". This is so that we // can easily dispatch on it later. struct calc_grammar; struct divides_rule : proto::divides<calc_grammar, calc_grammar> {}; // Use external transforms in calc_gramar struct calc_grammar : proto::or_< proto::when< proto::terminal<placeholder<proto::_> > , proto::functional::at(proto::_state, proto::_value) > , proto::when< proto::terminal<proto::convertible_to<double> > , proto::_value > , proto::when< proto::plus<calc_grammar, calc_grammar> , proto::_default<calc_grammar> > , proto::when< proto::minus<calc_grammar, calc_grammar> , proto::_default<calc_grammar> > , proto::when< proto::multiplies<calc_grammar, calc_grammar> , proto::_default<calc_grammar> > // Note that we don't specify how division nodes are // handled here. Proto::external_transform is a placeholder // for an actual transform. , proto::when< divides_rule , proto::external_transform > > {}; template<typename E> struct calc_expr; struct calc_domain : proto::domain<proto::generator<calc_expr> > {}; template<typename E> struct calc_expr : proto::extends<E, calc_expr<E>, calc_domain> { calc_expr(E const &e = E()) : calc_expr::proto_extends(e) {} }; calc_expr<proto::terminal<placeholder<mpl::int_<0> > >::type> _1; calc_expr<proto::terminal<placeholder<mpl::int_<1> > >::type> _2; // Use proto::external_transforms to map from named grammar rules to // transforms. struct non_checked_division : proto::external_transforms< proto::when< divides_rule, proto::_default<calc_grammar> > > {}; struct division_by_zero : std::exception {}; struct do_checked_divide : proto::callable { typedef int result_type; int operator()(int left, int right) const { if (right == 0) throw division_by_zero(); return left / right; } }; // Use proto::external_transforms again, this time to map the divides_rule // to a transforms that performs checked division. struct checked_division : proto::external_transforms< proto::when< divides_rule , do_checked_divide(calc_grammar(proto::_left), calc_grammar(proto::_right)) > > {}; int main() { non_checked_division non_checked; int result2 = calc_grammar()(_1 / _2, fusion::make_vector(6, 2), non_checked); BOOST_ASSERT(result2 == 3); try { checked_division checked; // This should throw int result3 = calc_grammar()(_1 / _2, fusion::make_vector(6, 0), checked); BOOST_ASSERT(false); // shouldn't get here! } catch(division_by_zero) { std::cout << "caught division by zero!\n"; } }
Proto was initially developed as part of Boost.Xpressive to simplify the job of transforming an expression template into an executable finite state machine capable of matching a regular expression. Since then, Proto has found application in the redesigned and improved Spirit-2 and the related Karma library. As a result of these efforts, Proto evolved into a generic and abstract grammar and tree transformation framework applicable in a wide variety of EDSL scenarios.
The grammar and tree transformation framework is modeled on Spirit's grammar and semantic action framework. The expression tree data structure is similar to Fusion data structures in many respects, and is interoperable with Fusion's iterators and algorithms.
        The syntax for the grammar-matching features of proto::matches<> is inspired by MPL's lambda expressions.
      
The idea for using function types for Proto's composite transforms is inspired by Aleksey Gurtovoy's "round" lambda notation.
Ren, D. and Erwig, M. 2006. A generic recursion toolbox for Haskell or: scrap your boilerplate systematically. In Proceedings of the 2006 ACM SIGPLAN Workshop on Haskell (Portland, Oregon, USA, September 17 - 17, 2006). Haskell '06. ACM, New York, NY, 13-24. DOI=http://doi.acm.org/10.1145/1159842.1159845
A technical paper about an earlier version of Proto was accepted into the ACM SIGPLAN Symposium on Library-Centric Software Design LCSD'07, and can be found at http://lcsd.cs.tamu.edu/2007/final/1/1_Paper.pdf. The tree transforms described in that paper differ from what exists today.
              A transform of the form R(A0,A1,...) (i.e., a function type) where
              proto::is_callable<R>::value is true.
              R is treated as a polymorphic
              function object and the arguments are treated as transforms that yield
              the arguments to the function object.
            
              In Proto, the term context refers to an object
              that can be passed, along with an expression to evaluate, to the proto::eval()
              function. The context determines how the expression is evaluated. All
              context structs define a nested eval<> template that, when instantiated
              with a node tag type (e.g., proto::tag::plus),
              is a binary polymorphic function object that accepts an expression
              of that type and the context object. In this way, contexts associate
              behaviors with expression nodes.
            
              In Proto, the term domain refers to a type that
              associates expressions within that domain with a generator
              for that domain and optionally a grammar for the
              domain. Domains are used primarily to imbue expressions within that
              domain with additional members and to restrict Proto's operator overloads
              such that expressions not conforming to the domain's grammar are never
              created. Domains are empty structs that inherit from proto::domain<>.
            
A programming language that targets a particular problem space by providing programming idioms, abstractions and constructs that match the constructs within that problem space.
A domain-specific language implemented as a library. The language in which the library is written is called the "host" language, and the language implemented by the library is called the "embedded" language.
              In Proto, an expression is a heterogeneous tree
              where each node is either an instantiation of boost::proto::expr<>, boost::proto::basic_expr<> or some type that is an extension
              (via boost::proto::extends<>
              or BOOST_PROTO_EXTENDS()) of such an instantiation.
            
A C++ technique using templates and operator overloading to cause expressions to build trees that represent the expression for lazy evaluation later, rather than evaluating the expression eagerly. Some C++ libraries use expression templates to build embedded domain-specific languages.
In Proto, a generator is a unary polymorphic function object that you specify when defining a domain. After constructing a new expression, Proto passes the expression to your domain's generator for further processing. Often, the generator wraps the expression in an extension wrapper that adds additional members to it.
              In Proto, a grammar is a type that describes a
              subset of Proto expression types. Expressions in a domain must conform
              to that domain's grammar. The proto::matches<> metafunction evaluates whether
              an expression type matches a grammar. Grammars are either primitives
              such as proto::_, composites such as proto::plus<>,
              control structures such as proto::or_<>, or some type derived from
              a grammar.
            
              A transform of the form R(A0,A1,...) (i.e., a function type) where
              proto::is_callable<R>::value is false.
              R is treated as the
              type of an object to construct and the arguments are treated as transforms
              that yield the parameters to the constructor.
            
              An instance of a class type with an overloaded function call operator
              and a nested result_type
              typedef or result<>
              template for calculating the return type of the function call operator.
            
A type that defines a kind of polymorphic function object that takes three arguments: expression, state, and data. Primitive transforms can be used to compose callable transforms and object transforms.
A sub-domain is a domain that declares another domain as its super-domain. Expressions in sub-domains can be combined with expressions in the super-domain, and the resulting expression is in the super-domain.
              Transforms are used to manipulate expression trees. They come in three
              flavors: primitive transforms, callable transforms, or object transforms.
              A transform Tproto::when<>, as in proto::when<proto::_, .
              Such a function object accepts expression, state,
              and data parameters, and computes a result from
              them.
            T>
[28] 
              It's not always possible to hold something by value. By default, proto::as_expr() makes an exception
              for functions, abstract types, and iostreams (types derived from std::ios_base). These objects are held
              by reference. All others are held by value, even arrays.
            
[29] This error message was generated with Microsoft Visual C++ 9.0. Different compilers will emit different messages with varying degrees of readability.