Messages can be sent to a counter party with the static Session::sendToTarget methods.
See this code in PYTHON, RUBY// send a message that already contains a BeginString, SenderCompID, and a TargetCompID static bool sendToTarget( Message&, const std::string& qualifier = "" ) throw(SessionNotFound&); // send a message based on the sessionID, convenient for use // in fromApp since it provides a session ID for incoming // messages static bool sendToTarget( Message&, const SessionID& ) throw(SessionNotFound&); // append a SenderCompID and TargetCompID before sending static bool sendToTarget( Message&, const SenderCompID&, const TargetCompID&, const std::string& qualifier = "" ) throw(SessionNotFound&); // pass SenderCompID and TargetCompID in as strings static bool sendToTarget( Message&, const std::string&, const std::string&, const std::string& qualifier = "" ) throw(SessionNotFound&);
Message constructors take in all required fields and adds the correct MsgType and BeginString for you. With the set method, the compiler will not let you add a field that is not a part of a message as defined by the spec.
void sendOrderCancelRequest()
{
  FIX41::OrderCancelRequest message(
    FIX::OrigClOrdID("123"),
    FIX::ClOrdID("321"),
    FIX::Symbol("LNUX"),
    FIX::Side(FIX::Side_BUY));
  message.set(FIX::Text("Cancel My Order!"));
  FIX::Session::sendToTarget(message, SenderCompID("TW"), TargetCompID("TARGET"));
}
    The setField method allows you to add any field to any message.
See this code in PYTHON, RUBY
void sendOrderCancelRequest()
{
  FIX::Message message;
  FIX::Header header& = message.getHeader();
  header.setField(FIX::BeginString("FIX.4.2"));
  header.setField(FIX::SenderCompID(TW));
  header.setField(FIX::TargetCompID("TARGET"));
  header.setField(FIX::MsgType(FIX::MsgType_OrderCancelRequest));
  message.setField(FIX::OrigClOrdID("123"));
  message.setField(FIX::ClOrdID("321"));
  message.setField(FIX::Symbol("LNUX"));
  message.setField(FIX::Side(FIX::Side_BUY));
  message.setField(FIX::Text("Cancel My Order!"));
  FIX::Session::sendToTarget(message);
}
    You can also use setField to pass in primitives.
See this code in PYTHON, RUBY
void sendOrderCancelRequest()
{
  FIX::Message message;
  // BeginString
  message.getHeader().setField(8, "FIX.4.2");
  // SenderCompID
  message.getHeader().setField(49, "TW");
  // TargetCompID, with enumeration
  message.getHeader().setField(FIX::FIELD::TargetCompID, "TARGET");
  // MsgType
  message.getHeader().setField(35, 'F');
  // OrigClOrdID
  message.setField(41, "123");
  // ClOrdID
  message.setField(11, "321");
  // Symbol
  message.setField(55, "LNUX");
  // Side, with value enumeration
  message.setField(54, FIX::Side_BUY);
  // Text
  message.setField(58, "Cancel My Order!");
  FIX::Session::sendToTarget(message);
}