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A timer service must meet the requirements for an I/O object service, as well as the additional requirements listed below.
        In the table below, X denotes
        a timer service class for time type Time
        and traits type TimeTraits,
        a denotes a value of type
        X, b
        denotes a value of type X::implementation_type,
        t denotes a value of type
        Time, d
        denotes a value of type TimeTraits::duration_type,
        e denotes a value of type
        error_code, and h denotes a value meeting WaitHandler requirements.
      
Table 39. TimerService requirements
| expression | return type | 
                  assertion/note | 
|---|---|---|
| 
                   | 
                  From IoObjectService
                  requirements. Implicitly cancels asynchronous wait operations,
                  as if by calling  | |
| 
 a.cancel(b, e); 
 | 
                   | 
                  Causes any outstanding asynchronous wait operations to complete
                  as soon as possible. Handlers for cancelled operations shall be
                  passed the error code  | 
| 
                   | 
                   | |
| 
 a.expires_at(b, t, e); 
 | 
                   | 
                  Implicitly cancels asynchronous wait operations, as if by calling
                   | 
| 
                   | 
                   | 
                  Returns a value equivalent to  | 
| 
 a.expires_from_now(b, d, e); 
 | 
                   | 
                  Equivalent to  | 
| 
 a.wait(b, e); 
 | 
                   | 
                  Sets  | 
| 
 a.async_wait(b, h); 
 | 
                  Initiates an asynchronous wait operation that is performed via
                  the  |