Waitable timer service requirements

A waitable 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 waitable timer service class for clock type Clock, where Clock meets the C++11 clock type requirements, a denotes a value of type X, b denotes a value of type X::implementation_type, t denotes a value of type Clock::time_point, d denotes a value of type Clock::duration, e denotes a value of type error_code, and h denotes a value meeting WaitHandler requirements.

Table 40. WaitableTimerService requirements

expression	return type	assertion/note pre/post-condition
`a.destroy(b);`		From IoObjectService requirements. Implicitly cancels asynchronous wait operations, as if by calling `a.cancel(b, e)`.
a.cancel(b, e);	`size_t`	Causes any outstanding asynchronous wait operations to complete as soon as possible. Handlers for cancelled operations shall be passed the error code `error::operation_aborted`. Sets `e` to indicate success or failure. Returns the number of operations that were cancelled.
`a.expires_at(b);`	`Clock::time_point`
a.expires_at(b, t, e);	`size_t`	Implicitly cancels asynchronous wait operations, as if by calling `a.cancel(b, e)`. Returns the number of operations that were cancelled. post: `a.expires_at(b) == t`.
`a.expires_from_now(b);`	`Clock::duration`	Returns a value equivalent to `a.expires_at(b) - Clock::now()`.
a.expires_from_now(b, d, e);	`size_t`	Equivalent to `a.expires_at(b, Clock::now() + d, e)`.
a.wait(b, e);	`error_code`	Sets `e` to indicate success or failure. Returns `e`. post: `!!e \|\| !(Clock::now() < a.expires_at(b))`.
a.async_wait(b, h);		Initiates an asynchronous wait operation that is performed via the `io_service` object `a.get_io_service()` and behaves according to asynchronous operation requirements. The handler shall be posted for execution only if the condition `!!ec \|\| !(Clock::now() < a.expires_at(b))` holds, where `ec` is the error code to be passed to the handler.