Timer service requirements

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### Timer service requirements

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 23. TimerService 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);` | `Time` |

    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);` | `TimeTraits::duration_type` | Returns a value equivalent to `TimeTraits::subtract(a.expires_at(b), TimeTraits::now())`.

    a.expires_from_now(b, d, e);

`size_t`

Equivalent to `a.expires_at(b, TimeTraits::add(TimeTraits::now(), d), e)`.

    a.wait(b, e);

`error_code`

Sets `e` to indicate success or failure. Returns `e`.
post: `!!e || !TimeTraits::lt(TimeTraits::now(), a.expires_at(b))`.

    a.async_wait(b, h);

Initiates an asynchronous wait operation that is performed via the `io_service` object `a.io_service()` and behaves according to asynchronous operation requirements.
The handler shall be posted for execution only if the condition `!!ec || !TimeTraits::lt(TimeTraits::now(), a.expires_at(b))` holds, where `ec` is the error code to be passed to the handler.