Signal set service requirements

A signal set 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 signal set service class, a denotes a value of type X, b denotes a value of type X::implementation_type, ec denotes a value of type error_code, n denotes a value of type int, and sh denotes a value meeting SignalHandler requirements.

Table 28. SignalSetService requirements

expression	return type	assertion/note pre/post-condition
`a.construct(b);`		From IoObjectService requirements.
`a.destroy(b);`		From IoObjectService requirements. Implicitly clears the registered signals as if by calling `a.clear(b, ec)`, then implicitly cancels outstanding asynchronous operations as if by calling `a.cancel(b, ec)`.
a.add(b, n, ec);	`error_code`
a.remove(b, n, ec);	`error_code`
a.clear(b, ec);	`error_code`
a.cancel(b, ec);	`error_code`
`a.async_wait(b, sh);`	`void`	pre: `a.is_open(b)`. Initiates an asynchronous operation to wait for the delivery of one of the signals registered for the signal set `b`. The operation is performed via the `io_service` object `a.get_io_service()` and behaves according to asynchronous operation requirements. If the operation completes successfully, the `SignalHandler` object `sh` is invoked with the number identifying the delivered signal. Otherwise it is invoked with `0`.