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##### websocket::stream::async_ping

Send a websocket ping control frame asynchronously.

###### Synopsis

    template<
        class PingHandler = net::default_completion_token_t<executor_type>>
    DEDUCED
    async_ping(
        ping_data const& payload,
        PingHandler&& handler = net::default_completion_token_t< executor_type >{});

###### Description

This function is used to asynchronously send a ping frame, which usually elicits an automatic pong control frame response from the peer.

- The ping frame is written.
- An error occurs.

The algorithm, known as a composed asynchronous operation, is implemented in terms of calls to the next layer's `async_write_some` function. The program must ensure that no other calls to `ping`, `pong`, `async_ping`, or `async_pong` are performed until this operation completes.

If a close frame is sent or received before the ping frame is sent, the error received by this completion handler will be `net::error::operation_aborted`.

###### Parameters

Name | Description
--- | ---
`payload` | The payload of the ping message, which may be empty. The implementation will not access the contents of this object after the initiating function returns.
`handler` | The completion handler to invoke when the operation completes. The implementation takes ownership of the handler by performing a decay-copy. The equivalent function signature of the handler must be:
    void handler(
        error_code const & ec     // Result of operation
    );
If the handler has an associated immediate executor, an immediate completion will be dispatched to it. Otherwise, the handler will not be invoked from within this function. Invocation of the handler will be performed by dispatching to the immediate executor. If no immediate executor is specified, this is equivalent to using `net::post`.

###### Per-Operation Cancellation

This asynchronous operation supports cancellation for the following net::cancellation_type values:

- `net::cancellation_type::terminal`
- `net::cancellation_type::total`

`total`cancellation succeeds if the operation is suspended due to ongoing control operations such as a ping/pong.

`terminal` cancellation succeeds when supported by the underlying stream.

`terminal` cancellation leaves the stream in an undefined state, so that only closing it is guaranteed to succeed.