#### async_write (5 of 8 overloads)

Start an asynchronous operation to write all of the supplied data to a stream.

    template<
        typename AsyncWriteStream,
        typename Allocator,
        typename WriteToken = default_completion_token_t<typename AsyncWriteStream::executor_type>>
    DEDUCED async_write(
        AsyncWriteStream & s,
        basic_streambuf< Allocator > & b,
        WriteToken && token = default_completion_token_t< typename AsyncWriteStream::executor_type >(),
        constraint_t< !is_completion_condition< decay_t< WriteToken > >::value >  = 0);

This function is used to asynchronously write a certain number of bytes of data to a stream. It is an initiating function for an asynchronous operation, and always returns immediately. The asynchronous operation will continue until one of the following conditions is true:

- All of the data in the supplied `basic_streambuf` has been written.
- An error occurred.

This operation is implemented in terms of zero or more calls to the stream's async_write_some function, and is known as a composed operation. The program must ensure that the stream performs no other write operations (such as async_write, the stream's async_write_some function, or any other composed operations that perform writes) until this operation completes.

###### Parameters

s

  The stream to which the data is to be written. The type must support the AsyncWriteStream concept.

b

  A `basic_streambuf` object from which data will be written. Ownership of the streambuf is retained by the caller, which must guarantee that it remains valid until the completion handler is called.

token

  The completion token that will be used to produce a completion handler, which will be called when the write completes. Potential completion tokens include `use_future`, `use_awaitable`, `yield_context`, or a function object with the correct completion signature. The function signature of the completion handler must be:

      void handler(
        // Result of operation.
        const asio::error_code& error,

        // Number of bytes written from the buffers. If an error
        // occurred, this will be less than the sum of the buffer sizes.
        std::size_t bytes_transferred
      );

  Regardless of whether the asynchronous operation completes immediately or not, the completion handler will not be invoked from within this function. On immediate completion, invocation of the handler will be performed in a manner equivalent to using `async_immediate`.

###### Completion Signature

    void(asio::error_code, std::size_t)

###### Per-Operation Cancellation

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

- `cancellation_type::terminal`
- `cancellation_type::partial`

if they are also supported by the `AsyncWriteStream` type's `async_write_some` operation.