#### async_write (6 of 6 overloads)

Start an asynchronous operation to write a certain amount of data to a stream.

    template<
        typename AsyncWriteStream,
        typename Allocator,
        typename CompletionCondition,
        typename WriteHandler>
    DEDUCED async_write(
        AsyncWriteStream & s,
        basic_streambuf< Allocator > & b,
        CompletionCondition completion_condition,
        WriteHandler && handler);

This function is used to asynchronously write a certain number of bytes of data to a stream. The function call 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.
- The completion_condition function object returns 0.

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 handler is called.

completion_condition

  The function object to be called to determine whether the write operation is complete. The signature of the function object must be:

      std::size_t completion_condition(
        // Result of latest async_write_some operation.
        const asio::error_code& error,

        // Number of bytes transferred so far.
        std::size_t bytes_transferred
      );

  A return value of 0 indicates that the write operation is complete. A non-zero return value indicates the maximum number of bytes to be written on the next call to the stream's async_write_some function.

handler

  The handler to be called when the write operation completes. Copies will be made of the handler as required. The function signature of the handler must be:

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

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

  Regardless of whether the asynchronous operation completes immediately or not, the handler will not be invoked from within this function. Invocation of the handler will be performed in a manner equivalent to using `asio::io_context::post()`.