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#### async_write (3 of 4 overloads)

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

    template<
        typename AsyncWriteStream,
        typename Allocator,
        typename WriteHandler>
    void async_write(
        AsyncWriteStream & s,
        basic_streambuf< Allocator > & b,
        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.
- 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 handler is called.

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 boost::system::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 `boost::asio::io_service::post()`.