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#### async_write (3 of 4 overloads)

Start an asynchronous operation to write a certain amount of 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 one or more calls to the stream's async_write_some function.

###### 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().