#### async_read (4 of 4 overloads)

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

    template<
        typename AsyncReadStream,
        typename Allocator,
        typename CompletionCondition,
        typename ReadHandler>
    void async_read(
        AsyncReadStream & s,
        basic_streambuf< Allocator > & b,
        CompletionCondition completion_condition,
        ReadHandler handler);

This function is used to asynchronously read a certain number of bytes of data from a stream. The function call always returns immediately. The asynchronous operation will continue until one of the following conditions is true:

- The completion_condition function object returns true.

This operation is implemented in terms of one or more calls to the stream's async_read_some function.

###### Parameters

s

  The stream from which the data is to be read. The type must support the AsyncReadStream concept.

b

  A basic_streambuf object into which the data will be read. 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 read operation is complete. The signature of the function object must be:

      bool completion_condition(
        const asio::error_code& error, // Result of latest read_some
                                                // operation.

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

  A return value of true indicates that the read operation is complete. False indicates that further calls to the stream's async_read_some function are required.

handler

  The handler to be called when the read 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 copied into the
                                                // buffers. If an error occurred,
                                                // this will be the  number of
                                                // bytes successfully transferred
                                                // prior to the error.
      );

  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_service::post().