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#### async_read (3 of 4 overloads)

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

    template<
        typename AsyncReadStream,
        typename Allocator,
        typename ReadHandler>
    void-or-deduced async_read(
        AsyncReadStream & s,
        basic_streambuf< Allocator > & b,
        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 supplied buffer is full (that is, it has reached maximum size).
- An error occurred.

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

###### 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.

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

###### Remarks

This overload is equivalent to calling:

    boost::asio::async_read(
        s, b,
        boost::asio::transfer_all(),
        handler);