#### async_read (6 of 6 overloads)

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

    template<
        typename AsyncReadStream,
        typename Allocator,
        typename CompletionCondition,
        typename ReadHandler>
    DEDUCED 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 supplied buffer is full (that is, it has reached maximum size).
- The completion_condition function object returns 0.

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.

completion_condition

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

      std::size_t completion_condition(
        // Result of latest async_read_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 read operation is complete. A non-zero return value indicates the maximum number of bytes to be read on the next call to the stream's async_read_some function.

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_context::post()`.