[/
 / Copyright (c) 2003-2022 Christopher M. Kohlhoff (chris at kohlhoff dot com)
 /
 / Distributed under the Boost Software License, Version 1.0. (See accompanying
 / file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
 /]

[section:compose Compositions as Asynchronous Operations]

Application developers may wish to package their own compositions as conforming
[link asio.overview.model.async_ops asynchronous operations] within the [link
asio.overview.model asynchronous model]. Doing so facilitates seamless
composition of these operations together with the operations already provided
by Asio.

While these operations may be written from scratch to conform with the [link
asio.reference.asynchronous_operations requirements on asynchronous
operations], Asio includes the [link asio.reference.async_compose
`async_compose`] function to simplify this process. The `async_compose`
implementation automatically provides an intermediate completion handler that
correctly propagates the [link asio.overview.model.associators associated
characteristics] and tracks outstanding work against the I/O executor and
completion executor.

The following example illustrates an asynchronous echo loop (i.e. read,
followed by write, and so on), expressed as a simple state machine.

  struct async_echo_implementation
  {
    tcp::socket& socket_;
    asio::mutable_buffer buffer_;
    enum { starting, reading, writing } state_;

    template <typename Self>
    void operator()(Self& self,
        asio::error_code error = {},
        std::size_t n = 0)
    {
      switch (state_)
      {
      case starting:
        state_ = reading;
        socket_.async_read_some(
            buffer_, std::move(self));
        break;
      case reading:
        if (error)
        {
          self.complete(error, 0);
        }
        else
        {
          state_ = writing;
          asio::async_write(socket_, buffer_,
              asio::transfer_exactly(n),
              std::move(self));
        }
        break;
      case writing:
        self.complete(error, n);
        break;
      }
    }
  };

This implementation is then used in an initiating function, which trivially
wraps `async_compose`:

  template <typename CompletionToken>
  auto async_echo(tcp::socket& socket,
      asio::mutable_buffer buffer,
      CompletionToken&& token) ->
    typename asio::async_result<
      typename std::decay<CompletionToken>::type,
        void(asio::error_code, std::size_t)>::return_type
  {
    return asio::async_compose<CompletionToken,
      void(asio::error_code, std::size_t)>(
        async_echo_implementation{socket, buffer,
          async_echo_implementation::starting},
        token, socket);
  }

Here, `async_compose` is first passed the function object that contains the
implementation of the composed asynchronous operation. The first argument to
the function object is a non-const reference to the enclosing intermediate
completion handler. The remaining arguments are any arguments that originate
from the completion handlers of any asynchronous operations performed by the
implementation.

The `async_compose` function is also passed the completion token, and zero or
more I/O objects or I/O executors for which outstanding work must be
maintained.

[heading See Also]

[link asio.reference.async_compose async_compose],
[link asio.examples.cpp11_examples.operations Operations examples (C++11)],
[link asio.examples.cpp14_examples.operations Operations examples (C++14)].

[endsect]
