### async_compose

Launch an asynchronous operation with a stateful implementation.

    template<
        typename CompletionToken,
        typename Signature,
        typename Implementation,
        typename... IoObjectsOrExecutors>
    DEDUCED async_compose(
        Implementation && implementation,
        type_identity_t< CompletionToken > & token,
        IoObjectsOrExecutors &&... io_objects_or_executors);

The async_compose function simplifies the implementation of composed asynchronous operations automatically wrapping a stateful function object with a conforming intermediate completion handler.

##### Parameters

implementation

  A 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.

token

  The completion token.

io_objects_or_executors

  Zero or more I/O objects or I/O executors for which outstanding work must be maintained.

##### Per-Operation Cancellation

By default, terminal per-operation cancellation is enabled for composed operations that are implemented using `async_compose`. To disable cancellation for the composed operation, or to alter its supported cancellation types, call the `self` object's `reset_cancellation_state` function.

##### Example:

    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;
        }
      }
    };

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

##### Requirements

Header: `asio/compose.hpp`

Convenience header: `asio.hpp`