text: //
  // composed_1.cpp
  // ~~~~~~~~~~~~~~
  //
  // Copyright (c) 2003-2018 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)
  //
  
  #include <asio/io_context.hpp>
  #include <asio/ip/tcp.hpp>
  #include <asio/use_future.hpp>
  #include <asio/write.hpp>
  #include <cstring>
  #include <iostream>
  #include <string>
  #include <type_traits>
  #include <utility>
  
  using asio::ip::tcp;
  
  //------------------------------------------------------------------------------
  
  // This is the simplest example of a composed asynchronous operation, where we
  // simply repackage an existing operation. The asynchronous operation
  // requirements are met by delegating responsibility to the underlying
  // operation.
  template <typename CompletionToken>
  auto async_write_message(tcp::socket& socket,
      const char* message, CompletionToken&& token)
    // The return type of the initiating function is deduced from the combination
    // of CompletionToken type and the completion handler's signature. When the
    // completion token is a simple callback, the return type is void. However,
    // when the completion token is asio::yield_context (used for stackful
    // coroutines) the return type would be std::size_t, and when the completion
    // token is asio::use_future it would be std::future<std::size_t>.
    -> typename asio::async_result<
      typename std::decay<CompletionToken>::type,
      void(std::error_code, std::size_t)>::return_type
  {
    // When delegating to the underlying operation we must take care to perfectly
    // forward the completion token. This ensures that our operation works
    // correctly with move-only function objects as callbacks, as well as other
    // completion token types.
    return asio::async_write(socket,
        asio::buffer(message, std::strlen(message)),
        std::forward<CompletionToken>(token));
  }
  
  //------------------------------------------------------------------------------
  
  void test_callback()
  {
    asio::io_context io_context;
  
    tcp::acceptor acceptor(io_context, {tcp::v4(), 55555});
    tcp::socket socket = acceptor.accept();
  
    // Test our asynchronous operation using a lambda as a callback.
    async_write_message(socket, "Testing callback\r\n",
        [](const std::error_code& error, std::size_t n)
        {
          if (!error)
          {
            std::cout << n << " bytes transferred\n";
          }
          else
          {
            std::cout << "Error: " << error.message() << "\n";
          }
        });
  
    io_context.run();
  }
  
  //------------------------------------------------------------------------------
  
  void test_future()
  {
    asio::io_context io_context;
  
    tcp::acceptor acceptor(io_context, {tcp::v4(), 55555});
    tcp::socket socket = acceptor.accept();
  
    // Test our asynchronous operation using the use_future completion token.
    // This token causes the operation's initiating function to return a future,
    // which may be used to synchronously wait for the result of the operation.
    std::future<std::size_t> f = async_write_message(
        socket, "Testing future\r\n", asio::use_future);
  
    io_context.run();
  
    try
    {
      // Get the result of the operation.
      std::size_t n = f.get();
      std::cout << n << " bytes transferred\n";
    }
    catch (const std::exception& e)
    {
      std::cout << "Error: " << e.what() << "\n";
    }
  }
  
  //------------------------------------------------------------------------------
  
  int main()
  {
    test_callback();
    test_future();
  }
  