text: //
  // composed_6.cpp
  // ~~~~~~~~~~~~~~
  //
  // Copyright (c) 2003-2020 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/executor_work_guard.hpp>
  #include <asio/io_context.hpp>
  #include <asio/ip/tcp.hpp>
  #include <asio/steady_timer.hpp>
  #include <asio/use_future.hpp>
  #include <asio/write.hpp>
  #include <functional>
  #include <iostream>
  #include <memory>
  #include <sstream>
  #include <string>
  #include <type_traits>
  #include <utility>
  
  using asio::ip::tcp;
  
  // NOTE: This example requires the new asio::async_initiate function. For
  // an example that works with the Networking TS style of completion tokens,
  // please see an older version of asio.
  
  //------------------------------------------------------------------------------
  
  // This composed operation shows composition of multiple underlying operations.
  // It automatically serialises a message, using its I/O streams insertion
  // operator, before sending it N times on the socket. To do this, it must
  // allocate a buffer for the encoded message and ensure this buffer's validity
  // until all underlying async_write operation complete. A one second delay is
  // inserted prior to each write operation, using a steady_timer.
  
  template <typename T, typename CompletionToken>
  auto async_write_messages(tcp::socket& socket,
      const T& message, std::size_t repeat_count,
      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 always void.
    // In this example, when the completion token is asio::yield_context
    // (used for stackful coroutines) the return type would be also be void, as
    // there is no non-error argument to the completion handler. When the
    // completion token is asio::use_future it would be std::future<void>.
    //
    // In C++14 we can omit the return type as it is automatically deduced from
    // the return type of asio::async_initiate.
  {
    // In addition to determining the mechanism by which an asynchronous
    // operation delivers its result, a completion token also determines the time
    // when the operation commences. For example, when the completion token is a
    // simple callback the operation commences before the initiating function
    // returns. However, if the completion token's delivery mechanism uses a
    // future, we might instead want to defer initiation of the operation until
    // the returned future object is waited upon.
    //
    // To enable this, when implementing an asynchronous operation we must
    // package the initiation step as a function object. The initiation function
    // object's call operator is passed the concrete completion handler produced
    // by the completion token. This completion handler matches the asynchronous
    // operation's completion handler signature, which in this example is:
    //
    //   void(std::error_code error)
    //
    // The initiation function object also receives any additional arguments
    // required to start the operation. (Note: We could have instead passed these
    // arguments in the lambda capture set. However, we should prefer to
    // propagate them as function call arguments as this allows the completion
    // token to optimise how they are passed. For example, a lazy future which
    // defers initiation would need to make a decay-copy of the arguments, but
    // when using a simple callback the arguments can be trivially forwarded
    // straight through.)
    auto initiation = [](auto&& completion_handler, tcp::socket& socket,
        std::unique_ptr<std::string> encoded_message, std::size_t repeat_count,
        std::unique_ptr<asio::steady_timer> delay_timer)
    {
      // In this example, the composed operation's intermediate completion
      // handler is implemented as a hand-crafted function object.
      struct intermediate_completion_handler
      {
        // The intermediate completion handler holds a reference to the socket as
        // it is used for multiple async_write operations, as well as for
        // obtaining the I/O executor (see get_executor below).
        tcp::socket& socket_;
  
        // The allocated buffer for the encoded message. The std::unique_ptr
        // smart pointer is move-only, and as a consequence our intermediate
        // completion handler is also move-only.
        std::unique_ptr<std::string> encoded_message_;
  
        // The repeat count remaining.
        std::size_t repeat_count_;
  
        // A steady timer used for introducing a delay.
        std::unique_ptr<asio::steady_timer> delay_timer_;
  
        // To manage the cycle between the multiple underlying asychronous
        // operations, our intermediate completion handler is implemented as a
        // state machine.
        enum { starting, waiting, writing } state_;
  
        // As our composed operation performs multiple underlying I/O operations,
        // we should maintain a work object against the I/O executor. This tells
        // the I/O executor that there is still more work to come in the future.
        asio::executor_work_guard<tcp::socket::executor_type> io_work_;
  
        // The user-supplied completion handler, called once only on completion
        // of the entire composed operation.
        typename std::decay<decltype(completion_handler)>::type handler_;
  
        // By having a default value for the second argument, this function call
        // operator matches the completion signature of both the async_write and
        // steady_timer::async_wait operations.
        void operator()(const std::error_code& error, std::size_t = 0)
        {
          if (!error)
          {
            switch (state_)
            {
            case starting:
            case writing:
              if (repeat_count_ > 0)
              {
                --repeat_count_;
                state_ = waiting;
                delay_timer_->expires_after(std::chrono::seconds(1));
                delay_timer_->async_wait(std::move(*this));
                return; // Composed operation not yet complete.
              }
              break; // Composed operation complete, continue below.
            case waiting:
              state_ = writing;
              asio::async_write(socket_,
                  asio::buffer(*encoded_message_), std::move(*this));
              return; // Composed operation not yet complete.
            }
          }
  
          // This point is reached only on completion of the entire composed
          // operation.
  
          // We no longer have any future work coming for the I/O executor.
          io_work_.reset();
  
          // Deallocate the encoded message before calling the user-supplied
          // completion handler.
          encoded_message_.reset();
  
          // Call the user-supplied handler with the result of the operation.
          handler_(error);
        }
  
        // It is essential to the correctness of our composed operation that we
        // preserve the executor of the user-supplied completion handler. With a
        // hand-crafted function object we can do this by defining a nested type
        // executor_type and member function get_executor. These obtain the
        // completion handler's associated executor, and default to the I/O
        // executor - in this case the executor of the socket - if the completion
        // handler does not have its own.
        using executor_type = asio::associated_executor_t<
            typename std::decay<decltype(completion_handler)>::type,
            tcp::socket::executor_type>;
  
        executor_type get_executor() const noexcept
        {
          return asio::get_associated_executor(
              handler_, socket_.get_executor());
        }
  
        // Although not necessary for correctness, we may also preserve the
        // allocator of the user-supplied completion handler. This is achieved by
        // defining a nested type allocator_type and member function
        // get_allocator. These obtain the completion handler's associated
        // allocator, and default to std::allocator<void> if the completion
        // handler does not have its own.
        using allocator_type = asio::associated_allocator_t<
            typename std::decay<decltype(completion_handler)>::type,
            std::allocator<void>>;
  
        allocator_type get_allocator() const noexcept
        {
          return asio::get_associated_allocator(
              handler_, std::allocator<void>{});
        }
      };
  
      // Initiate the underlying async_write operation using our intermediate
      // completion handler.
      auto encoded_message_buffer = asio::buffer(*encoded_message);
      asio::async_write(socket, encoded_message_buffer,
          intermediate_completion_handler{
            socket, std::move(encoded_message),
            repeat_count, std::move(delay_timer),
            intermediate_completion_handler::starting,
            asio::make_work_guard(socket.get_executor()),
            std::forward<decltype(completion_handler)>(completion_handler)});
    };
  
    // Encode the message and copy it into an allocated buffer. The buffer will
    // be maintained for the lifetime of the composed asynchronous operation.
    std::ostringstream os;
    os << message;
    std::unique_ptr<std::string> encoded_message(new std::string(os.str()));
  
    // Create a steady_timer to be used for the delay between messages.
    std::unique_ptr<asio::steady_timer> delay_timer(
        new asio::steady_timer(socket.get_executor()));
  
    // The asio::async_initiate function takes:
    //
    // - our initiation function object,
    // - the completion token,
    // - the completion handler signature, and
    // - any additional arguments we need to initiate the operation.
    //
    // It then asks the completion token to create a completion handler (i.e. a
    // callback) with the specified signature, and invoke the initiation function
    // object with this completion handler as well as the additional arguments.
    // The return value of async_initiate is the result of our operation's
    // initiating function.
    //
    // Note that we wrap non-const reference arguments in std::reference_wrapper
    // to prevent incorrect decay-copies of these objects.
    return asio::async_initiate<
      CompletionToken, void(std::error_code)>(
        initiation, token, std::ref(socket),
        std::move(encoded_message), repeat_count,
        std::move(delay_timer));
  }
  
  //------------------------------------------------------------------------------
  
  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_messages(socket, "Testing callback\r\n", 5,
        [](const std::error_code& error)
        {
          if (!error)
          {
            std::cout << "Messages sent\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<void> f = async_write_messages(
        socket, "Testing future\r\n", 5, asio::use_future);
  
    io_context.run();
  
    try
    {
      // Get the result of the operation.
      f.get();
      std::cout << "Messages sent\n";
    }
    catch (const std::exception& e)
    {
      std::cout << "Error: " << e.what() << "\n";
    }
  }
  
  //------------------------------------------------------------------------------
  
  int main()
  {
    test_callback();
    test_future();
  }
  