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doc/html/boost_asio/example/cpp11/operations/composed_4.cpp

//
// composed_4.cpp
// ~~~~~~~~~~~~~~
//
// 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)
//

#include <boost/asio/bind_executor.hpp>
#include <boost/asio/deferred.hpp>
#include <boost/asio/io_context.hpp>
#include <boost/asio/ip/tcp.hpp>
#include <boost/asio/use_future.hpp>
#include <boost/asio/write.hpp>
#include <cstring>
#include <functional>
#include <iostream>
#include <string>
#include <type_traits>
#include <utility>

using boost::asio::ip::tcp;

// NOTE: This example requires the new boost::asio::async_initiate function. For
// an example that works with the Networking TS style of completion tokens,
// please see an older version of asio.

//------------------------------------------------------------------------------

// In this composed operation we repackage an existing operation, but with a
// different completion handler signature. We will also intercept an empty
// message as an invalid argument, and propagate the corresponding error to the
// user. The asynchronous operation requirements are met by delegating
// responsibility to the underlying operation.

// 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.
struct async_write_message_initiation
{
  // 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(boost::system::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 as members in the initiaton function object. 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.)
  template <typename CompletionHandler>
  void operator()(CompletionHandler&& completion_handler,
      tcp::socket& socket, const char* message) const
  {
    // The post operation has a completion handler signature of:
    //
    //   void()
    //
    // and the async_write operation has a completion handler signature of:
    //
    //   void(boost::system::error_code error, std::size n)
    //
    // Both of these operations' completion handler signatures differ from our
    // operation's completion handler signature. We will adapt our completion
    // handler to these signatures by using std::bind, which drops the
    // additional arguments.
    //
    // However, it is essential to the correctness of our composed operation
    // that we preserve the executor of the user-supplied completion handler.
    // The std::bind function will not do this for us, so we must do this by
    // first obtaining the completion handler's associated executor (defaulting
    // to the I/O executor - in this case the executor of the socket - if the
    // completion handler does not have its own) ...
    auto executor = boost::asio::get_associated_executor(
        completion_handler, socket.get_executor());

    // ... and then binding this executor to our adapted completion handler
    // using the boost::asio::bind_executor function.
    std::size_t length = std::strlen(message);
    if (length == 0)
    {
      boost::asio::post(
          boost::asio::bind_executor(executor,
            std::bind(std::forward<CompletionHandler>(completion_handler),
              boost::asio::error::invalid_argument)));
    }
    else
    {
      boost::asio::async_write(socket,
          boost::asio::buffer(message, length),
          boost::asio::bind_executor(executor,
            std::bind(std::forward<CompletionHandler>(completion_handler),
              std::placeholders::_1)));
    }
  }
};

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:
  //
  // - the CompletionToken type,
  // - the completion handler signature, and
  // - the asynchronous operation's initiation function object.
  //
  // When the completion token is a simple callback, the return type is always
  // void. In this example, when the completion token is boost::asio::yield_context
  // (used for stackful coroutines) the return type would also be void, as
  // there is no non-error argument to the completion handler. When the
  // completion token is boost::asio::use_future it would be std::future<void>. When
  // the completion token is boost::asio::deferred, the return type differs for each
  // asynchronous operation.
  //
  // In C++11 we deduce the type from the call to boost::asio::async_initiate.
  -> decltype(
      boost::asio::async_initiate<
        CompletionToken, void(boost::system::error_code)>(
          async_write_message_initiation(),
          token, std::ref(socket), message))
{
  // The boost::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 boost::asio::async_initiate<
    CompletionToken, void(boost::system::error_code)>(
      async_write_message_initiation(),
      token, std::ref(socket), message);
}

//------------------------------------------------------------------------------

void test_callback()
{
  boost::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, "",
      [](const boost::system::error_code& error)
      {
        if (!error)
        {
          std::cout << "Message sent\n";
        }
        else
        {
          std::cout << "Error: " << error.message() << "\n";
        }
      });

  io_context.run();
}

//------------------------------------------------------------------------------

void test_deferred()
{
  boost::asio::io_context io_context;

  tcp::acceptor acceptor(io_context, {tcp::v4(), 55555});
  tcp::socket socket = acceptor.accept();

  // Test our asynchronous operation using the deferred completion token. This
  // token causes the operation's initiating function to package up the
  // operation and its arguments to return a function object, which may then be
  // used to launch the asynchronous operation.
  auto op = async_write_message(socket, "", boost::asio::deferred);

  // Launch the operation using a lambda as a callback.
  std::move(op)(
      [](const boost::system::error_code& error)
      {
        if (!error)
        {
          std::cout << "Message sent\n";
        }
        else
        {
          std::cout << "Error: " << error.message() << "\n";
        }
      });

  io_context.run();
}

//------------------------------------------------------------------------------

void test_future()
{
  boost::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_message(
      socket, "", boost::asio::use_future);

  io_context.run();

  try
  {
    // Get the result of the operation.
    f.get();
    std::cout << "Message sent\n";
  }
  catch (const std::exception& e)
  {
    std::cout << "Exception: " << e.what() << "\n";
  }
}

//------------------------------------------------------------------------------

int main()
{
  test_callback();
  test_deferred();
  test_future();
}