asio/example/cpp20/operations/composed_2.cpp

//
// composed_2.cpp
// ~~~~~~~~~~~~~~
//
// Copyright (c) 2003-2025 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/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 <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.

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

// This next simplest example of a composed asynchronous operation involves
// repackaging multiple operations but choosing to invoke just one of them. All
// of these underlying operations have the same completion signature. The
// asynchronous operation requirements are met by delegating responsibility to
// the underlying operations.

template <
  boost::asio::completion_token_for<void(boost::system::error_code, std::size_t)>
    CompletionToken>
auto async_write_message(tcp::socket& socket,
    const char* message, bool allow_partial_write,
    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 void.
  // However, when the completion token is boost::asio::yield_context (used for
  // stackful coroutines) the return type would be std::size_t, and when the
  // completion token is boost::asio::use_future it would be std::future<std::size_t>.
  // When the completion token is boost::asio::deferred, the return type differs for
  // each asynchronous operation.
  //
  // In C++20 we can omit the return type as it is automatically deduced from
  // the return type of boost::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(boost::system::error_code error, std::size_t)
  //
  // 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 = [](
      boost::asio::completion_handler_for<void(boost::system::error_code, std::size_t)>
        auto&& completion_handler,
      tcp::socket& socket,
      const char* message,
      bool allow_partial_write)
  {
    if (allow_partial_write)
    {
      // When delegating to an underlying operation we must take care to
      // perfectly forward the completion handler. This ensures that our
      // operation works correctly with move-only function objects as
      // callbacks.
      return socket.async_write_some(
          boost::asio::buffer(message, std::strlen(message)),
          std::forward<decltype(completion_handler)>(completion_handler));
    }
    else
    {
      // As above, we must perfectly forward the completion handler when calling
      // the alternate underlying operation.
      return boost::asio::async_write(socket,
          boost::asio::buffer(message, std::strlen(message)),
          std::forward<decltype(completion_handler)>(completion_handler));
    }
  };

  // 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, std::size_t)>(
      initiation, token, std::ref(socket), message, allow_partial_write);
}

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

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, "Testing callback\r\n", false,
      [](const boost::system::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_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 with its arguments to return a function object, which may then be
  // used to launch the asynchronous operation.
  boost::asio::async_operation auto op = async_write_message(
      socket, "Testing deferred\r\n", false, boost::asio::deferred);

  // Launch the operation using a lambda as a callback.
  std::move(op)(
      [](const boost::system::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()
{
  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<std::size_t> f = async_write_message(
      socket, "Testing future\r\n", false, boost::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_deferred();
  test_future();
}