Boost C++ Libraries of the most highly regarded and expertly designed C++ library projects in the world. Herb Sutter and Andrei Alexandrescu, C++ Coding Standards

This is the documentation for an old version of Boost. Click here to view this page for the latest version.


// composed_5.cpp
// ~~~~~~~~~~~~~~
// Copyright (c) 2003-2021 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

#include <boost/asio/io_context.hpp>
#include <boost/asio/ip/tcp.hpp>
#include <boost/asio/use_future.hpp>
#include <boost/asio/write.hpp>
#include <functional>
#include <iostream>
#include <memory>
#include <sstream>
#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 composed operation automatically serialises a message, using its I/O
// streams insertion operator, before sending it on the socket. To do this, it
// must allocate a buffer for the encoded message and ensure this buffer's
// validity until the underlying async_write operation completes.

// 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, std::unique_ptr<std::string> encoded_message) const
    // In this example, the composed operation's intermediate completion
    // handler is implemented as a hand-crafted function object, rather than
    // using a lambda or std::bind.
    struct intermediate_completion_handler
      // The intermediate completion handler holds a reference to the socket so
      // that it can obtain 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 user-supplied completion handler.
      typename std::decay<CompletionHandler>::type handler_;

      // The function call operator matches the completion signature of the
      // async_write operation.
      void operator()(const boost::system::error_code& error, std::size_t /*n*/)
        // Deallocate the encoded message before calling the user-supplied
        // completion handler.

        // Call the user-supplied handler with the result of the operation.
        // The arguments must match the completion signature of our composed
        // operation.

      // 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 = boost::asio::associated_executor_t<
          typename std::decay<CompletionHandler>::type,

      executor_type get_executor() const noexcept
        return boost::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 = boost::asio::associated_allocator_t<
          typename std::decay<CompletionHandler>::type,

      allocator_type get_allocator() const noexcept
        return boost::asio::get_associated_allocator(
            handler_, std::allocator<void>{});

    // Initiate the underlying async_write operation using our intermediate
    // completion handler.
    auto encoded_message_buffer = boost::asio::buffer(*encoded_message);
    boost::asio::async_write(socket, encoded_message_buffer,
        intermediate_completion_handler{socket, std::move(encoded_message),

template <typename T, typename CompletionToken>
auto async_write_message(tcp::socket& socket,
    const T& 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 always void.
  // In this example, when the completion token is boost::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 boost::asio::use_future it would be std::future<void>.
  -> typename boost::asio::async_result<
    typename std::decay<CompletionToken>::type,
  // Encode the message and copy it into an allocated buffer. The buffer will
  // be maintained for the lifetime of the asynchronous operation.
  std::ostringstream os;
  os << message;
  std::unique_ptr<std::string> encoded_message(new std::string(os.str()));

  // 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), std::move(encoded_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, 123456,
      [](const boost::system::error_code& error)
        if (!error)
          std::cout << "Message sent\n";
          std::cout << "Error: " << error.message() << "\n";


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, 654.321, boost::asio::use_future);;

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


int main()