[DEV] add v1.76.0

doc/html/boost_asio/example/cpp14/operations/composed_1.cpp

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+//
+// composed_1.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 http://www.boost.org/LICENSE_1_0.txt)
+//
+
+#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;
+
+//------------------------------------------------------------------------------
+
+// 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 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>.
+  //
+  // In C++14 we can omit the return type as it is automatically deduced from
+  // the return type of our underlying asynchronous operation
+{
+  // 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 boost::asio::async_write(socket,
+      boost::asio::buffer(message, std::strlen(message)),
+      std::forward<CompletionToken>(token));
+}
+
+//------------------------------------------------------------------------------
+
+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",
+      [](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", 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_future();
+}

doc/html/boost_asio/example/cpp14/operations/composed_2.cpp

@@ -0,0 +1,131 @@
+//
+// composed_2.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 http://www.boost.org/LICENSE_1_0.txt)
+//
+
+#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;
+
+//------------------------------------------------------------------------------
+
+// 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 <typename 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 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 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>.
+  //
+  // In C++14 we can omit the return type as it is automatically deduced from
+  // the return type of our underlying asynchronous operation
+{
+  // As the return type of the initiating function is deduced solely from the
+  // CompletionToken and completion signature, we know that two different
+  // asynchronous operations having the same completion signature will produce
+  // the same return type, when passed the same CompletionToken. This allows us
+  // to trivially delegate to alternate implementations.
+  if (allow_partial_write)
+  {
+    // When delegating to an 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 socket.async_write_some(
+        boost::asio::buffer(message, std::strlen(message)),
+        std::forward<CompletionToken>(token));
+  }
+  else
+  {
+    // As above, we must perfectly forward the completion token when calling
+    // the alternate underlying operation.
+    return boost::asio::async_write(socket,
+        boost::asio::buffer(message, std::strlen(message)),
+        std::forward<CompletionToken>(token));
+  }
+}
+
+//------------------------------------------------------------------------------
+
+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_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_future();
+}

doc/html/boost_asio/example/cpp14/operations/composed_3.cpp

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+//
+// composed_3.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 http://www.boost.org/LICENSE_1_0.txt)
+//
+
+#include <boost/asio/bind_executor.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. 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 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>.
+  //
+  // In C++14 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)
+  //
+  // 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, const char* message)
+  {
+    // The async_write operation has a completion handler signature of:
+    //
+    //   void(boost::system::error_code error, std::size n)
+    //
+    // This differs from our operation's signature in that it is also passed
+    // the number of bytes transferred as an argument of type std::size_t. We
+    // will adapt our completion handler to async_write's completion handler
+    // signature by using std::bind, which drops the additional argument.
+    //
+    // 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.
+    boost::asio::async_write(socket,
+        boost::asio::buffer(message, std::strlen(message)),
+        boost::asio::bind_executor(executor,
+          std::bind(std::forward<decltype(completion_handler)>(
+              completion_handler), std::placeholders::_1)));
+  };
+
+  // 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)>(
+      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, "Testing callback\r\n",
+      [](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, "Testing future\r\n", boost::asio::use_future);
+
+  io_context.run();
+
+  // Get the result of the operation.
+  try
+  {
+    // Get the result of the operation.
+    f.get();
+    std::cout << "Message sent\n";
+  }
+  catch (const std::exception& e)
+  {
+    std::cout << "Error: " << e.what() << "\n";
+  }
+}
+
+//------------------------------------------------------------------------------
+
+int main()
+{
+  test_callback();
+  test_future();
+}

doc/html/boost_asio/example/cpp14/operations/composed_4.cpp

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+//
+// composed_4.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 http://www.boost.org/LICENSE_1_0.txt)
+//
+
+#include <boost/asio/bind_executor.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.
+
+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 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>.
+  //
+  // In C++14 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)
+  //
+  // 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, const char* message)
+  {
+    // 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<decltype(completion_handler)>(
+                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<decltype(completion_handler)>(
+                completion_handler), std::placeholders::_1)));
+    }
+  };
+
+  // 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)>(
+      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_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_future();
+}

doc/html/boost_asio/example/cpp14/operations/composed_5.cpp

@@ -0,0 +1,238 @@
+//
+// 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 http://www.boost.org/LICENSE_1_0.txt)
+//
+
+#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.
+
+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>.
+  //
+  // In C++14 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)
+  //
+  // 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)
+  {
+    // 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<decltype(completion_handler)>::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.
+        encoded_message_.reset();
+
+        // Call the user-supplied handler with the result of the operation.
+        // The arguments must match the completion signature of our composed
+        // 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 = boost::asio::associated_executor_t<
+          typename std::decay<decltype(completion_handler)>::type,
+          tcp::socket::executor_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<decltype(completion_handler)>::type,
+          std::allocator<void>>;
+
+      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),
+          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 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)>(
+      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";
+        }
+        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, 654.321, 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_future();
+}

doc/html/boost_asio/example/cpp14/operations/composed_6.cpp

@@ -0,0 +1,298 @@
+//
+// composed_6.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 http://www.boost.org/LICENSE_1_0.txt)
+//
+
+#include <boost/asio/executor_work_guard.hpp>
+#include <boost/asio/io_context.hpp>
+#include <boost/asio/ip/tcp.hpp>
+#include <boost/asio/steady_timer.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 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 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>.
+  //
+  // In C++14 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)
+  //
+  // 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<boost::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<boost::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.
+      typename std::decay<decltype(boost::asio::prefer(
+            std::declval<tcp::socket::executor_type>(),
+            boost::asio::execution::outstanding_work.tracked))>::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 boost::system::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;
+            boost::asio::async_write(socket_,
+                boost::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.
+
+        // 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 = boost::asio::associated_executor_t<
+          typename std::decay<decltype(completion_handler)>::type,
+          tcp::socket::executor_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<decltype(completion_handler)>::type,
+          std::allocator<void>>;
+
+      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),
+          repeat_count, std::move(delay_timer),
+          intermediate_completion_handler::starting,
+          boost::asio::prefer(socket.get_executor(),
+              boost::asio::execution::outstanding_work.tracked),
+          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<boost::asio::steady_timer> delay_timer(
+      new boost::asio::steady_timer(socket.get_executor()));
+
+  // 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)>(
+      initiation, token, std::ref(socket),
+      std::move(encoded_message), repeat_count,
+      std::move(delay_timer));
+}
+
+//------------------------------------------------------------------------------
+
+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_messages(socket, "Testing callback\r\n", 5,
+      [](const boost::system::error_code& error)
+      {
+        if (!error)
+        {
+          std::cout << "Messages 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_messages(
+      socket, "Testing future\r\n", 5, boost::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();
+}

doc/html/boost_asio/example/cpp14/operations/composed_7.cpp

@@ -0,0 +1,219 @@
+//
+// composed_7.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 http://www.boost.org/LICENSE_1_0.txt)
+//
+
+#include <boost/asio/compose.hpp>
+#include <boost/asio/io_context.hpp>
+#include <boost/asio/ip/tcp.hpp>
+#include <boost/asio/steady_timer.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_compose 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 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>.
+  //
+  // In C++14 we can omit the return type as it is automatically deduced from
+  // the return type of boost::asio::async_initiate.
+{
+  // 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<boost::asio::steady_timer> delay_timer(
+      new boost::asio::steady_timer(socket.get_executor()));
+
+  // To manage the cycle between the multiple underlying asychronous
+  // operations, our implementation is a state machine.
+  enum { starting, waiting, writing };
+
+  // The boost::asio::async_compose function takes:
+  //
+  // - our asynchronous operation implementation,
+  // - the completion token,
+  // - the completion handler signature, and
+  // - any I/O objects (or executors) used by the operation
+  //
+  // It then wraps our implementation, which is implemented here as a state
+  // machine in a lambda, in an intermediate completion handler that meets the
+  // requirements of a conforming asynchronous operation. This includes
+  // tracking outstanding work against the I/O executors associated with the
+  // operation (in this example, this is the socket's executor).
+  //
+  // The first argument to our lambda is a reference to the enclosing
+  // intermediate completion handler. This intermediate completion handler is
+  // provided for us by the boost::asio::async_compose function, and takes care
+  // of all the details required to implement a conforming asynchronous
+  // operation. When calling an underlying asynchronous operation, we pass it
+  // this enclosing intermediate completion handler as the completion token.
+  //
+  // All arguments to our lambda after the first must be defaulted to allow the
+  // state machine to be started, as well as to allow the completion handler to
+  // match the completion signature of both the async_write and
+  // steady_timer::async_wait operations.
+  return boost::asio::async_compose<
+    CompletionToken, void(boost::system::error_code)>(
+      [
+        // The implementation holds a reference to the socket as it is used for
+        // multiple async_write operations.
+        &socket,
+
+        // The allocated buffer for the encoded message. The std::unique_ptr
+        // smart pointer is move-only, and as a consequence our lambda
+        // implementation is also move-only.
+        encoded_message = std::move(encoded_message),
+
+        // The repeat count remaining.
+        repeat_count,
+
+        // A steady timer used for introducing a delay.
+        delay_timer = std::move(delay_timer),
+
+        // To manage the cycle between the multiple underlying asychronous
+        // operations, our implementation is a state machine.
+        state = starting
+      ]
+      (
+        auto& self,
+        const boost::system::error_code& error = {},
+        std::size_t = 0
+      ) mutable
+      {
+        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(self));
+              return; // Composed operation not yet complete.
+            }
+            break; // Composed operation complete, continue below.
+          case waiting:
+            state = writing;
+            boost::asio::async_write(socket,
+                boost::asio::buffer(*encoded_message), std::move(self));
+            return; // Composed operation not yet complete.
+          }
+        }
+
+        // This point is reached only on completion of the entire composed
+        // operation.
+
+        // Deallocate the encoded message and delay timer before calling the
+        // user-supplied completion handler.
+        encoded_message.reset();
+        delay_timer.reset();
+
+        // Call the user-supplied handler with the result of the operation.
+        self.complete(error);
+      },
+      token, socket);
+}
+
+//------------------------------------------------------------------------------
+
+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_messages(socket, "Testing callback\r\n", 5,
+      [](const boost::system::error_code& error)
+      {
+        if (!error)
+        {
+          std::cout << "Messages 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_messages(
+      socket, "Testing future\r\n", 5, boost::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();
+}

doc/html/boost_asio/example/cpp14/operations/composed_8.cpp

@@ -0,0 +1,212 @@
+//
+// composed_8.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 http://www.boost.org/LICENSE_1_0.txt)
+//
+
+#include <boost/asio/compose.hpp>
+#include <boost/asio/coroutine.hpp>
+#include <boost/asio/io_context.hpp>
+#include <boost/asio/ip/tcp.hpp>
+#include <boost/asio/steady_timer.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_compose 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,
+// using asio's stackless coroutines support to express the flow of control. 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.
+
+#include <boost/asio/yield.hpp>
+
+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 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>.
+  //
+  // In C++14 we can omit the return type as it is automatically deduced from
+  // the return type of boost::asio::async_initiate.
+{
+  // 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<boost::asio::steady_timer> delay_timer(
+      new boost::asio::steady_timer(socket.get_executor()));
+
+  // The boost::asio::async_compose function takes:
+  //
+  // - our asynchronous operation implementation,
+  // - the completion token,
+  // - the completion handler signature, and
+  // - any I/O objects (or executors) used by the operation
+  //
+  // It then wraps our implementation, which is implemented here as a stackless
+  // coroutine in a lambda, in an intermediate completion handler that meets the
+  // requirements of a conforming asynchronous operation. This includes
+  // tracking outstanding work against the I/O executors associated with the
+  // operation (in this example, this is the socket's executor).
+  //
+  // The first argument to our lambda is a reference to the enclosing
+  // intermediate completion handler. This intermediate completion handler is
+  // provided for us by the boost::asio::async_compose function, and takes care
+  // of all the details required to implement a conforming asynchronous
+  // operation. When calling an underlying asynchronous operation, we pass it
+  // this enclosing intermediate completion handler as the completion token.
+  //
+  // All arguments to our lambda after the first must be defaulted to allow the
+  // state machine to be started, as well as to allow the completion handler to
+  // match the completion signature of both the async_write and
+  // steady_timer::async_wait operations.
+  return boost::asio::async_compose<
+    CompletionToken, void(boost::system::error_code)>(
+      [
+        // The implementation holds a reference to the socket as it is used for
+        // multiple async_write operations.
+        &socket,
+
+        // The allocated buffer for the encoded message. The std::unique_ptr
+        // smart pointer is move-only, and as a consequence our lambda
+        // implementation is also move-only.
+        encoded_message = std::move(encoded_message),
+
+        // The repeat count remaining.
+        repeat_count,
+
+        // A steady timer used for introducing a delay.
+        delay_timer = std::move(delay_timer),
+
+        // The coroutine state.
+        coro = boost::asio::coroutine()
+      ]
+      (
+        auto& self,
+        const boost::system::error_code& error = {},
+        std::size_t = 0
+      ) mutable
+      {
+        reenter (coro)
+        {
+          while (repeat_count > 0)
+          {
+            --repeat_count;
+
+            delay_timer->expires_after(std::chrono::seconds(1));
+            yield delay_timer->async_wait(std::move(self));
+            if (error)
+              break;
+
+            yield boost::asio::async_write(socket,
+                boost::asio::buffer(*encoded_message), std::move(self));
+            if (error)
+              break;
+          }
+
+          // Deallocate the encoded message and delay timer before calling the
+          // user-supplied completion handler.
+          encoded_message.reset();
+          delay_timer.reset();
+
+          // Call the user-supplied handler with the result of the operation.
+          self.complete(error);
+        }
+      },
+      token, socket);
+}
+
+#include <boost/asio/unyield.hpp>
+
+//------------------------------------------------------------------------------
+
+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_messages(socket, "Testing callback\r\n", 5,
+      [](const boost::system::error_code& error)
+      {
+        if (!error)
+        {
+          std::cout << "Messages 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_messages(
+      socket, "Testing future\r\n", 5, boost::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();
+}