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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 <asio/bind_executor.hpp>
+#include <asio/io_context.hpp>
+#include <asio/ip/tcp.hpp>
+#include <asio/use_future.hpp>
+#include <asio/write.hpp>
+#include <cstring>
+#include <functional>
+#include <iostream>
+#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.
+
+//------------------------------------------------------------------------------
+
+// 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 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, const char* message)
+ {
+ // The async_write operation has a completion handler signature of:
+ //
+ // void(std::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 = asio::get_associated_executor(
+ completion_handler, socket.get_executor());
+
+ // ... and then binding this executor to our adapted completion handler
+ // using the asio::bind_executor function.
+ asio::async_write(socket,
+ asio::buffer(message, std::strlen(message)),
+ asio::bind_executor(executor,
+ std::bind(std::forward<decltype(completion_handler)>(
+ completion_handler), std::placeholders::_1)));
+ };
+
+ // 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), message);
+}
+
+//------------------------------------------------------------------------------
+
+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_message(socket, "Testing callback\r\n",
+ [](const std::error_code& error)
+ {
+ if (!error)
+ {
+ std::cout << "Message 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_message(
+ socket, "Testing future\r\n", 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();
+}