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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 <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. 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(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 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(std::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 = 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.
+ std::size_t length = std::strlen(message);
+ if (length == 0)
+ {
+ asio::post(
+ asio::bind_executor(executor,
+ std::bind(std::forward<CompletionHandler>(completion_handler),
+ asio::error::invalid_argument)));
+ }
+ else
+ {
+ asio::async_write(socket,
+ asio::buffer(message, length),
+ 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 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>.
+ -> typename asio::async_result<
+ typename std::decay<CompletionToken>::type,
+ void(std::error_code)>::return_type
+{
+ // 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)>(
+ async_write_message_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, "",
+ [](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, "", 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();
+}