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+//
+// 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 <asio/io_context.hpp>
+#include <asio/ip/tcp.hpp>
+#include <asio/use_future.hpp>
+#include <asio/write.hpp>
+#include <cstring>
+#include <iostream>
+#include <string>
+#include <type_traits>
+#include <utility>
+
+using 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 asio::yield_context (used for stackful
+ // coroutines) the return type would be std::size_t, and when the completion
+ // token is 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(
+ 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 asio::async_write(socket,
+ asio::buffer(message, std::strlen(message)),
+ std::forward<CompletionToken>(token));
+ }
+}
+
+//------------------------------------------------------------------------------
+
+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", false,
+ [](const std::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()
+{
+ 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, 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();
+}