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+//
+// 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 <asio/io_context.hpp>
+#include <asio/ip/tcp.hpp>
+#include <asio/use_future.hpp>
+#include <asio/write.hpp>
+#include <functional>
+#include <iostream>
+#include <memory>
+#include <sstream>
+#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.
+
+//------------------------------------------------------------------------------
+
+// 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(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, 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 std::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 = asio::associated_executor_t<
+ typename std::decay<CompletionHandler>::type,
+ tcp::socket::executor_type>;
+
+ executor_type get_executor() const noexcept
+ {
+ return 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 = asio::associated_allocator_t<
+ typename std::decay<CompletionHandler>::type,
+ std::allocator<void>>;
+
+ allocator_type get_allocator() const noexcept
+ {
+ return asio::get_associated_allocator(
+ handler_, std::allocator<void>{});
+ }
+ };
+
+ // Initiate the underlying async_write operation using our intermediate
+ // completion handler.
+ auto encoded_message_buffer = asio::buffer(*encoded_message);
+ asio::async_write(socket, encoded_message_buffer,
+ intermediate_completion_handler{socket, std::move(encoded_message),
+ std::forward<CompletionHandler>(completion_handler)});
+ }
+};
+
+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 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
+{
+ // 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 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), std::move(encoded_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, 123456,
+ [](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, 654.321, 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();
+}