doc/html/boost_asio/example/cpp14/operations/composed_5.cpp
// // 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(); }