doc/html/boost_asio/example/cpp11/operations/composed_6.cpp
// // composed_6.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/executor_work_guard.hpp> #include <boost/asio/io_context.hpp> #include <boost/asio/ip/tcp.hpp> #include <boost/asio/steady_timer.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 shows composition of multiple underlying operations. // It automatically serialises a message, using its I/O streams insertion // operator, before sending it N times on the socket. To do this, it must // allocate a buffer for the encoded message and ensure this buffer's validity // until all underlying async_write operation complete. A one second delay is // inserted prior to each write operation, using a steady_timer. // 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(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 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, std::size_t repeat_count, std::unique_ptr<boost::asio::steady_timer> delay_timer) const { // In this example, the composed operation's intermediate completion // handler is implemented as a hand-crafted function object. struct intermediate_completion_handler { // The intermediate completion handler holds a reference to the socket as // it is used for multiple async_write operations, as well as for // obtaining 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 repeat count remaining. std::size_t repeat_count_; // A steady timer used for introducing a delay. std::unique_ptr<boost::asio::steady_timer> delay_timer_; // To manage the cycle between the multiple underlying asychronous // operations, our intermediate completion handler is implemented as a // state machine. enum { starting, waiting, writing } state_; // As our composed operation performs multiple underlying I/O operations, // we should maintain a work object against the I/O executor. This tells // the I/O executor that there is still more work to come in the future. typename std::decay<decltype(boost::asio::prefer( std::declval<tcp::socket::executor_type>(), boost::asio::execution::outstanding_work.tracked))>::type io_work_; // The user-supplied completion handler, called once only on completion // of the entire composed operation. typename std::decay<CompletionHandler>::type handler_; // By having a default value for the second argument, this function call // operator matches the completion signature of both the async_write and // steady_timer::async_wait operations. void operator()(const boost::system::error_code& error, std::size_t = 0) { if (!error) { switch (state_) { case starting: case writing: if (repeat_count_ > 0) { --repeat_count_; state_ = waiting; delay_timer_->expires_after(std::chrono::seconds(1)); delay_timer_->async_wait(std::move(*this)); return; // Composed operation not yet complete. } break; // Composed operation complete, continue below. case waiting: state_ = writing; boost::asio::async_write(socket_, boost::asio::buffer(*encoded_message_), std::move(*this)); return; // Composed operation not yet complete. } } // This point is reached only on completion of the entire composed // operation. // 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. 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<CompletionHandler>::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<CompletionHandler>::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), repeat_count, std::move(delay_timer), intermediate_completion_handler::starting, boost::asio::prefer(socket.get_executor(), boost::asio::execution::outstanding_work.tracked), std::forward<CompletionHandler>(completion_handler)}); } }; template <typename T, typename CompletionToken> auto async_write_messages(tcp::socket& socket, const T& message, std::size_t repeat_count, 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>. -> typename boost::asio::async_result< typename std::decay<CompletionToken>::type, void(boost::system::error_code)>::return_type { // Encode the message and copy it into an allocated buffer. The buffer will // be maintained for the lifetime of the composed asynchronous operation. std::ostringstream os; os << message; std::unique_ptr<std::string> encoded_message(new std::string(os.str())); // Create a steady_timer to be used for the delay between messages. std::unique_ptr<boost::asio::steady_timer> delay_timer( new boost::asio::steady_timer(socket.get_executor())); // 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)>( async_write_message_initiation(), token, std::ref(socket), std::move(encoded_message), repeat_count, std::move(delay_timer)); } //------------------------------------------------------------------------------ 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_messages(socket, "Testing callback\r\n", 5, [](const boost::system::error_code& error) { if (!error) { std::cout << "Messages 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_messages( socket, "Testing future\r\n", 5, boost::asio::use_future); io_context.run(); try { // Get the result of the operation. f.get(); std::cout << "Messages sent\n"; } catch (const std::exception& e) { std::cout << "Error: " << e.what() << "\n"; } } //------------------------------------------------------------------------------ int main() { test_callback(); test_future(); }