| // |
| // 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(); |
| } |