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Diffstat (limited to '3rdparty/asio/src/examples/cpp20/operations/composed_4.cpp')
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diff --git a/3rdparty/asio/src/examples/cpp20/operations/composed_4.cpp b/3rdparty/asio/src/examples/cpp20/operations/composed_4.cpp new file mode 100644 index 00000000000..069f57ce1ac --- /dev/null +++ b/3rdparty/asio/src/examples/cpp20/operations/composed_4.cpp @@ -0,0 +1,247 @@ +// +// composed_4.cpp +// ~~~~~~~~~~~~~~ +// +// Copyright (c) 2003-2024 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/bind_executor.hpp> +#include <asio/deferred.hpp> +#include <asio/io_context.hpp> +#include <asio/ip/tcp.hpp> +#include <asio/use_future.hpp> +#include <asio/write.hpp> +#include <cstring> +#include <functional> +#include <iostream> +#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. + +//------------------------------------------------------------------------------ + +// In this composed operation we repackage an existing operation, but with a +// different completion handler signature. We will also intercept an empty +// message as an invalid argument, and propagate the corresponding error to the +// user. The asynchronous operation requirements are met by delegating +// responsibility to the underlying operation. + +template < + asio::completion_token_for<void(std::error_code)> CompletionToken> +auto async_write_message(tcp::socket& socket, + const char* message, CompletionToken&& token) + // The return type of the initiating function is deduced from the combination + // of: + // + // - the CompletionToken type, + // - the completion handler signature, and + // - the asynchronous operation's initiation function object. + // + // 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 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>. When + // the completion token is asio::deferred, the return type differs for each + // asynchronous operation. + // + // In C++20 we can omit the return type as it is automatically deduced from + // the return type of 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(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 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 = []( + asio::completion_handler_for<void(std::error_code)> + auto&& completion_handler, + tcp::socket& socket, + const char* message) + { + // The post operation has a completion handler signature of: + // + // void() + // + // and the async_write operation has a completion handler signature of: + // + // void(std::error_code error, std::size n) + // + // Both of these operations' completion handler signatures differ from our + // operation's completion handler signature. We will adapt our completion + // handler to these signatures by using std::bind, which drops the + // additional arguments. + // + // However, it is essential to the correctness of our composed operation + // that we preserve the executor of the user-supplied completion handler. + // The std::bind function will not do this for us, so we must do this by + // first obtaining the completion handler's associated executor (defaulting + // to the I/O executor - in this case the executor of the socket - if the + // completion handler does not have its own) ... + auto executor = asio::get_associated_executor( + completion_handler, socket.get_executor()); + + // ... and then binding this executor to our adapted completion handler + // using the asio::bind_executor function. + std::size_t length = std::strlen(message); + if (length == 0) + { + asio::post( + asio::bind_executor(executor, + std::bind(std::forward<decltype(completion_handler)>( + completion_handler), asio::error::invalid_argument))); + } + else + { + asio::async_write(socket, + asio::buffer(message, length), + asio::bind_executor(executor, + std::bind(std::forward<decltype(completion_handler)>( + completion_handler), std::placeholders::_1))); + } + }; + + // 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)>( + initiation, token, std::ref(socket), 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, "", + [](const std::error_code& error) + { + if (!error) + { + std::cout << "Message sent\n"; + } + else + { + std::cout << "Error: " << error.message() << "\n"; + } + }); + + io_context.run(); +} + +//------------------------------------------------------------------------------ + +void test_deferred() +{ + asio::io_context io_context; + + tcp::acceptor acceptor(io_context, {tcp::v4(), 55555}); + tcp::socket socket = acceptor.accept(); + + // Test our asynchronous operation using the deferred completion token. This + // token causes the operation's initiating function to package up the + // operation with its arguments to return a function object, which may then be + // used to launch the asynchronous operation. + asio::async_operation auto op = + async_write_message(socket, "", asio::deferred); + + // Launch the operation using a lambda as a callback. + std::move(op)( + [](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, "", 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_deferred(); + test_future(); +} |