diff options
Diffstat (limited to '3rdparty/asio/src/examples/cpp14/operations/composed_5.cpp')
-rw-r--r-- | 3rdparty/asio/src/examples/cpp14/operations/composed_5.cpp | 238 |
1 files changed, 238 insertions, 0 deletions
diff --git a/3rdparty/asio/src/examples/cpp14/operations/composed_5.cpp b/3rdparty/asio/src/examples/cpp14/operations/composed_5.cpp new file mode 100644 index 00000000000..560773217f6 --- /dev/null +++ b/3rdparty/asio/src/examples/cpp14/operations/composed_5.cpp @@ -0,0 +1,238 @@ +// +// 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. + +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>. + // + // In C++14 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 = [](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 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<decltype(completion_handler)>::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<decltype(completion_handler)>::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<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 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), + 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(); +} |