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- //
- // composed_4.cpp
- // ~~~~~~~~~~~~~~
- //
- // Copyright (c) 2003-2019 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/bind_executor.hpp>
- #include <boost/asio/io_context.hpp>
- #include <boost/asio/ip/tcp.hpp>
- #include <boost/asio/use_future.hpp>
- #include <boost/asio/write.hpp>
- #include <cstring>
- #include <functional>
- #include <iostream>
- #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.
- //------------------------------------------------------------------------------
- // 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 <typename 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 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, 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(boost::system::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 = boost::asio::get_associated_executor(
- completion_handler, socket.get_executor());
- // ... and then binding this executor to our adapted completion handler
- // using the boost::asio::bind_executor function.
- std::size_t length = std::strlen(message);
- if (length == 0)
- {
- boost::asio::post(
- boost::asio::bind_executor(executor,
- std::bind(std::forward<decltype(completion_handler)>(
- completion_handler), boost::asio::error::invalid_argument)));
- }
- else
- {
- boost::asio::async_write(socket,
- boost::asio::buffer(message, length),
- boost::asio::bind_executor(executor,
- std::bind(std::forward<decltype(completion_handler)>(
- completion_handler), std::placeholders::_1)));
- }
- };
- // 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), 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, "",
- [](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, "", 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();
- }
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