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- //
- // composed_5.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/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.
- // 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) const
- {
- // 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<CompletionHandler>::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<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),
- std::forward<CompletionHandler>(completion_handler)});
- }
- };
- 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>.
- -> 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 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)>(
- async_write_message_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();
- }
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