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- //
- // composed_6.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/executor_work_guard.hpp>
- #include <boost/asio/io_context.hpp>
- #include <boost/asio/ip/tcp.hpp>
- #include <boost/asio/steady_timer.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 shows composition of multiple underlying operations.
- // It automatically serialises a message, using its I/O streams insertion
- // operator, before sending it N times on the socket. To do this, it must
- // allocate a buffer for the encoded message and ensure this buffer's validity
- // until all underlying async_write operation complete. A one second delay is
- // inserted prior to each write operation, using a steady_timer.
- template <typename T, typename CompletionToken>
- auto async_write_messages(tcp::socket& socket,
- const T& message, std::size_t repeat_count,
- 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, std::size_t repeat_count,
- std::unique_ptr<boost::asio::steady_timer> delay_timer)
- {
- // In this example, the composed operation's intermediate completion
- // handler is implemented as a hand-crafted function object.
- struct intermediate_completion_handler
- {
- // The intermediate completion handler holds a reference to the socket as
- // it is used for multiple async_write operations, as well as for
- // obtaining 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 repeat count remaining.
- std::size_t repeat_count_;
- // A steady timer used for introducing a delay.
- std::unique_ptr<boost::asio::steady_timer> delay_timer_;
- // To manage the cycle between the multiple underlying asychronous
- // operations, our intermediate completion handler is implemented as a
- // state machine.
- enum { starting, waiting, writing } state_;
- // As our composed operation performs multiple underlying I/O operations,
- // we should maintain a work object against the I/O executor. This tells
- // the I/O executor that there is still more work to come in the future.
- boost::asio::executor_work_guard<tcp::socket::executor_type> io_work_;
- // The user-supplied completion handler, called once only on completion
- // of the entire composed operation.
- typename std::decay<decltype(completion_handler)>::type handler_;
- // By having a default value for the second argument, this function call
- // operator matches the completion signature of both the async_write and
- // steady_timer::async_wait operations.
- void operator()(const boost::system::error_code& error, std::size_t = 0)
- {
- if (!error)
- {
- switch (state_)
- {
- case starting:
- case writing:
- if (repeat_count_ > 0)
- {
- --repeat_count_;
- state_ = waiting;
- delay_timer_->expires_after(std::chrono::seconds(1));
- delay_timer_->async_wait(std::move(*this));
- return; // Composed operation not yet complete.
- }
- break; // Composed operation complete, continue below.
- case waiting:
- state_ = writing;
- boost::asio::async_write(socket_,
- boost::asio::buffer(*encoded_message_), std::move(*this));
- return; // Composed operation not yet complete.
- }
- }
- // This point is reached only on completion of the entire composed
- // operation.
- // We no longer have any future work coming for the I/O executor.
- io_work_.reset();
- // 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.
- 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),
- repeat_count, std::move(delay_timer),
- intermediate_completion_handler::starting,
- boost::asio::make_work_guard(socket.get_executor()),
- 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 composed asynchronous operation.
- std::ostringstream os;
- os << message;
- std::unique_ptr<std::string> encoded_message(new std::string(os.str()));
- // Create a steady_timer to be used for the delay between messages.
- std::unique_ptr<boost::asio::steady_timer> delay_timer(
- new boost::asio::steady_timer(socket.get_executor()));
- // 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), repeat_count,
- std::move(delay_timer));
- }
- //------------------------------------------------------------------------------
- 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_messages(socket, "Testing callback\r\n", 5,
- [](const boost::system::error_code& error)
- {
- if (!error)
- {
- std::cout << "Messages 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_messages(
- socket, "Testing future\r\n", 5, boost::asio::use_future);
- io_context.run();
- try
- {
- // Get the result of the operation.
- f.get();
- std::cout << "Messages sent\n";
- }
- catch (const std::exception& e)
- {
- std::cout << "Error: " << e.what() << "\n";
- }
- }
- //------------------------------------------------------------------------------
- int main()
- {
- test_callback();
- test_future();
- }
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