244 lines
9.5 KiB
C++
244 lines
9.5 KiB
C++
//
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// composed_5.cpp
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// ~~~~~~~~~~~~~~
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//
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// Copyright (c) 2003-2019 Christopher M. Kohlhoff (chris at kohlhoff dot com)
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//
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// Distributed under the Boost Software License, Version 1.0. (See accompanying
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// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
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//
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#include <boost/asio/io_context.hpp>
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#include <boost/asio/ip/tcp.hpp>
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#include <boost/asio/use_future.hpp>
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#include <boost/asio/write.hpp>
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#include <functional>
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#include <iostream>
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#include <memory>
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#include <sstream>
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#include <string>
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#include <type_traits>
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#include <utility>
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using boost::asio::ip::tcp;
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// NOTE: This example requires the new boost::asio::async_initiate function. For
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// an example that works with the Networking TS style of completion tokens,
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// please see an older version of asio.
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//------------------------------------------------------------------------------
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// This composed operation automatically serialises a message, using its I/O
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// streams insertion operator, before sending it on the socket. To do this, it
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// must allocate a buffer for the encoded message and ensure this buffer's
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// validity until the underlying async_write operation completes.
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// In addition to determining the mechanism by which an asynchronous operation
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// delivers its result, a completion token also determines the time when the
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// operation commences. For example, when the completion token is a simple
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// callback the operation commences before the initiating function returns.
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// However, if the completion token's delivery mechanism uses a future, we
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// might instead want to defer initiation of the operation until the returned
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// future object is waited upon.
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//
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// To enable this, when implementing an asynchronous operation we must package
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// the initiation step as a function object.
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struct async_write_message_initiation
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{
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// The initiation function object's call operator is passed the concrete
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// completion handler produced by the completion token. This completion
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// handler matches the asynchronous operation's completion handler signature,
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// which in this example is:
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//
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// void(boost::system::error_code error)
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//
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// The initiation function object also receives any additional arguments
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// required to start the operation. (Note: We could have instead passed these
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// arguments as members in the initiaton function object. However, we should
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// prefer to propagate them as function call arguments as this allows the
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// completion token to optimise how they are passed. For example, a lazy
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// future which defers initiation would need to make a decay-copy of the
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// arguments, but when using a simple callback the arguments can be trivially
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// forwarded straight through.)
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template <typename CompletionHandler>
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void operator()(CompletionHandler&& completion_handler,
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tcp::socket& socket, std::unique_ptr<std::string> encoded_message) const
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{
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// In this example, the composed operation's intermediate completion
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// handler is implemented as a hand-crafted function object, rather than
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// using a lambda or std::bind.
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struct intermediate_completion_handler
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{
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// The intermediate completion handler holds a reference to the socket so
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// that it can obtain the I/O executor (see get_executor below).
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tcp::socket& socket_;
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// The allocated buffer for the encoded message. The std::unique_ptr
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// smart pointer is move-only, and as a consequence our intermediate
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// completion handler is also move-only.
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std::unique_ptr<std::string> encoded_message_;
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// The user-supplied completion handler.
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typename std::decay<CompletionHandler>::type handler_;
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// The function call operator matches the completion signature of the
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// async_write operation.
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void operator()(const boost::system::error_code& error, std::size_t /*n*/)
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{
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// Deallocate the encoded message before calling the user-supplied
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// completion handler.
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encoded_message_.reset();
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// Call the user-supplied handler with the result of the operation.
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// The arguments must match the completion signature of our composed
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// operation.
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handler_(error);
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}
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// It is essential to the correctness of our composed operation that we
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// preserve the executor of the user-supplied completion handler. With a
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// hand-crafted function object we can do this by defining a nested type
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// executor_type and member function get_executor. These obtain the
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// completion handler's associated executor, and default to the I/O
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// executor - in this case the executor of the socket - if the completion
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// handler does not have its own.
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using executor_type = boost::asio::associated_executor_t<
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typename std::decay<CompletionHandler>::type,
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tcp::socket::executor_type>;
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executor_type get_executor() const noexcept
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{
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return boost::asio::get_associated_executor(
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handler_, socket_.get_executor());
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}
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// Although not necessary for correctness, we may also preserve the
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// allocator of the user-supplied completion handler. This is achieved by
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// defining a nested type allocator_type and member function
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// get_allocator. These obtain the completion handler's associated
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// allocator, and default to std::allocator<void> if the completion
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// handler does not have its own.
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using allocator_type = boost::asio::associated_allocator_t<
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typename std::decay<CompletionHandler>::type,
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std::allocator<void>>;
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allocator_type get_allocator() const noexcept
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{
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return boost::asio::get_associated_allocator(
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handler_, std::allocator<void>{});
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}
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};
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// Initiate the underlying async_write operation using our intermediate
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// completion handler.
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auto encoded_message_buffer = boost::asio::buffer(*encoded_message);
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boost::asio::async_write(socket, encoded_message_buffer,
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intermediate_completion_handler{socket, std::move(encoded_message),
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std::forward<CompletionHandler>(completion_handler)});
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}
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};
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template <typename T, typename CompletionToken>
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auto async_write_message(tcp::socket& socket,
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const T& message, CompletionToken&& token)
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// The return type of the initiating function is deduced from the combination
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// of CompletionToken type and the completion handler's signature. When the
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// completion token is a simple callback, the return type is always void.
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// In this example, when the completion token is boost::asio::yield_context
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// (used for stackful coroutines) the return type would be also be void, as
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// there is no non-error argument to the completion handler. When the
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// completion token is boost::asio::use_future it would be std::future<void>.
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-> typename boost::asio::async_result<
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typename std::decay<CompletionToken>::type,
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void(boost::system::error_code)>::return_type
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{
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// Encode the message and copy it into an allocated buffer. The buffer will
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// be maintained for the lifetime of the asynchronous operation.
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std::ostringstream os;
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os << message;
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std::unique_ptr<std::string> encoded_message(new std::string(os.str()));
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// The boost::asio::async_initiate function takes:
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//
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// - our initiation function object,
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// - the completion token,
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// - the completion handler signature, and
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// - any additional arguments we need to initiate the operation.
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//
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// It then asks the completion token to create a completion handler (i.e. a
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// callback) with the specified signature, and invoke the initiation function
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// object with this completion handler as well as the additional arguments.
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// The return value of async_initiate is the result of our operation's
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// initiating function.
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//
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// Note that we wrap non-const reference arguments in std::reference_wrapper
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// to prevent incorrect decay-copies of these objects.
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return boost::asio::async_initiate<
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CompletionToken, void(boost::system::error_code)>(
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async_write_message_initiation(), token,
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std::ref(socket), std::move(encoded_message));
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}
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//------------------------------------------------------------------------------
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void test_callback()
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{
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boost::asio::io_context io_context;
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tcp::acceptor acceptor(io_context, {tcp::v4(), 55555});
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tcp::socket socket = acceptor.accept();
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// Test our asynchronous operation using a lambda as a callback.
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async_write_message(socket, 123456,
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[](const boost::system::error_code& error)
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{
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if (!error)
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{
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std::cout << "Message sent\n";
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}
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else
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{
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std::cout << "Error: " << error.message() << "\n";
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}
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});
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io_context.run();
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}
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//------------------------------------------------------------------------------
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void test_future()
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{
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boost::asio::io_context io_context;
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tcp::acceptor acceptor(io_context, {tcp::v4(), 55555});
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tcp::socket socket = acceptor.accept();
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// Test our asynchronous operation using the use_future completion token.
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// This token causes the operation's initiating function to return a future,
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// which may be used to synchronously wait for the result of the operation.
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std::future<void> f = async_write_message(
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socket, 654.321, boost::asio::use_future);
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io_context.run();
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try
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{
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// Get the result of the operation.
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f.get();
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std::cout << "Message sent\n";
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}
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catch (const std::exception& e)
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{
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std::cout << "Exception: " << e.what() << "\n";
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}
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}
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//------------------------------------------------------------------------------
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int main()
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{
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test_callback();
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test_future();
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}
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