187 lines
7.3 KiB
C++
187 lines
7.3 KiB
C++
//
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// composed_3.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/bind_executor.hpp>
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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 <cstring>
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#include <functional>
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#include <iostream>
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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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// In this composed operation we repackage an existing operation, but with a
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// different completion handler signature. The asynchronous operation
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// requirements are met by delegating responsibility to the underlying
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// operation.
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template <typename CompletionToken>
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auto async_write_message(tcp::socket& socket,
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const char* 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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//
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// In C++14 we can omit the return type as it is automatically deduced from
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// the return type of boost::asio::async_initiate.
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{
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// In addition to determining the mechanism by which an asynchronous
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// operation delivers its result, a completion token also determines the time
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// when the operation commences. For example, when the completion token is a
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// simple callback the operation commences before the initiating function
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// returns. However, if the completion token's delivery mechanism uses a
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// future, we might instead want to defer initiation of the operation until
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// the returned future object is waited upon.
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//
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// To enable this, when implementing an asynchronous operation we must
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// package the initiation step as a function object. The initiation function
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// object's call operator is passed the concrete completion handler produced
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// by the completion token. This completion handler matches the asynchronous
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// operation's completion handler signature, 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 in the lambda capture set. However, we should prefer to
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// propagate them as function call arguments as this allows the completion
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// token to optimise how they are passed. For example, a lazy future which
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// defers initiation would need to make a decay-copy of the arguments, but
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// when using a simple callback the arguments can be trivially forwarded
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// straight through.)
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auto initiation = [](auto&& completion_handler,
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tcp::socket& socket, const char* message)
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{
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// The async_write operation has a completion handler signature of:
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//
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// void(boost::system::error_code error, std::size n)
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//
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// This differs from our operation's signature in that it is also passed
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// the number of bytes transferred as an argument of type std::size_t. We
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// will adapt our completion handler to async_write's completion handler
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// signature by using std::bind, which drops the additional argument.
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//
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// However, it is essential to the correctness of our composed operation
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// that we preserve the executor of the user-supplied completion handler.
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// The std::bind function will not do this for us, so we must do this by
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// first obtaining the completion handler's associated executor (defaulting
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// to the I/O executor - in this case the executor of the socket - if the
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// completion handler does not have its own) ...
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auto executor = boost::asio::get_associated_executor(
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completion_handler, socket.get_executor());
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// ... and then binding this executor to our adapted completion handler
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// using the boost::asio::bind_executor function.
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boost::asio::async_write(socket,
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boost::asio::buffer(message, std::strlen(message)),
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boost::asio::bind_executor(executor,
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std::bind(std::forward<decltype(completion_handler)>(
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completion_handler), std::placeholders::_1)));
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};
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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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initiation, token, std::ref(socket), 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, "Testing callback\r\n",
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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, "Testing future\r\n", boost::asio::use_future);
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io_context.run();
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// Get the result of the operation.
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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 << "Error: " << 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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