asio/example/cpp14/operations/composed_3.cpp

187 lines
7.3 KiB
C++

//
// composed_3.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. 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 async_write operation has a completion handler signature of:
//
// void(boost::system::error_code error, std::size n)
//
// This differs from our operation's signature in that it is also passed
// the number of bytes transferred as an argument of type std::size_t. We
// will adapt our completion handler to async_write's completion handler
// signature by using std::bind, which drops the additional argument.
//
// 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.
boost::asio::async_write(socket,
boost::asio::buffer(message, std::strlen(message)),
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, "Testing callback\r\n",
[](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, "Testing future\r\n", boost::asio::use_future);
io_context.run();
// Get the result of the operation.
try
{
// Get the result of the operation.
f.get();
std::cout << "Message sent\n";
}
catch (const std::exception& e)
{
std::cout << "Error: " << e.what() << "\n";
}
}
//------------------------------------------------------------------------------
int main()
{
test_callback();
test_future();
}