asio/example/cpp11/operations/composed_4.cpp

208 lines
7.8 KiB
C++

//
// composed_4.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. We will also intercept an empty
// message as an invalid argument, and propagate the corresponding error to the
// user. The asynchronous operation requirements are met by delegating
// responsibility to the underlying operation.
// 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, const char* message) const
{
// The post operation has a completion handler signature of:
//
// void()
//
// and the async_write operation has a completion handler signature of:
//
// void(boost::system::error_code error, std::size n)
//
// Both of these operations' completion handler signatures differ from our
// operation's completion handler signature. We will adapt our completion
// handler to these signatures by using std::bind, which drops the
// additional arguments.
//
// 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.
std::size_t length = std::strlen(message);
if (length == 0)
{
boost::asio::post(
boost::asio::bind_executor(executor,
std::bind(std::forward<CompletionHandler>(completion_handler),
boost::asio::error::invalid_argument)));
}
else
{
boost::asio::async_write(socket,
boost::asio::buffer(message, length),
boost::asio::bind_executor(executor,
std::bind(std::forward<CompletionHandler>(completion_handler),
std::placeholders::_1)));
}
}
};
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>.
-> typename boost::asio::async_result<
typename std::decay<CompletionToken>::type,
void(boost::system::error_code)>::return_type
{
// 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), 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, "",
[](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, "", 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();
}