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//
// composed_4.cpp
// ~~~~~~~~~~~~~~
//
// Copyright (c) 2003-2018 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/io_context.hpp>
#include <boost/asio/ip/tcp.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;
//------------------------------------------------------------------------------
// This composed operation automatically serialises a message, using its I/O
// streams insertion operator, before sending it on the socket. To do this, it
// must allocate a buffer for the encoded message and ensure this buffer's
// validity until the underlying async_write operation completes.
template <typename T, typename CompletionToken>
auto async_write_message(tcp::socket& socket,
const T& 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
{
// Define a type alias for the concrete completion handler, as we will use
// the type in several places in the implementation below.
using completion_handler_type =
typename boost::asio::async_completion<CompletionToken,
void(boost::system::error_code)>::completion_handler_type;
// In this example, the composed operation's intermediate completion handler
// is implemented as a hand-crafted function object, rather than a lambda.
struct intermediate_completion_handler
{
// The intermediate completion handler holds a reference to the socket so
// that it can obtain 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 user-supplied completion handler.
completion_handler_type handler_;
// The function call operator matches the completion signature of the
// async_write operation.
void operator()(const boost::system::error_code& error, std::size_t /*n*/)
{
// 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.
// The arguments must match the completion signature of our composed
// 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<
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<
completion_handler_type, std::allocator<void>>;
allocator_type get_allocator() const noexcept
{
return boost::asio::get_associated_allocator(
handler_, std::allocator<void>{});
}
};
// The boost::asio::async_completion object takes the completion token and
// from it creates:
//
// - completion.completion_handler:
// A completion handler (i.e. a callback) with the specified signature.
//
// - completion.result:
// An object from which we obtain the result of the initiating function.
boost::asio::async_completion<CompletionToken,
void(boost::system::error_code)> completion(token);
// Encode the message and copy it into an allocated buffer. The buffer will
// be maintained for the lifetime of the asynchronous operation.
std::ostringstream os;
os << message;
std::unique_ptr<std::string> encoded_message(new std::string(os.str()));
// Initiate the underlying async_write operation using our intermediate
// completion handler.
boost::asio::async_write(socket, boost::asio::buffer(*encoded_message),
intermediate_completion_handler{socket, std::move(encoded_message),
std::move(completion.completion_handler)});
// Finally, we return the result of the initiating function.
return completion.result.get();
}
//------------------------------------------------------------------------------
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();
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();
}
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