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0c340c786a
The experimental::deferred completion token takes a call to an
asynchronous operation's initiating function and turns it into a
function object that accepts a completion token. For example:
auto deferred_op =
timer.async_wait(
boost::asio::experimental::deferred);
...
std::move(deferred_op)(
[](std::error_code ec){ ... });
or
auto deferred_op =
timer.async_wait(
boost::asio::experimental::deferred);
...
std::future<void> =
std::move(deferred_op)(
boost::asio::use_future);
The deferred token also supports chaining, to create simple
compositions:
auto deferred_op =
timer.async_wait(
boost::asio::experimental::deferred(
[&](std::error_code ec)
{
timer.expires_after(
std::chrono::seconds(1));
return timer.async_wait(
boost::asio::experimental::deferred);
});
...
std::future<void> = std::move(deferred_op)(boost::asio::use_future);
74 lines
1.9 KiB
C++
74 lines
1.9 KiB
C++
//
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// deferred_7.cpp
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// ~~~~~~~~~~~~~~
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//
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// Copyright (c) 2003-2021 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.hpp>
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#include <boost/asio/experimental/append.hpp>
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#include <boost/asio/experimental/deferred.hpp>
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#include <iostream>
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using boost::asio::experimental::append;
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using boost::asio::experimental::deferred;
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template <typename CompletionToken>
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auto async_wait_twice(boost::asio::steady_timer& timer, CompletionToken&& token)
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{
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return deferred.values(&timer)
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| deferred(
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[](boost::asio::steady_timer* timer)
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{
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timer->expires_after(std::chrono::seconds(1));
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return timer->async_wait(append(deferred, timer));
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}
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)
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| deferred(
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[](boost::system::error_code ec, boost::asio::steady_timer* timer)
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{
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std::cout << "first timer wait finished: " << ec.message() << "\n";
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timer->expires_after(std::chrono::seconds(1));
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return deferred.when(!ec)
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.then(timer->async_wait(deferred))
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.otherwise(deferred.values(ec));
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}
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)
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| deferred(
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[](boost::system::error_code ec)
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{
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std::cout << "second timer wait finished: " << ec.message() << "\n";
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return deferred.when(!ec)
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.then(deferred.values(42))
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.otherwise(deferred.values(0));
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}
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)
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| std::forward<CompletionToken>(token);
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}
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int main()
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{
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boost::asio::io_context ctx;
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boost::asio::steady_timer timer(ctx);
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timer.expires_after(std::chrono::seconds(1));
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async_wait_twice(
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timer,
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[](int result)
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{
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std::cout << "result is " << result << "\n";
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}
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);
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// Uncomment the following line to trigger an error in async_wait_twice.
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//timer.cancel();
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ctx.run();
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return 0;
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}
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