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<title>Timer.1 - Using a timer synchronously</title>
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<div class="titlepage"><div><div><h3 class="title">
<a name="asio.tutorial.tuttimer1"></a><a class="link" href="tuttimer1.html" title="Timer.1 - Using a timer synchronously">Timer.1 - Using a timer synchronously</a>
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<p>
This tutorial program introduces asio by showing how to perform a blocking
wait on a timer.
</p>
<p>
We start by including the necessary header files.
</p>
<p>
All of the asio classes can be used by simply including the <code class="computeroutput">"asio.hpp"</code>
header file.
</p>
<pre class="programlisting">#include <iostream>
#include <asio.hpp>
</pre>
<p>
All programs that use asio need to have at least one I/O execution context,
such as an <a class="link" href="../reference/io_context.html" title="io_context">io_context</a> or
<a class="link" href="../reference/thread_pool.html" title="thread_pool">thread_pool</a> object. An
I/O execution context provides access to I/O functionality. We declare an
object of type <a class="link" href="../reference/io_context.html" title="io_context">io_context</a>
first thing in the main function.
</p>
<pre class="programlisting">int main()
{
asio::io_context io;
</pre>
<p>
Next we declare an object of type asio::steady_timer. The core asio classes
that provide I/O functionality (or as in this case timer functionality) always
take a reference to an io_context as their first constructor argument. The
second argument to the constructor sets the timer to expire 5 seconds from
now.
</p>
<pre class="programlisting"> asio::steady_timer t(io, asio::chrono::seconds(5));
</pre>
<p>
In this simple example we perform a blocking wait on the timer. That is,
the call to <a class="link" href="../reference/basic_waitable_timer/wait.html" title="basic_waitable_timer::wait">steady_timer::wait()</a>
will not return until the timer has expired, 5 seconds after it was created
(i.e. not from when the wait starts).
</p>
<p>
A timer is always in one of two states: "expired" or "not
expired". If the <a class="link" href="../reference/basic_waitable_timer/wait.html" title="basic_waitable_timer::wait">steady_timer::wait()</a>
function is called on an expired timer, it will return immediately.
</p>
<pre class="programlisting"> t.wait();
</pre>
<p>
Finally we print the obligatory <code class="computeroutput">"Hello, world!"</code> message
to show when the timer has expired.
</p>
<pre class="programlisting"> std::cout << "Hello, world!" << std::endl;
return 0;
}
</pre>
<p>
See the <a class="link" href="tuttimer1/src.html" title="Source listing for Timer.1">full source listing</a>
</p>
<p>
Return to the <a class="link" href="../tutorial.html" title="Tutorial">tutorial index</a>
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<p>
Next: <a class="link" href="tuttimer2.html" title="Timer.2 - Using a timer asynchronously">Timer.2 - Using a timer asynchronously</a>
</p>
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<td align="right"><div class="copyright-footer">Copyright © 2003-2020 Christopher M.
Kohlhoff<p>
Distributed under the Boost Software License, Version 1.0. (See accompanying
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
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