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<title>Timer.3 - Binding arguments to a handler</title>
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<div class="titlepage"><div><div><h3 class="title">
<a name="asio.tutorial.tuttimer3"></a><a class="link" href="tuttimer3.html" title="Timer.3 - Binding arguments to a handler">Timer.3 - Binding arguments
to a handler</a>
</h3></div></div></div>
<p>
In this tutorial we will modify the program from tutorial Timer.2 so that
the timer fires once a second. This will show how to pass additional parameters
to your handler function.
</p>
<pre class="programlisting">#include &lt;iostream&gt;
#include &lt;asio.hpp&gt;
#include &lt;boost/bind/bind.hpp&gt;
</pre>
<p>
To implement a repeating timer using asio you need to change the timer's
expiry time in your callback function, and to then start a new asynchronous
wait. Obviously this means that the callback function will need to be able
to access the timer object. To this end we add two new parameters to the
<code class="computeroutput">print</code> function:
</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
A pointer to a timer object.
</li>
<li class="listitem">
A counter so that we can stop the program when the timer fires for the
sixth time.
</li>
</ul></div>
<pre class="programlisting">void print(const asio::error_code&amp; /*e*/,
asio::steady_timer* t, int* count)
{
</pre>
<p>
As mentioned above, this tutorial program uses a counter to stop running
when the timer fires for the sixth time. However you will observe that there
is no explicit call to ask the io_context to stop. Recall that in tutorial
Timer.2 we learnt that the <a class="link" href="../reference/io_context/run.html" title="io_context::run">io_context::run()</a>
function completes when there is no more "work" to do. By not starting
a new asynchronous wait on the timer when <code class="computeroutput">count</code> reaches 5, the
io_context will run out of work and stop running.
</p>
<pre class="programlisting"> if (*count &lt; 5)
{
std::cout &lt;&lt; *count &lt;&lt; std::endl;
++(*count);
</pre>
<p>
Next we move the expiry time for the timer along by one second from the previous
expiry time. By calculating the new expiry time relative to the old, we can
ensure that the timer does not drift away from the whole-second mark due
to any delays in processing the handler.
</p>
<pre class="programlisting"> t-&gt;expires_at(t-&gt;expiry() + asio::chrono::seconds(1));
</pre>
<p>
Then we start a new asynchronous wait on the timer. As you can see, the
<a class="link" href="boost_bind.html" title="boost::bind">boost::bind</a> function is used
to associate the extra parameters with your callback handler. The <a class="link" href="../reference/basic_waitable_timer/async_wait.html" title="basic_waitable_timer::async_wait">steady_timer::async_wait()</a>
function expects a handler function (or function object) with the signature
<code class="computeroutput">void(const asio::error_code&amp;)</code>. Binding the additional parameters
converts your <code class="computeroutput">print</code> function into a function object that matches
the signature correctly.
</p>
<p>
See the <a href="http://www.boost.org/libs/bind/bind.html" target="_top">Boost.Bind
documentation</a> for more information on how to use <a class="link" href="boost_bind.html" title="boost::bind">boost::bind</a>
.
</p>
<p>
In this example, the asio::placeholders::error argument to <a class="link" href="boost_bind.html" title="boost::bind">boost::bind</a>
is a named placeholder for the error object passed to the handler. When initiating
the asynchronous operation, and if using <a class="link" href="boost_bind.html" title="boost::bind">boost::bind</a>
, you must specify only the arguments that match the handler's parameter
list. In tutorial Timer.4 you will see that this placeholder may be elided
if the parameter is not needed by the callback handler.
</p>
<pre class="programlisting"> t-&gt;async_wait(boost::bind(print,
asio::placeholders::error, t, count));
}
}
int main()
{
asio::io_context io;
</pre>
<p>
A new <code class="computeroutput">count</code> variable is added so that we can stop the program
when the timer fires for the sixth time.
</p>
<pre class="programlisting"> int count = 0;
asio::steady_timer t(io, asio::chrono::seconds(1));
</pre>
<p>
As in Step 4, when making the call to <a class="link" href="../reference/basic_waitable_timer/async_wait.html" title="basic_waitable_timer::async_wait">steady_timer::async_wait()</a>
from <code class="computeroutput">main</code> we bind the additional parameters needed for the
<code class="computeroutput">print</code> function.
</p>
<pre class="programlisting"> t.async_wait(boost::bind(print,
asio::placeholders::error, &amp;t, &amp;count));
io.run();
</pre>
<p>
Finally, just to prove that the <code class="computeroutput">count</code> variable was being used
in the <code class="computeroutput">print</code> handler function, we will print out its new value.
</p>
<pre class="programlisting"> std::cout &lt;&lt; "Final count is " &lt;&lt; count &lt;&lt; std::endl;
return 0;
}
</pre>
<p>
See the <a class="link" href="tuttimer3/src.html" title="Source listing for Timer.3">full source listing</a>
</p>
<p>
Return to the <a class="link" href="../tutorial.html" title="Tutorial">tutorial index</a>
</p>
<p>
Previous: <a class="link" href="tuttimer2.html" title="Timer.2 - Using a timer asynchronously">Timer.2 - Using a timer
asynchronously</a>
</p>
<p>
Next: <a class="link" href="tuttimer4.html" title="Timer.4 - Using a member function as a handler">Timer.4 - Using a member function
as a handler</a>
</p>
</div>
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
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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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