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<a name="asio.tutorial.tuttimer3"></a><a class="link" href="tuttimer3.html" title="Timer.3 - Binding arguments to a completion handler">Timer.3 - Binding arguments
to a completion 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"><span class="special">#</span><span class="identifier">include</span> <span class="special">&lt;</span><span class="identifier">iostream</span><span class="special">&gt;</span>
<span class="special">#</span><span class="identifier">include</span> <span class="special">&lt;</span><span class="identifier">asio</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="special">#</span><span class="identifier">include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">bind</span><span class="special">/</span><span class="identifier">bind</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
</pre>
<p>
To implement a repeating timer using asio you need to change the timer's
expiry time in your completion handler, and to then start a new asynchronous
wait. Obviously this means that the completion handler will need to be able
to access the timer object. To this end we add two new parameters to the
<code class="computeroutput"><span class="identifier">print</span></code> function:
</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
a pointer to a timer object; and
</li>
<li class="listitem">
a counter so that we can stop the program when the timer fires for the
sixth time
</li>
</ul></div>
<p>
at the end of the parameter list.
</p>
<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">print</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">asio</span><span class="special">::</span><span class="identifier">error_code</span><span class="special">&amp;</span> <span class="comment">/*e*/</span><span class="special">,</span>
<span class="identifier">asio</span><span class="special">::</span><span class="identifier">steady_timer</span><span class="special">*</span> <span class="identifier">t</span><span class="special">,</span> <span class="keyword">int</span><span class="special">*</span> <span class="identifier">count</span><span class="special">)</span>
<span class="special">{</span>
</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"><span class="identifier">count</span></code>
reaches 5, the io_context will run out of work and stop running.
</p>
<pre class="programlisting"> <span class="keyword">if</span> <span class="special">(*</span><span class="identifier">count</span> <span class="special">&lt;</span> <span class="number">5</span><span class="special">)</span>
<span class="special">{</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="special">*</span><span class="identifier">count</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
<span class="special">++(*</span><span class="identifier">count</span><span class="special">);</span>
</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"> <span class="identifier">t</span><span class="special">-&gt;</span><span class="identifier">expires_at</span><span class="special">(</span><span class="identifier">t</span><span class="special">-&gt;</span><span class="identifier">expiry</span><span class="special">()</span> <span class="special">+</span> <span class="identifier">asio</span><span class="special">::</span><span class="identifier">chrono</span><span class="special">::</span><span class="identifier">seconds</span><span class="special">(</span><span class="number">1</span><span class="special">));</span>
</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 completion 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"><span class="keyword">void</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">asio</span><span class="special">::</span><span class="identifier">error_code</span><span class="special">&amp;)</span></code>. Binding the additional parameters
converts your <code class="computeroutput"><span class="identifier">print</span></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 completion handler.
</p>
<pre class="programlisting"> <span class="identifier">t</span><span class="special">-&gt;</span><span class="identifier">async_wait</span><span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">bind</span><span class="special">(</span><span class="identifier">print</span><span class="special">,</span>
<span class="identifier">asio</span><span class="special">::</span><span class="identifier">placeholders</span><span class="special">::</span><span class="identifier">error</span><span class="special">,</span> <span class="identifier">t</span><span class="special">,</span> <span class="identifier">count</span><span class="special">));</span>
<span class="special">}</span>
<span class="special">}</span>
<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
<span class="special">{</span>
<span class="identifier">asio</span><span class="special">::</span><span class="identifier">io_context</span> <span class="identifier">io</span><span class="special">;</span>
</pre>
<p>
A new <code class="computeroutput"><span class="identifier">count</span></code> variable is added
so that we can stop the program when the timer fires for the sixth time.
</p>
<pre class="programlisting"> <span class="keyword">int</span> <span class="identifier">count</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span>
<span class="identifier">asio</span><span class="special">::</span><span class="identifier">steady_timer</span> <span class="identifier">t</span><span class="special">(</span><span class="identifier">io</span><span class="special">,</span> <span class="identifier">asio</span><span class="special">::</span><span class="identifier">chrono</span><span class="special">::</span><span class="identifier">seconds</span><span class="special">(</span><span class="number">1</span><span class="special">));</span>
</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"><span class="identifier">main</span></code> we bind the additional
parameters needed for the <code class="computeroutput"><span class="identifier">print</span></code>
function.
</p>
<pre class="programlisting"> <span class="identifier">t</span><span class="special">.</span><span class="identifier">async_wait</span><span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">bind</span><span class="special">(</span><span class="identifier">print</span><span class="special">,</span>
<span class="identifier">asio</span><span class="special">::</span><span class="identifier">placeholders</span><span class="special">::</span><span class="identifier">error</span><span class="special">,</span> <span class="special">&amp;</span><span class="identifier">t</span><span class="special">,</span> <span class="special">&amp;</span><span class="identifier">count</span><span class="special">));</span>
<span class="identifier">io</span><span class="special">.</span><span class="identifier">run</span><span class="special">();</span>
</pre>
<p>
Finally, just to prove that the <code class="computeroutput"><span class="identifier">count</span></code>
variable was being used in the <code class="computeroutput"><span class="identifier">print</span></code>
handler function, we will print out its new value.
</p>
<pre class="programlisting"> <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"Final count is "</span> <span class="special">&lt;&lt;</span> <span class="identifier">count</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
<span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
<span class="special">}</span>
</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 completion handler">Timer.4 - Using a member function
as a completion handler</a>
</p>
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<div class="copyright-footer">Copyright © 2003-2023 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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