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<title>Timer.4 - Using a member function as a handler</title>
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<div class="titlepage"><div><div><h3 class="title">
<a name="asio.tutorial.tuttimer4"></a><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>
</h3></div></div></div>
<p>
In this tutorial we will see how to use a class member function as a callback
handler. The program should execute identically to the tutorial program from
tutorial Timer.3.
</p>
<pre class="programlisting">#include <iostream>
#include <asio.hpp>
#include <boost/bind/bind.hpp>
</pre>
<p>
Instead of defining a free function <code class="computeroutput">print</code> as the callback handler,
as we did in the earlier tutorial programs, we now define a class called
<code class="computeroutput">printer</code>.
</p>
<pre class="programlisting">class printer
{
public:
</pre>
<p>
The constructor of this class will take a reference to the io_context object
and use it when initialising the <code class="computeroutput">timer_</code> member. The counter
used to shut down the program is now also a member of the class.
</p>
<pre class="programlisting"> printer(asio::io_context& io)
: timer_(io, asio::chrono::seconds(1)),
count_(0)
{
</pre>
<p>
The <a class="link" href="boost_bind.html" title="boost::bind">boost::bind</a> function
works just as well with class member functions as with free functions. Since
all non-static class member functions have an implicit <code class="computeroutput">this</code>
parameter, we need to bind <code class="computeroutput">this</code> to the function. As in tutorial
Timer.3, <a class="link" href="boost_bind.html" title="boost::bind">boost::bind</a> converts
our callback handler (now a member function) into a function object that
can be invoked as though it has the signature <code class="computeroutput">void(const asio::error_code&)</code>.
</p>
<p>
You will note that the asio::placeholders::error placeholder is not specified
here, as the <code class="computeroutput">print</code> member function does not accept an error
object as a parameter.
</p>
<pre class="programlisting"> timer_.async_wait(boost::bind(&printer::print, this));
}
</pre>
<p>
In the class destructor we will print out the final value of the counter.
</p>
<pre class="programlisting"> ~printer()
{
std::cout << "Final count is " << count_ << std::endl;
}
</pre>
<p>
The <code class="computeroutput">print</code> member function is very similar to the <code class="computeroutput">print</code>
function from tutorial Timer.3, except that it now operates on the class
data members instead of having the timer and counter passed in as parameters.
</p>
<pre class="programlisting"> void print()
{
if (count_ < 5)
{
std::cout << count_ << std::endl;
++count_;
timer_.expires_at(timer_.expiry() + asio::chrono::seconds(1));
timer_.async_wait(boost::bind(&printer::print, this));
}
}
private:
asio::steady_timer timer_;
int count_;
};
</pre>
<p>
The <code class="computeroutput">main</code> function is much simpler than before, as it now declares
a local <code class="computeroutput">printer</code> object before running the io_context as normal.
</p>
<pre class="programlisting">int main()
{
asio::io_context io;
printer p(io);
io.run();
return 0;
}
</pre>
<p>
See the <a class="link" href="tuttimer4/src.html" title="Source listing for Timer.4">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="tuttimer3.html" title="Timer.3 - Binding arguments to a handler">Timer.3 - Binding arguments
to a handler</a>
</p>
<p>
Next: <a class="link" href="tuttimer5.html" title="Timer.5 - Synchronising handlers in multithreaded programs">Timer.5 - Synchronising handlers
in multithreaded programs</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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