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  26 <div class="titlepage"><div><div><h3 class="title">
  27 <a name="boost_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
  28       function as a handler</a>
  29 </h3></div></div></div>
  30 <p>
  31         In this tutorial we will see how to use a class member function as a callback
  32         handler. The program should execute identically to the tutorial program from
  33         tutorial Timer.3.
  34       </p>
  35 <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>
  36 <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">asio</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
  37 <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">hpp</span><span class="special">&gt;</span>
  38 <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">date_time</span><span class="special">/</span><span class="identifier">posix_time</span><span class="special">/</span><span class="identifier">posix_time</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
  39 </pre>
  40 <p>
  41         Instead of defining a free function <code class="computeroutput"><span class="identifier">print</span></code>
  42         as the callback handler, as we did in the earlier tutorial programs, we now
  43         define a class called <code class="computeroutput"><span class="identifier">printer</span></code>.
  44       </p>
  45 <pre class="programlisting"><span class="keyword">class</span> <span class="identifier">printer</span>
  46 <span class="special">{</span>
  47 <span class="keyword">public</span><span class="special">:</span>
  48 </pre>
  49 <p>
  50         The constructor of this class will take a reference to the io_service object
  51         and use it when initialising the <code class="computeroutput"><span class="identifier">timer_</span></code>
  52         member. The counter used to shut down the program is now also a member of
  53         the class.
  54       </p>
  55 <pre class="programlisting">  <span class="identifier">printer</span><span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">io_service</span><span class="special">&amp;</span> <span class="identifier">io</span><span class="special">)</span>
  56     <span class="special">:</span> <span class="identifier">timer_</span><span class="special">(</span><span class="identifier">io</span><span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">posix_time</span><span class="special">::</span><span class="identifier">seconds</span><span class="special">(</span><span class="number">1</span><span class="special">)),</span>
  57       <span class="identifier">count_</span><span class="special">(</span><span class="number">0</span><span class="special">)</span>
  58   <span class="special">{</span>
  59 </pre>
  60 <p>
  61         The boost::bind() function works just as well with class member functions
  62         as with free functions. Since all non-static class member functions have
  63         an implicit <code class="computeroutput"><span class="keyword">this</span></code> parameter,
  64         we need to bind <code class="computeroutput"><span class="keyword">this</span></code> to the
  65         function. As in tutorial Timer.3, boost::bind() converts our callback handler
  66         (now a member function) into a function object that can be invoked as though
  67         it has the signature <code class="computeroutput"><span class="keyword">void</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">system</span><span class="special">::</span><span class="identifier">error_code</span><span class="special">&amp;)</span></code>.
  68       </p>
  69 <p>
  70         You will note that the boost::asio::placeholders::error placeholder is not
  71         specified here, as the <code class="computeroutput"><span class="identifier">print</span></code>
  72         member function does not accept an error object as a parameter.
  73       </p>
  74 <pre class="programlisting">    <span class="identifier">timer_</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">(&amp;</span><span class="identifier">printer</span><span class="special">::</span><span class="identifier">print</span><span class="special">,</span> <span class="keyword">this</span><span class="special">));</span>
  75   <span class="special">}</span>
  76 </pre>
  77 <p>
  78         In the class destructor we will print out the final value of the counter.
  79       </p>
  80 <pre class="programlisting">  <span class="special">~</span><span class="identifier">printer</span><span class="special">()</span>
  81   <span class="special">{</span>
  82     <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="string">"\n"</span><span class="special">;</span>
  83   <span class="special">}</span>
  84 </pre>
  85 <p>
  86         The <code class="computeroutput"><span class="identifier">print</span></code> member function
  87         is very similar to the <code class="computeroutput"><span class="identifier">print</span></code>
  88         function from tutorial Timer.3, except that it now operates on the class
  89         data members instead of having the timer and counter passed in as parameters.
  90       </p>
  91 <pre class="programlisting">  <span class="keyword">void</span> <span class="identifier">print</span><span class="special">()</span>
  92   <span class="special">{</span>
  93     <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>
  94     <span class="special">{</span>
  95       <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">count_</span> <span class="special">&lt;&lt;</span> <span class="string">"\n"</span><span class="special">;</span>
  96       <span class="special">++</span><span class="identifier">count_</span><span class="special">;</span>
  97
  98       <span class="identifier">timer_</span><span class="special">.</span><span class="identifier">expires_at</span><span class="special">(</span><span class="identifier">timer_</span><span class="special">.</span><span class="identifier">expires_at</span><span class="special">()</span> <span class="special">+</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">posix_time</span><span class="special">::</span><span class="identifier">seconds</span><span class="special">(</span><span class="number">1</span><span class="special">));</span>
  99       <span class="identifier">timer_</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">(&amp;</span><span class="identifier">printer</span><span class="special">::</span><span class="identifier">print</span><span class="special">,</span> <span class="keyword">this</span><span class="special">));</span>
 100     <span class="special">}</span>
 101   <span class="special">}</span>
 102
 103 <span class="keyword">private</span><span class="special">:</span>
 104   <span class="identifier">boost</span><span class="special">::</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">deadline_timer</span> <span class="identifier">timer_</span><span class="special">;</span>
 105   <span class="keyword">int</span> <span class="identifier">count_</span><span class="special">;</span>
 106 <span class="special">};</span>
 107 </pre>
 108 <p>
 109         The <code class="computeroutput"><span class="identifier">main</span></code> function is much
 110         simpler than before, as it now declares a local <code class="computeroutput"><span class="identifier">printer</span></code>
 111         object before running the io_service as normal.
 112       </p>
 113 <pre class="programlisting"><span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
 114 <span class="special">{</span>
 115   <span class="identifier">boost</span><span class="special">::</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">io_service</span> <span class="identifier">io</span><span class="special">;</span>
 116   <span class="identifier">printer</span> <span class="identifier">p</span><span class="special">(</span><span class="identifier">io</span><span class="special">);</span>
 117   <span class="identifier">io</span><span class="special">.</span><span class="identifier">run</span><span class="special">();</span>
 118
 119   <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
 120 <span class="special">}</span>
 121 </pre>
 122 <p>
 123         See the <a class="link" href="tuttimer4/src.html" title="Source listing for Timer.4">full source listing</a>
 124       </p>
 125 <p>
 126         Return to the <a class="link" href="../tutorial.html" title="Tutorial">tutorial index</a>
 127       </p>
 128 <p>
 129         Previous: <a class="link" href="tuttimer3.html" title="Timer.3 - Binding arguments to a handler">Timer.3 - Binding
 130         arguments to a handler</a>
 131       </p>
 132 <p>
 133         Next: <a class="link" href="tuttimer5.html" title="Timer.5 - Synchronising handlers in multithreaded programs">Timer.5 - Synchronising
 134         handlers in multithreaded programs</a>
 135       </p>
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 139 <td align="right"><div class="copyright-footer">Copyright &#169; 2003-2014 Christopher M. Kohlhoff<p>
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