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| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="boost_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"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">iostream</span><span class="special">></span> |
| <span class="preprocessor">#include</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">hpp</span><span class="special">></span> |
| <span class="preprocessor">#include</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">hpp</span><span class="special">></span> |
| <span class="preprocessor">#include</span> <span class="special"><</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">></span> |
| </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"><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. |
| </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"><span class="keyword">void</span> <span class="identifier">print</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">&</span> <span class="comment">/*e*/</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">deadline_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_service to stop. Recall that in tutorial |
| Timer.2 we learnt that the <a class="link" href="../reference/io_service/run.html" title="io_service::run">io_service::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_service 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"><</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"><<</span> <span class="special">*</span><span class="identifier">count</span> <span class="special"><<</span> <span class="string">"\n"</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">-></span><span class="identifier">expires_at</span><span class="special">(</span><span class="identifier">t</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> |
| </pre> |
| <p> |
| Then we start a new asynchronous wait on the timer. As you can see, the boost::bind() |
| function is used to associate the extra parameters with your callback handler. |
| The <a class="link" href="../reference/basic_deadline_timer/async_wait.html" title="basic_deadline_timer::async_wait">deadline_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">boost</span><span class="special">::</span><span class="identifier">system</span><span class="special">::</span><span class="identifier">error_code</span><span class="special">&)</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 boost::bind(). |
| </p> |
| <p> |
| In this example, the boost::asio::placeholders::error argument to boost::bind() |
| is a named placeholder for the error object passed to the handler. When initiating |
| the asynchronous operation, and if using boost::bind(), 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"> <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">boost</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">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> |
| </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">boost</span><span class="special">::</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">deadline_timer</span> <span class="identifier">t</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> |
| </pre> |
| <p> |
| As in Step 4, when making the call to <a class="link" href="../reference/basic_deadline_timer/async_wait.html" title="basic_deadline_timer::async_wait">deadline_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">boost</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">&</span><span class="identifier">t</span><span class="special">,</span> <span class="special">&</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"><<</span> <span class="string">"Final count is "</span> <span class="special"><<</span> <span class="identifier">count</span> <span class="special"><<</span> <span class="string">"\n"</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 handler">Timer.4 - Using a member |
| function as a handler</a> |
| </p> |
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| <td align="right"><div class="copyright-footer">Copyright © 2003 - 2010 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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