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| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="boost_asio.tutorial.tuttimer2"></a><a class="link" href="tuttimer2.html" title="Timer.2 - Using a timer asynchronously">Timer.2 - Using a timer |
| asynchronously</a> |
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| <p> |
| This tutorial program demonstrates how to use asio's asynchronous callback |
| functionality by modifying the program from tutorial Timer.1 to perform an |
| asynchronous wait on the timer. |
| </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">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> |
| Using asio's asynchronous functionality means having a callback function |
| that will be called when an asynchronous operation completes. In this program |
| we define a function called <code class="computeroutput"><span class="identifier">print</span></code> |
| to be called when the asynchronous wait finishes. |
| </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">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="special">{</span> |
| <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"Hello, world!\n"</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> |
| |
| <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">5</span><span class="special">));</span> |
| </pre> |
| <p> |
| Next, instead of doing a blocking wait as in tutorial Timer.1, we call the |
| <a class="link" href="../reference/basic_deadline_timer/async_wait.html" title="basic_deadline_timer::async_wait">deadline_timer::async_wait()</a> |
| function to perform an asynchronous wait. When calling this function we pass |
| the <code class="computeroutput"><span class="identifier">print</span></code> callback handler |
| that was defined above. |
| </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">print</span><span class="special">);</span> |
| </pre> |
| <p> |
| Finally, we must call the <a class="link" href="../reference/io_service/run.html" title="io_service::run">io_service::run()</a> |
| member function on the io_service object. |
| </p> |
| <p> |
| The asio library provides a guarantee that callback handlers will only be |
| called from threads that are currently calling <a class="link" href="../reference/io_service/run.html" title="io_service::run">io_service::run()</a>. |
| Therefore unless the <a class="link" href="../reference/io_service/run.html" title="io_service::run">io_service::run()</a> |
| function is called the callback for the asynchronous wait completion will |
| never be invoked. |
| </p> |
| <p> |
| The <a class="link" href="../reference/io_service/run.html" title="io_service::run">io_service::run()</a> |
| function will also continue to run while there is still "work" |
| to do. In this example, the work is the asynchronous wait on the timer, so |
| the call will not return until the timer has expired and the callback has |
| completed. |
| </p> |
| <p> |
| It is important to remember to give the io_service some work to do before |
| calling <a class="link" href="../reference/io_service/run.html" title="io_service::run">io_service::run()</a>. |
| For example, if we had omitted the above call to <a class="link" href="../reference/basic_deadline_timer/async_wait.html" title="basic_deadline_timer::async_wait">deadline_timer::async_wait()</a>, |
| the io_service would not have had any work to do, and consequently <a class="link" href="../reference/io_service/run.html" title="io_service::run">io_service::run()</a> would |
| have returned immediately. |
| </p> |
| <pre class="programlisting"> <span class="identifier">io</span><span class="special">.</span><span class="identifier">run</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="tuttimer2/src.html" title="Source listing for Timer.2">full source listing</a> |
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| Return to the <a class="link" href="../tutorial.html" title="Tutorial">tutorial index</a> |
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| Previous: <a class="link" href="tuttimer1.html" title="Timer.1 - Using a timer synchronously">Timer.1 - Using a |
| timer synchronously</a> |
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| Next: <a class="link" href="tuttimer3.html" title="Timer.3 - Binding arguments to a handler">Timer.3 - Binding arguments |
| to a handler</a> |
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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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