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167 lines
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<title>Timer.3 - Binding arguments to a handler</title>
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<div class="titlepage"><div><div><h3 class="title">
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<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
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to a handler</a>
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</h3></div></div></div>
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<p>
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In this tutorial we will modify the program from tutorial Timer.2 so that
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the timer fires once a second. This will show how to pass additional parameters
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to your handler function.
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</p>
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<pre class="programlisting">#include <iostream>
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#include <boost/asio.hpp>
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#include <boost/bind/bind.hpp>
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</pre>
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<p>
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To implement a repeating timer using asio you need to change the timer's
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expiry time in your callback function, and to then start a new asynchronous
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wait. Obviously this means that the callback function will need to be able
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to access the timer object. To this end we add two new parameters to the
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<code class="computeroutput">print</code> function:
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</p>
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<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
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<li class="listitem">
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A pointer to a timer object.
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</li>
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<li class="listitem">
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A counter so that we can stop the program when the timer fires for the
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sixth time.
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</li>
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</ul></div>
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<pre class="programlisting">void print(const boost::system::error_code& /*e*/,
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boost::asio::steady_timer* t, int* count)
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{
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</pre>
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<p>
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As mentioned above, this tutorial program uses a counter to stop running
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when the timer fires for the sixth time. However you will observe that there
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is no explicit call to ask the io_context to stop. Recall that in tutorial
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Timer.2 we learnt that the <a class="link" href="../reference/io_context/run.html" title="io_context::run">io_context::run()</a>
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function completes when there is no more "work" to do. By not starting
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a new asynchronous wait on the timer when <code class="computeroutput">count</code> reaches 5, the
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io_context will run out of work and stop running.
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</p>
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<pre class="programlisting"> if (*count < 5)
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{
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std::cout << *count << std::endl;
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++(*count);
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</pre>
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<p>
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Next we move the expiry time for the timer along by one second from the previous
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expiry time. By calculating the new expiry time relative to the old, we can
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ensure that the timer does not drift away from the whole-second mark due
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to any delays in processing the handler.
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</p>
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<pre class="programlisting"> t->expires_at(t->expiry() + boost::asio::chrono::seconds(1));
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</pre>
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<p>
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Then we start a new asynchronous wait on the timer. As you can see, the boost::bind()
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function is used to associate the extra parameters with your callback handler.
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The <a class="link" href="../reference/basic_waitable_timer/async_wait.html" title="basic_waitable_timer::async_wait">steady_timer::async_wait()</a>
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function expects a handler function (or function object) with the signature
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<code class="computeroutput">void(const boost::system::error_code&)</code>. Binding the additional
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parameters converts your <code class="computeroutput">print</code> function into a function object
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that matches the signature correctly.
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</p>
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<p>
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See the <a href="http://www.boost.org/libs/bind/bind.html" target="_top">Boost.Bind
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documentation</a> for more information on how to use boost::bind().
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</p>
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<p>
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In this example, the boost::asio::placeholders::error argument to boost::bind()
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is a named placeholder for the error object passed to the handler. When initiating
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the asynchronous operation, and if using boost::bind(), you must specify
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only the arguments that match the handler's parameter list. In tutorial Timer.4
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you will see that this placeholder may be elided if the parameter is not
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needed by the callback handler.
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</p>
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<pre class="programlisting"> t->async_wait(boost::bind(print,
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boost::asio::placeholders::error, t, count));
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}
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}
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int main()
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{
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boost::asio::io_context io;
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</pre>
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<p>
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A new <code class="computeroutput">count</code> variable is added so that we can stop the program
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when the timer fires for the sixth time.
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</p>
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<pre class="programlisting"> int count = 0;
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boost::asio::steady_timer t(io, boost::asio::chrono::seconds(1));
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</pre>
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<p>
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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>
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from <code class="computeroutput">main</code> we bind the additional parameters needed for the
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<code class="computeroutput">print</code> function.
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</p>
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<pre class="programlisting"> t.async_wait(boost::bind(print,
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boost::asio::placeholders::error, &t, &count));
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io.run();
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</pre>
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<p>
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Finally, just to prove that the <code class="computeroutput">count</code> variable was being used
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in the <code class="computeroutput">print</code> handler function, we will print out its new value.
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</p>
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<pre class="programlisting"> std::cout << "Final count is " << count << std::endl;
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return 0;
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}
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</pre>
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<p>
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See the <a class="link" href="tuttimer3/src.html" title="Source listing for Timer.3">full source listing</a>
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</p>
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<p>
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Return to the <a class="link" href="../tutorial.html" title="Tutorial">tutorial index</a>
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</p>
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<p>
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Previous: <a class="link" href="tuttimer2.html" title="Timer.2 - Using a timer asynchronously">Timer.2 - Using a
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timer asynchronously</a>
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</p>
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<p>
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Next: <a class="link" href="tuttimer4.html" title="Timer.4 - Using a member function as a handler">Timer.4 - Using a member
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function as a handler</a>
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</p>
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<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
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<td align="left"></td>
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<td align="right"><div class="copyright-footer">Copyright © 2003-2021 Christopher
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M. Kohlhoff<p>
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Distributed under the Boost Software License, Version 1.0. (See accompanying
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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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