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