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POObserver pattern using weak_ptr
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  1. POObserver pattern using weak_ptr
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    copied!<p>I'm trying to write a safe <code>Subject</code> class from the <a href="http://en.wikipedia.org/wiki/Observer_pattern" rel="nofollow noreferrer">observer pattern</a>. I want to know if using <code>weak_ptr</code> is the best way to store <code>IObserver</code> instances in such a way that:</p> <ul> <li>It is not possible to use an <code>IObserver</code> instance after it has been free'd.</li> <li>The <code>Subject</code> class does not hold on to <code>IObserver</code> references that should be free'd (<a href="http://en.wikipedia.org/wiki/Lapsed_listener_problem" rel="nofollow noreferrer">lapsed listener problem</a>).</li> <li>The <code>Subject</code> class must be thread safe.</li> </ul> <p>Unfortunately, our coding standards say that we're not allowed to use boost. I guess I was a bad person in a previous life. Fortunately, I am allowed to use C++11 (what is shipped with Visual Studio 2012).</p> <p>Here is a sample <code>Observer</code> class.</p> <pre class="lang-cpp prettyprint-override"><code>// Observer interface that supports notify() method class IObserver { public: virtual void notify() const = 0; virtual ~IObserver() {} }; // Concrete observer implementation that prints a message class Observer : public IObserver { public: Observer( const std::string&amp; message) : m_message( message ){} void notify() const { printf( "%s\r\n", m_message.c_str() ); } private: std::string m_message; }; </code></pre> <p>And here is the <code>Subject</code> class. </p> <pre class="lang-cpp prettyprint-override"><code>// Subject which registers observers and notifies them as needed. class Subject { public: // Use shared_ptr to guarantee the observer is valid right now void registerObserver( const std::shared_ptr&lt;IObserver&gt;&amp; o ) { std::lock_guard&lt;std::mutex&gt; guard( m_observersMutex ); m_observers.push_back( o ); } void unregisterObserver( const std::shared_ptr&lt;IObserver&gt;&amp; o ) { std::lock_guard&lt;std::mutex&gt; guard( m_observersMutex ); // Code to remove the observer from m_observersMutex } // This is a method that is run in its own thread that notifies observers of some event void doNotify() { std::lock_guard&lt;std::mutex&gt; guard( m_observersMutex ); // Notify any valid observers of events. std::for_each( m_observers.cbegin(), m_observers.cend(), []( const std::weak_ptr&lt;IObserver&gt;&amp; o ) { auto observer = o.lock(); if ( observer ) { observer-&gt;notify(); } } ); // Remove any dead observers. These are ones which have expired(). m_observers.erase( std::remove_if( m_observers.begin(), m_observers.end(), []( const std::weak_ptr&lt;IObserver&gt;&amp; o ) { return o.expired(); } ), m_observers.end() ); } private: std::vector&lt;std::weak_ptr&lt;IObserver&gt;&gt; m_observers; std::mutex m_observersMutex; }; </code></pre> <p>Here's some code that exercises <code>Subject</code>:</p> <pre class="lang-cpp prettyprint-override"><code>int main(int argc, wchar_t* argv[]) { Subject subject; auto observerHello = std::make_shared&lt;Observer&gt;( "Hello world" ); subject.registerObserver( observerHello ); { // Create a scope to show unregistration. auto observerBye = std::make_shared&lt;Observer&gt;( "Good bye" ); subject.registerObserver( observerBye ); subject.doNotify(); } printf( "%s\r\n", "Observer good bye is now be destructed" ); subject.doNotify(); return 0; } </code></pre> <p>Is my usage of <code>weak_ptr</code> thread-safe? From here <a href="https://stackoverflow.com/a/2160422/1517648">https://stackoverflow.com/a/2160422/1517648</a> I <em>think</em> it is.</p> <p>Is this a legitimate way of solving the lapsed listener problem?</p>
 

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