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copied!<p>Virtual destructors are useful when you can delete an instance of a derived class through a pointer to base class:</p> <pre><code>class Base { // some virtual methods }; class Derived : public Base { ~Derived() { // Do some important cleanup } }; </code></pre> <p>Here, you'll notice that I didn't declare Base's destructor to be <code>virtual</code>. Now, let's have a look at the following snippet:</p> <pre><code>Base *b = new Derived(); // use b delete b; // Here's the problem! </code></pre> <p>Since Base's destructor is not <code>virtual</code> and <code>b</code> is a <code>Base*</code> pointing to a <code>Derived</code> object, <code>delete b</code> has <a href="https://stackoverflow.com/q/2397984/20984">undefined behaviour</a>:</p> <blockquote> <p>[In <code>delete b</code>], if the static type of the object to be deleted is different from its dynamic type, the static type shall be a base class of the dynamic type of the object to be deleted and <strong>the static type shall have a virtual destructor or the behavior is undefined</strong>.</p> </blockquote> <p>In most implementations, the call to the destructor will be resolved like any non-virtual code, meaning that the destructor of the base class will be called but not the one of the derived class, resulting in a resources leak.</p> <p>To sum up, always make base classes' destructors <code>virtual</code> when they're meant to be manipulated polymorphically.</p> <p>If you want to prevent the deletion of an instance through a base class pointer, you can make the base class destructor protected and nonvirtual; by doing so, the compiler won't let you call <code>delete</code> on a base class pointer.</p> <p>You can learn more about virtuality and virtual base class destructor in <a href="http://www.gotw.ca/publications/mill18.htm" rel="noreferrer">this article from Herb Sutter</a>.</p>

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