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POshared_ptr magic :)
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copied!<p><a href="https://stackoverflow.com/questions/3899688/default-virtual-dtor/3899726#3899726">Mr. Lidström and I had an argument</a> :)</p> <p>Mr. Lidström's claim is that a construct <code>shared_ptr<Base> p(new Derived);</code> doesn't require Base to have a virtual destructor:</p> <blockquote> <p><strong>Armen Tsirunyan</strong>: "Really? Will the <em>shared_ptr</em> clean up correctly? Could you please in this case demonstrate how that effect could be implemented?"</p> <p><strong>Daniel Lidström</strong>: "The <em>shared_ptr</em> uses its own destructor to delete the Concrete instance. This is known as RAII within the C++ community. My advice is that you learn all you can about RAII. It will make your C++ coding so much easier when you use RAII in all situations."</p> <p><strong>Armen Tsirunyan</strong>: "I know about RAII, and I also know that eventually the <em>shared_ptr</em> destructor may delete the stored px when pn reaches 0. But if px had static type pointer to <code>Base</code> and dynamic type pointer to <code>Derived</code>, then unless <code>Base</code> has a virtual destructor, this will result in undefined behavior. Correct me if I am wrong."</p> <p><strong>Daniel Lidström</strong>: "The <em>shared_ptr</em> knows the static type is Concrete. It knows this since I passed it in its constructor! Seems a bit like magic, but I can assure you it is by design and extremely nice."</p> </blockquote> <p>So, judge us. How is it possible (if it is) to implement <em>shared_ptr</em> without requiring polymorphic classes to have virtual destructor? Thanks in advance</p>

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