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Looking at qwidget.h, I found the destructor as below:
~QWidget();
I was wondering why this is not declared as virtual
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Yes, it is possible to have a pure virtual destructor. Pure virtual destructors are legal in standard C++ and one of the most important things to remember is that if a class contains a pure virtual destructor, it must provide a function body for the pure virtual destructor.
A virtual destructor is used to free up the memory space allocated by the derived class object or instance while deleting instances of the derived class using a base class pointer object.
The implicitly-declared destructor is virtual (because the base class has a virtual destructor) and the lookup for the deallocation function (operator delete()) results in a call to ambiguous, deleted, or inaccessible function.
Virtual keyword for destructor is necessary when you want different destructors should follow proper order while objects is being deleted through base class pointer.
The destructor is virtual, because QWidget derives from QObject which does have a virtual destructor. Why it's not declared virtual in the code is a either style issue or a harmless mistake. I would have declared it virtual myself.
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