13.9. Virtual Destructors

A problem can occur when using polymorphism to process dynamically allocated objects of a class hierarchy. So far you have seen nonvirtual destructors—destructors that are not declared with keyword virtual. If a derived-class object with a nonvirtual destructor is destroyed explicitly by applying the delete operator to a base-class pointer to the object, the C++ standard specifies that the behavior is undefined.

The simple solution to this problem is to create a virtual destructor (i.e., a destructor that is declared with keyword virtual) in the base class. This makes all derived-class destructors virtual even though they do not have the same name as the base-class destructor. Now, if an object in the hierarchy is destroyed explicitly by applying the delete operator to a base-class pointer, the destructor for the appropriate class is called based on the object to which the base-class pointer points. Remember, when a derived-class object is destroyed, the base-class part of the derived-class object is also destroyed, so it is important for the destructors of both the derived class and base class to execute. The base-class destructor automatically executes after the derived-class destructor.