Virtual destructors are needed when class hierarchies are used and when polymorphism is used in the program.
The problem is how to select the correct destructor when an object referenced by a pointer not of its own class is deleted. <cpp> // FIRST SCENARIO class Base { public:
~Base() {}
virtual void f() {}
};
class Derived : public Base { public:
~Derived() {}
void f() {}
};
void main() {
Base *b = new Derived(); b->f(); // ok: calls Derived::f() delete b; // problems: calls Base::~Base()
} </cpp>
Defining the destructor virtual in Base solves the problem and allows the correct destructor to be selected: <cpp> // SECOND SCENARIO class Base { public:
virtual ~Base() {} // virtual destructor
virtual void f() {}
};
class Derived : public Base { public:
~Derived() {}
void f() {}
};
void main() {
Base *b = new Derived(); b->f(); // ok: calls Derived::f() delete b; // ok: calls Derived::~Derived()
} </cpp>
"Virtual" does not mean a destructor is any less real: it means only that the correct one to call will be determined at run time (i.e., depending on the object being destroyed).