Why should the destructor of base classes be virtual?

Question

in C++: Why should the destructor of base classes be virtual?

Answer 1

For situations like this:

class A
{
    virtual ~A();
};

class B:A
{
    ~B();
};

A *a = new B(); //legal, since it's a downcast
delete a; //Unless the destructor is virtual, ~A() is called here instead of ~B().