When C + + destructors need to be declared as virtual functions

And look at the example below

Class Deriver:public Base
{

...;

};

Base *PB = new Deriver;
Delete PB; When the child class contains a resource that needs to be freed,
If the virtual ~base () is not declared, a memory leak can occur.

In code design, if a derived class has its own space for application in heap, and it uses a derived class to convert to a base class, be sure to declare a virtual destructor. If you do not meet both of these conditions, do not declare a virtual destructor (declaring a virtual destructor and declaring a virtual member function will occupy a virtual pointer).

In the process of code design, it is best to explicitly declare both the base class and the derived class destructor as virtual functions.

Make sure that when you use a base class pointer that points to a derived class object to release the memory space of a derived class object,
The destructor of a derived class can be called normally.

It's called polymorphism.

If you point to a derived class object with a pointer to a base class. When you delete this object, only the destructor of the base class is executed and the derived class does not perform the destructor. The destructor of a derived class is performed only when the destructor of the base class is set to a virtual function.
If you need to delete objects from the base class, the destructor of the object needs to be virtual