Thread: Virtual & Pure virtual destructors

I don't know why you said the code crashes (do you mean it won't compile or you get a runtime explosion?), it compiled/ran okay for me.

A pure virtual function (it can be any function, doesn't have to be the destructor) is a function that declares that you can not instantiate an object of that type. In other words, you declare a pure virtual function in a class that is strictly intended to be inherited and not used itself. The way you make a function pure virtual is to make it virtual and after the declaration add "= 0" In addition, if you inherit from such a base class, you are required to override the pure virtual function. BTW, this is what's referred to as an 'abstract base class'
For example:

Code:

class BaseShape {
public:
  virtual int GetArea() = 0;//this function is pure virtual
};

class Square : public BaseShape {
public:
  virtual int GetArea() { return 100; }//function must be defined
};

class Triangle : public BaseShape {
public:
  virtual int GetArea() { return 100; }//function must be defined
};

int main() {
  //BaseShape bs;//this line won't compile. can't instantiate it!
  Square *sq = new Square;
  delete sq;

  return 0;
}

The reason you should make destructors virtual when they are being inherited from is that if you don't, destructors might not be called (depending on usage).
For example:

Code:

class A {
public:
  virtual ~A() { cout << "A\n"; }
};

class B {
public:
  ~B() { cout << "B\n"; }
};

int main() {
  A *pA = new B;
  delete pA;
  
  return 0;

If class A's destructor wasn't virtual, ~B() would not be called upon destruction.

HTH