We have two classes
class A{
public :
A(){ cout<<”Base constructor of class A “;
~A(){ cout<<”Base destructor of class A “;};
Class B : public A{
public :
B(){ cout<<”Derived constructor of class B” ;}
~B(){ cout<<”Derived desstructor of class B“};
};
Now in main , we have different way to create object;
int main(){
// Case 1 –> Object Creation
//Proper constructor and destructor will be called , nothing need to be done
A objA;
B objB ;
//Case 2—> Pointer with same class dynamic object
A *pA = new A;
B *pB = new B;
….
…..
delete pA ;delete pB; // no issue ,work properly and destructor of A & B class will be called
//Case 3 –> Base class pointer , derived class memory
//in such case , proper destructor will not be called
A *pBase = new B; ///constructor of B & A is called
---------
……..
delete pBase; //Destructor of A is called
return 0;
}
Case 3 Output
Base constructor of class A
Derived constructor of class B
Derived Destructor of class B
---------------------------------------------------------------
Here Base class destructor is not called.
The only problem case is Base Class pointer with derived class memory.
while we assign the memory by statement A *pBase = new B; compiler know that it will be B class , so proper constructor is called.
when we called delete pBase , it does not know which class memory it hold in run time. That s why we should have a mechanism so that system can call destructor of class B at run time.
to achieve the same , we need to create the destructor as virtual .Once we declare destructor as virtual system automatically delete proper memory & call the destructor of class B.
to resolve the issue of dynamic memory allocation of derived class in Base class pointer & avoid Memory leak , we need to have virtual destructor.
so we need to define the class like this
class A{
public :
A(){ cout<<”Base constructor of class A “;
virtual ~A(){ cout<<”Base destructor of class A “;};
Class B : public A{
public :
B(){ cout<<”Derived constructor of class B” ;}
~B(){ cout<<”Derived desstructor of class B“};
};
int main(){
//Case 3 –> Base class pointer , derived class memory
A *pBase = new B;
---------
……..
delete pBase; //Destructor of A is called
return 0;
}
Case 3 Output
Base constructor of class A
Derived constructor of class B
Derived Destructor of class B
Base destructor of class A
