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Invoke Virtual Function in Constructor or Destructor


Case 1: 

 class Dog {

 public:
Dog() 
{
cout << "Dog Born" << endl;
}

 virtual void bark() 
{
cout << "Dog barking" << endl;
}

 void SeeCat()
{
bark();  // first it check bark() in YellowDog else invokes in Dog class
}
};

 class YellowDog : public Dog 
{
public:
YellowDog()
{
cout << "Yellow Dog Born" << endl;
}

 virtual void bark()  // virtual keyword is optional
       {

cout << "Yellow barking" << endl;
}
};

 int main() {
YellowDog d;
d.SeeCat();
}
Output: 

 Dog Born
Yellow Dog Born

Yellow Dog barking
Case 2: why Virtual function in constructor  should be avoided
class Dog {

 public:
Dog() 
{
cout << "Dog Born" << endl;
bark();  Since construtor of yellowdog is not yet constructed, so bark of Dog class gets called.
}

 virtual void bark() 
{
cout << "Dog barking" << endl;
}

 void SeeCat()
{
bark();
}
};

 class YellowDog : public Dog 
{

 public:
YellowDog()
{
cout << "Yellow Dog Born" << endl;
}

 virtual void bark()
{
cout << "Yellow Dog barking" << endl;
}
};

int main() {
YellowDog d;
d.SeeCat();

}

 Output: 

 Dog Born
Dog barking
Yellow Dog Born

Yellow Dog barking

Case 3: why Virtual function in destructor  should be avoided

 public:
Dog() 
{
cout << "Dog Born" << endl;
}

 virtual void bark() 
{
cout << "Dog barking" << endl;
}

 void SeeCat()
{
bark();
}

 ~Dog()
{
bark(); // It invokes bark of Dog class because Yellow Dog is already been destructed.
}
};

 class YellowDog : public Dog 
{

 public:
YellowDog()
{
cout << "Yellow Dog Born" << endl;
}

 virtual void bark()
{
cout << "Yellow Dog barking" << endl;
}
};


 int main() {
YellowDog d;
d.SeeCat();

}
Output: 

 Dog Born
Yellow Dog B
Yellow Dog b

Dog barking
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