Inheritance storing address of derived

Question

I am just trying to understand this piece of code below but could not understand these 2 lines at the end.

ptr = &D;
ptr->show(); 

I am actually on my way to study polymorphism concept. And this is just basics of that. So please explain me how it is working and why ptr->show() is calling function in base.

#include <iostream>
using namespace std;

class Base {
    public:
        void show() 
        {
            cout << "This is the base\n";
        }
};

class Derived : public Base {
    public:
        void show() 
        {
                cout << "This is the derived\n";
        }
};


int main() 
{
   Base B;
   B.show(); 
   Derived D;
   D.show();

   Base *ptr = &B;
   ptr->show(); 

   ptr = &D;
  ptr->show(); 

   return 0;
}

Output

This is the base

This is the derived

This is the base

This is the base

Answer 1

Base is a simple class.

Derived is a class that inherit from Base. (We will say that Derived is the daughter of Base, and Base is the mother of Derived)

Let's see the effects of inheritences :

• All methods in the mother is available in the daughter
• If a method is defined in both class (Mother & daughter) the first one will be used. Exept if the keyword virtual is applied to the method.
• You can manipulate a daughter class with a Mother class pointer

B is an instance from Base

Base B;

Call method show of class Base

B.show();

D is an instance from Derived

Derived D;

Call method show of class Derived

   D.show();

Create a pointer to Base class intialized to the address of B object (Base class)

  Base *ptr = &B;

Call method show of Class Base, cause ptr is a Base pointer

   ptr->show(); 

Initialise ptr pointer with the address of D object (Derived class)

   ptr = &D;

Call method show of Class Base, cause ptr is a Base pointer

  ptr->show(); 

Why is it working like that? Why does he did not call Derived show method?

Imagine compilator algorithm...

You told to him : "Please call the methods show! For class pointed by ptr"

So it look after the type of value pointed by ptr (Class Base) and look after the method show. Does the function show have the keyword virtual?

• No? So i call it now!

• Yes? So i am going throught all daughter and take the last definition of show and then call it!

Do you get it?

If you have any question you are wwelcome buddy!

Answer 2

Member functions are called accordingly to the static type of the pointer. If you would define function show as virtual in this case it were called when ptr assigned by address of the derived class due to indirect call using vptr.

As you declared ptr as Base *ptr that is its static type is Base then the compiler call member function show defined in class Base.