CODEWITHSUNDEEP
c++inheritance
avinash pandey
avinash pandey

Reputation: 1381

Calling function using base class pointer?

Hey I am just trying to figure out the concept of using base class pointer to call methods of derived class, but then I came across this problem.

#include <iostream>

using namespace std;

class A {
    int a;
public:
    A() {}
    A(int x) :a(x) {}
    void show() {
        cout<<"Hi this is base class ::\nvalue of a is "<<a<<endl;
    }
};

class B:public A {
    int b;
public:
    B() {}
    B(int x) : b(x) {}
    void show() {
        cout<<"Hi this is derived class::\n value of b is "<<b<<endl;
    }
};

int main() {
    A a(20), *a_ptr;
    B b(10), *b_ptr;
    a_ptr = &b;
    a.show();
    a_ptr->show();
    return 0;
}

a.show() outputs:

Hi this is base class ::
value of a is 20

as accepted but a_ptr->show() outputs garbage value

Hi this is base class ::
value of a is -1121246592

Can anybody explain why this is happening..

Upvotes: 0

Views: 905

Answers (2)

Steephen
Steephen

Reputation: 15824

In simple you didn't initialize your base class data member when you are calling a derived class constructor so it is showing garbage value; since it will call base class default constructor.

If you change your base class default constructor as follows, you will get expected result.

 A():a(10) {}

Upvotes: 0

Amnon Shochot
Amnon Shochot

Reputation: 9356

This is because in order to have polymorphism you have to define show() as a virtual function, i.e. in class A:

virtual void show();

As this is not the case in your code the compiler will statically bind the call a_ptr->show(); to class A implementation of show which prints the member a which is not initialized by class B constructor.

Upvotes: 3

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