There are 2 problems with your code described below.

Problem 1

You have a method prod with the same name as the data member prod in class B.

To solve this change, you can either change the name of the method or the data member so that they're not the same as shown below.

Problem 2

The code has undefined behavior because you're using the value of the uninitialized data member b.

To solve this make sure that b is initialized(or assigned) before using its value as shown below. Similarly, prod is uninitialized.

Additionally, you were creating a separate object obj and calling the member functions on that object. That obj is different from the subobject A inside B object obj2. This has been fixed and highlighted using comments in the below program so that you can see what changes have been made.

class A { private: long int a; public: long int b,x; void set_a(){ std::cout<<"Enter variable A's value (integer) \nAnswer: "; std::cin>>a; x=a; } void display_a(){ std::cout<<"\n value for Variable A: "<<x<<std::endl; } void getdata(){ std::cout<<"Enter variable B's value (integer) \nAnswer: "; std::cin>>b; } }; class B:public A { private: long int prod; public: //-------vvvvvvv------------->name changed to product so that it is different from prod data member void product(){ //call these methods here inside product instead of calling them outside on a separater `A` object set_a(); getdata(); display_a(); prod = x*b; } void display(){ std::cout<<"Displaying product of variables A and B \nProduct: "<<prod<<std::endl; } }; int main() { //no need to create a separate `A` object B obj2; obj2.product(); obj2.display(); } 

Demo

In the above program the call to member functions set_a, getdata and dispaly_a is made from inside the member function product instead of calling them on a separate A object.