Accessing a base class member in derived class

Question

I have a simple class as below

class A {
protected:
    int x;
};
        
class B : public A {
public:
    int y;

    void sety(int d) {
        y = d;
    }

    int gety() {
        return y;
    }
};
    
int main() {
    B obj;
    obj.sety(10);
    cout << obj.gety();
    getch();
}

How can I set the value of the protected instance variable A::x from an instance of the derived class B without creating an instance of class A.

EDIT: Can we access the value of A::x using the object of B? Like obj.x?

Answer 1

B is an A, so creating an instance of B is creating an instance of A. That being said, I'm not sure what your actual question is, so here's some code that will hopefully clarify things:

class A
{
protected:
    int x;
};

class B : public A
{
public:
    int y;

    int gety() const { return y; }
    void sety(int d) { y = d; }

    int getx() const { return x; }
    void setx(int d) { x = d; }
};

int main()
{
    B obj;

    // compiles cleanly because B::sety/gety are public
    obj.sety(10);
    std::cout << obj.gety() << '\n';

    // compiles cleanly because B::setx/getx are public, even though
    // they touch A::x which is protected
    obj.setx(42);
    std::cout << obj.getx() << '\n';

    // compiles cleanly because B::y is public
    obj.y = 20;
    std::cout << obj.y << '\n';

    // compilation errors because A::x is protected
    obj.x = 84;
    std::cout << obj.x << '\n';
}

obj can access A::x just as an instance of A could, because obj is implicitly an instance of A.