Virtual functions pick the real type

shape is a Shape *, but it points at a Square on the heap. Because area is virtual, the call runs Square::area. Watch which frame appears on the stack.

delete shape; also runs Square's destructor, because the base destructor is virtual.

#include <iostream>

struct Shape {
    virtual ~Shape() = default;
    virtual int area() const { return 0; }
};

struct Square : Shape {
    int side;
    explicit Square(int s) : side(s) {}
    int area() const override { return side * side; }
};

int main() {
    Shape *shape = new Square(5);
    int a = shape->area();
    std::cout << a << "\n";
    delete shape;
    return 0;
}

Output:

25

From the lesson: Inheritance and polymorphism