How can I call a derived const function defined in base class while I have a same named one in sub class?

user_14570152 0
Viewed 78

The code looks like this.

Why p1->display() causes a build error?

How can I call display() const by a Derived ptr?

#include <iostream>
using namespace std;

class Base
{
    public:
        virtual void display() const
        {
            std::cout << "Base display const!" << std::endl;
        }
        virtual ~Base(){}
};

class Derive: public Base
{
    public:
        virtual void display()
        {
            std::cout << "Derive display!"<< std::endl;
        }
        virtual ~Derive(){}
};

int main()
{
    const Base *pb1 = new Derive();
    pb1->display(); // Base display const!
    Base *pb2 = new Derive();
    pb2->display(); // Base display const!
    const Derive *p1 = new Derive();
    //p1->display(); // Build Error
    Derive *p2 = new Derive();
    p2->display();  // Derive display!
    return 0;
}
polymorphism inheritance c++
1 Answers

The non-const display() in Derive is hiding the const display() in Base.

Add this in Derive to bring it in:

using Base::display;

Demo

Note that you now have two virtual overloads that may be overridden separately.

class Last : public Derive {
public:
    void display() override { std::cout << "non-const overridden\n"; }
    void display() const override { std::cout << "const overridden\n"; }
};

int main() {
    Derive* l1 = new Last;
    l1->display();                 // prints non-const overridden
    const Derive* l2 = new Last;   // prints const overridden
    l2->display();
}
user_75822470
Related