CODEWITHSUNDEEP
c++inheritancepolymorphism
weberc2
weberc2

Reputation: 7948

c++ make the base class use overridden methods in the child class

I'm trying to override a method of the base class used by another method in the base class; however, when the derived class invokes the using-method of the base class, the derived used-method is never executed, but instead the base class's used-method is called. Here's an example:

#include <iostream>
using namespace std;
class Base {
public:
    Base() {}
    virtual ~Base() {}
    void printLeft() { cout << this->getLeft(); }
    int getLeft() { return 0; }
};

class Derived: public Base {
public:
    Derived() {}
    virtual ~Derived() {}
    int getLeft() { return 1; }
};
int main(int argc, char *argv[]) {
    Derived d = Derived();
    d.printLeft();
}

Running main() prints 0, indicating Base's getLeft() method was used rather than the derived object's method.

How can I change this code so Derived::getLeft() is called when called from an instance of Derived?

Upvotes: 2

Views: 4237

Answers (2)

JohnMcG
JohnMcG

Reputation: 8815

For non-virtual functions, the static type of the object is used to determine which class's method is called.

In your case, you are calling getLeft() from Base::printLeft(). The type of this is Base*, so the function called will be Base::getLeft().

The way around this is to use the virtual keyword. In that case, the virtual function table will be used to determine which version of getLeft() to call, in this case Derived.

You can trigger this behavior by prepending virtual to the declarations of printLeft in both Base and Derived.

Upvotes: 1

Nick
Nick

Reputation: 25828

You just need to make getLeft virtual:

class Base {
public:
    Base() {}
    virtual ~Base() {}
    void printLeft() { cout << this->getLeft(); }
    virtual int getLeft() { return 0; }
};

By default in C++ , member functions are not virtual. That is, you can't override them in a subclass.

Upvotes: 6

Related Questions