What is the mechanism of method overriding (override)…

The method overriding mechanism in C++ allows a derived class to provide its own implementation of a method that is already declared in its parent class. This is a key element of runtime polymorphism.

For successful overriding, the following conditions must be met:

The method in the base class must be declared as virtual.
The method name in the derived class must match the method name in the base class.
The argument list of the methods must be identical (including constness).
The return type must be either the same or covariant (for pointers or references).
Qualifiers (e.g., const, volatile) must match.
Both functions must belong to the same class hierarchy (base and derived).
The accessibility of the method (public, protected, private) can be changed, but usually remains the same or more accessible.

The keyword override (available since C++11) is not mandatory for overriding, but its use is highly recommended as it forces the compiler to check whether the method in the derived class actually overrides a virtual method in the base class. This helps to identify errors such as typos in the method name or signature mismatch.

Example:

#include <iostream>

class Base {
public:
    virtual void display() { // Virtual method
        std::cout << "Base class display" << std::endl;
    }
};

class Derived : public Base {
public:
    void display() override { // Overriding method using override
        std::cout << "Derived class display" << std::endl;
    }
};

int main() {
    Base* basePtr = new Derived();
    basePtr->display(); // Calls the version from Derived thanks to polymorphism

    delete basePtr;
    return 0;
}

In this example, when display() is called through a pointer to the base class pointing to a derived class object, the overridden version from the Derived class is invoked.

What is the mechanism of method overriding (override) in C++?