How to know when function has to be virtual?

Question

While describing a class, how to know when function has to be virtual?

I know what virtual function means, but I just can't figure out when I should make them virtual

Thanks

Answer 1

Functions should be virtual if you want to invoke them polymorphically. In other words, virtual function express behavior that can be customized in subclasses. For instance, consider the following class:

class Car {
public:
    const std::string & getId() const;
    virtual void startEngine() = 0;
private:
    std::string id;
}

Every car has an Id, and there is no customization allowed here, so there is no point in making it virtual. Subclasses should not be able to modify that property. How the engine is started however, depends on the specific car, and can in fact not be defined globally. But we do know that every car has to start the engine before we can drive, so we define it as a pure virtual function.

An important guideline for when to make a function virtual, and when not, is given by the Non-Virtual Interface idiom.