How to enforce overriding a virtual function in subclasses

Question

Let's imagine I have the following code:

class Base {
  virtual void myFunc();
};

class Subclass1 : public Base { // OK
  virtual void myFunc() override; // overrides the base function
};

class Subclass2 : public Base { // How to make this fail ?
  // does not declare an override for myFunc
};

Is there any way in C++ to make compilation fail because Subclass2 does not implement its variant of myFunc() ? I know how to make the opposite, that is to make compilation fail because Subclass1 overrides myFunc() (by using final), but I'm not aware of any mechanic to achieve what I want in C++11.

I'm curious about possibilities in later C++ standards as well.

Make the function in `Base` pure virtual: `virtual void myFunc() = 0;` — Igor Tandetnik, Sep 06 '20 at 21:47
There's nothing in C++ that will require a subclass to always override a method in the superclass. The only requirement is that a pure virtual function must be overridden either by the class being constructed, or one of the intermediate subclasses. That's the only way that C++ works. — Sam Varshavchik, Sep 06 '20 at 21:48
Does this answer your question? [how to implement Interfaces in C++?](https://stackoverflow.com/questions/9756893/how-to-implement-interfaces-in-c) — Den-Jason, Sep 06 '20 at 22:14

score 4 · Answer 1 · edited Sep 06 '20 at 23:42

You can force one subclass level to override a method, by making it 'pure' (virtual method()=0;), but you cannot force multiple levels to each override it again - and that should really not be your concern.

In other words, if you provide a base (or derived) class to an enduser, you can force him to override (define) a method in his derived class. If that user wants to derive from his derived class again, it’s his business to decide if the next (further) derived class needs a different override or not.

How to enforce overriding a virtual function in subclasses

1 Answers1