Notes

Stops further overriding or inheriting.

class Base {
    virtual void update() final;   // no derived class can override this
};

class Sealed final : public Base { };  // nothing can inherit from Sealed at all

What happens if you try anyway

class Derived : public Base {
    void update() override;   // compile error: overriding final function 'update'
};

class Illegal : public Sealed { };   // compile error: base 'Sealed' is marked 'final'

Both fail at compile time, with the compiler pointing at the exact violation — not a runtime surprise, not a silent no-op.

Why bother

Communicates intent to readers (“this is the end of the line for this behavior”), and gives the compiler a guarantee it can sometimes optimize around — a final virtual call might get devirtualized (turned back into a direct call) since there’s no possible override left to dispatch to.

Related

Virtual functions and polymorphism