Is there a way to allow inheritance only for final classes in C++

Question

I have a base class that I want to be inherited from only by classes marked as final (or classes that are not themselves inherited from).

The underlying idea is that I want to forbid inheritance from this class by other superclasses, at compile time.

For example:

class O { /* Some compile time check here */ };
class A final : O {}; // OK
class B : O {}; // Not OK

I know this could be done in class A by using something like:

if ( std::is_base_of<O,A>::value ) static_assert( std::is_final<A>::value );

But this would need to be written in every single class. I'd like this check to be in class O (but I don't know if that's even possible).

Thank you

Answer 1

You can use CRTP and std::is_final together.

#include <type_traits>

template <typename CRTP>
class Base {
public:
    ~Base() {
        static_assert( std::is_final<CRTP>::value );
    }
};

// This will trip a compile time static_assert when the class is instantiated.
class Derived : Base<Derived> { 
};

class DerivedAndFinal final : Base<DerivedAndFinal> {
};

int main() {
    Derived d;
    (void)d;
    DerivedAndFinal daf;
    (void)daf;
}