I'm trying to implement CRTP with C 20 concept. The key is to prevent the parent class template from being instantiated by any type argument except its subclass.

Compiler complains about the self-referencing CRTP_Root in the requires-clause:

#include <concepts>

//error: ‘CRTP_Root’ was not declared in this scope
//     | requires std::derived_from<derived_type, CRTP_Root<derived_type>>

template<typename derived_type>
requires std::derived_from<derived_type, CRTP_Root<derived_type>>
class CRTP_Root {

};

class CRTP_child : public CRTP_Root<CRTP_child> {

};

Since requires-clause must be part of the declaration, there seems no way to write a forward declaration of such self-referring constrained class template. Is there any workaround?

CodePudding user response：

If you just want to prevent the parent class template from being instantiated by any type argument except its subclass. one possible solution might be making the CRTP_Root's ctor private, and use friend to decorate derived_type

template<typename derived_type>
class CRTP_Root {
public:
  // polymorphic interfaces
private:
  CRTP_Root() {}
  friend derived_type;
};

class CRTP_child : public CRTP_Root<CRTP_child> {

};

CodePudding user response：

You could use CRTP to refer to yourself in the constraints.

template<typename derived_type, template <typename> typename tml_base>
requires std::derived_from<derived_type, tml_base<derived_type>>
struct CRTP_Helper {};

template<typename derived_type>
class CRTP_Root : CRTP_Helper<derived_type, CRTP_Root> {};

The problem here is you can't check it because the derived class is incomplete at that time.

See the error message in Demo.