# Extra Clang Tools 21.1.8 documentation

## clang-tidy - bugprone-crtp-constructor-accessibility

# bugprone-crtp-constructor-accessibility

Detects error-prone Curiously Recurring Template Pattern usage, when the CRTP can be constructed outside itself and the derived class.

The CRTP is an idiom, in which a class derives from a template class, where itself is the template argument. It should be ensured that if a class is intended to be a base class in this idiom, it can only be instantiated if the derived class is its template argument.

Example:

    template <typename T> class CRTP {
    private:
      CRTP() = default;
      friend T;
    };

    class Derived : CRTP<Derived> {};

Below can be seen some common mistakes that will allow the breaking of the idiom.

If the constructor of a class intended to be used in a CRTP is public, then it allows users to construct that class on its own.

Example:

    template <typename T> class CRTP {
    public:
      CRTP() = default;
    };

    class Good : CRTP<Good> {};
    Good GoodInstance;

    CRTP<int> BadInstance;

If the constructor is protected, the possibility of an accidental instantiation is prevented, however it can fade an error, when a different class is used as the template parameter instead of the derived one.

Example:

    template <typename T> class CRTP {
    protected:
      CRTP() = default;
    };

    class Good : CRTP<Good> {};
    Good GoodInstance;

    class Bad : CRTP<Good> {};
    Bad BadInstance;

To ensure that no accidental instantiation happens, the best practice is to make the constructor private and declare the derived class as friend. Note that as a tradeoff, this also gives the derived class access to every other private members of the CRTP. However, constructors can still be public or protected if they are deleted.

Example:

    template <typename T> class CRTP {
      CRTP() = default;
      friend T;
    };

    class Good : CRTP<Good> {};
    Good GoodInstance;

    class Bad : CRTP<Good> {};
    Bad CompileTimeError;

    CRTP<int> AlsoCompileTimeError;

Limitations:

- The check is not supported below C++11

- The check does not handle when the derived class is passed as a variadic template argument

- Accessible functions that can construct the CRTP, like factory functions are not checked

The check also suggests a fix-its in some cases.