Question

Is there a way to force derived types to define a static constexpr in C++? I have a base class, and I want to force every derived class to define a static const bool has_property.

I tried doing this with CRTP (so that each derived class gets its own static const):

template <typename T, class MyClass>
struct Base {
    T data;
    static constexpr bool has_property;
};

template <typename T>
struct Derived : public Base<T, Derived<T> > {
    static constexpr bool has_property = false;
};

But the compiler complains that Base::has_property is not initialized.

How can I accomplish this?

Answer 1

We could insert a static_assert into the constructor of Base:

template <typename T, class MyClass>
struct Base {
    T data;

    Base() {
        static_assert(std::is_same<
                        typename std::decay<
                          decltype(MyClass::has_property)
                        >::type, bool
                      >::value, 
                      "Must be a bool");
        static_assert(MyClass::has_property || true, 
                      "Must be constexpr");
    }
};

This check will only work when the derived class is instantiated, and the default constructor of Base is used. The constexpr check doesn’t work in g++ 4.7 at the moment.

---

Alternatively, you could use a type traits instead of a constexpr static member, e.g.

template <typename T>
struct has_property;

template <typename T>
struct has_property<Derived<T>> : std::true_type {};

// use as: has_property<X>::value.