Ensure derived class implements static method

Question

I want to ensure, that a derived class implements a specific static method. I think doing so should be possible using static_assert, std::is_same, decltype, CRTP and maybe m

Answer 1

This is a common problem when using CRTP: Base<Derived> is instantiated at the point where it is encountered in Derived's list of bases, at which time Derived is not yet a complete type since the rest of its declaration hasn't been parsed yet. There are various workarounds. For static_assert, you need to delay instantiation of the assertion until Derived is complete. One way to do so is to put the assertion in a member function of Base that you know must be instantiated - the destructor is always a good choice (Live at Coliru):

template <class T>
class Base 
{
public:
    ~Base() {
        static_assert(std::is_same<decltype(T::foo(1)), int>::value, "ERROR STRING");
    }
};

class Derived : public Base<Derived>
{
public:
    static int foo(int) { return 42; };
};

Addressing question #2: C::* is the syntax for "pointer to member of class C." So int(*)(int) is "pointer to function taking a single int parameter and returning int", and int(C::*)(int) is analogously "pointer to member function of C taking a single int parameter and returning int." The monstrosity

typename C::const_iterator(C::*)() const

would translate to "pointer to constant member function of C taking no parameters and returning C::const_iterator" where of course the typename is necessary to indicate that the dependent name C::const_iterator is a type.