Initializing base class static const variable with derived class variable, via CRTP

Question

The following program compiles and runs in Cygwin GCC 4.8.2 but not in MSCV 2008 (or MSVC 2013 either)

#include <iostream>

template<typename T>
struct Base
{
    static const unsigned BAR = T::FOO;

    static void func()
    {
        std::cout << BAR << std::endl;
    }
};

template<typename T>
struct Derived : Base<Derived<T> >
{
    static const unsigned FOO = 42;
};

int main()
{   
    Derived<int>::func();
}

MSVC error

Test.cpp(6) : error C2039: 'FOO' : is not a member of 'Derived<T>'
        with
        [
            T=int
        ]
        Test.cpp(16) : see reference to class template instantiation 'Base<T>' being compiled
        with
        [
            T=Derived<int>
        ]
        Test.cpp(24) : see reference to class template instantiation 'Derived<T>' being compiled
        with
        [
            T=int
        ]
Test.cpp(6) : error C2065: 'FOO' : undeclared identifier
Test.cpp(6) : error C2057: expected constant expression

Is this program compliant or is this an MSCV issue? Also, is there a workaround?

Answer 1

EDIT 2 : it is enough to reject initialization after Derived class declaration.

The only way I could have MSVC and Clang compile your program is :

• reject Base::BAR initialization after Derived class declaration
• use a concrete implementation of Derived to initialize BAR

Edit : I think it is normal because in your code the compiler has no mean when compiling Base to know that T::FOO will exist and will be a constant value.

Current source is :

#include <iostream>

template<typename T>
struct Base
{
    static const unsigned BAR;

    static void func()
    {
        std::cout << BAR << std::endl;
    }
};

template<typename T>
struct Derived : Base<Derived<T> >
{
    static const unsigned FOO = 42;
};

template<typename T>
const unsigned Base<T>::BAR = T::FOO;

int main()
{   
    Derived<int>::func();
}