Question

Consider the following simplified template meta-programming code that implements an Angle class that is internally storing the modulo 360 degrees reduced value.

#include <iostream>
#include <typeinfo>

template<int N, int D>
struct Modulus
{
        static auto const value = N % D;
};

template<int N>
struct Angle
{
        static auto const value = Modulus<N, 360>::value; // ERROR
        //static int const value = Modulus<N, 360>::value;  // OK
        //static auto const value = N % 360;                // OK

        typedef Angle<value> type;
};

int main()
{
        std::cout << typeid(Angle<30>::type).name() << "\n";
        std::cout << typeid(Angle<390>::type).name() << "\n";

        return 0;
}

Output on Ideone

With Visual C++ 2010 Express, I can do static auto const = Modulus<N, 360>::value, but with MinGW gcc 4.7.2 (Nuwen distro) or Ideone (gcc 4.5.1) I have to either explicitly denote the type as static int const value = Modulus<N, 360>::value or I have to use auto with the full modular expression as static auto const value = N % 360;.

Question: Which compiler is correct acccording to the new C++11 Standard?

Answer 1

The code is valid. Visual C++ is right to accept it and gcc is wrong to reject it (for completeness, Clang 3.1 also accepts the code). The specification states that (C++11 7.1.6.4[dcl.spec.auto]/4):

The auto type-specifier can also be used…in declaring a static data member with a brace-or-equal-initializer that appears within the member-specification of a class definition.

Your value is a static data member. It has a brace-or-equal-initializer (that is the = Modulus<N, 360>::value part of the declaration), and the initializer appears within the member-specification of the class definition (i.e., it’s what mortals might call an “inline initializer”).