[C++]T::value か規定値を返すメタ関数

value_getter.cpp

#include <iostream>
#include <type_traits>

using namespace std;

template <typename T, typename Ignore = void>
struct value_getter {
    static const int value = 0;
};

template <typename T>
struct value_getter<T, typename enable_if<(T::value != 0)>::type> {
    static const decltype(T::value) value = T::value;
};

struct A { static const int value = 10; };
struct B { enum { value = 1 }; };
struct C {};

int main() {
    cout << value_getter<A>::value << endl; // 10
    cout << value_getter<B>::value << endl; // 1
    cout << value_getter<C>::value << endl; // 0
}