c++ - Static constexpr int vs old-fashioned enum: when and why?

Question

This is maybe a basic question, but I cannot see the response by myself right now.

Consider the following code:

template<bool b>
struct T {
    static constexpr int value = (b ? 42 : 0);
};

template<bool b>
struct U {
    enum { value = (b ? 42 : 0) };
};

int main() {
    static_assert(T<true>::value == 42, "!");
    static_assert(T<false>::value == 0, "!");
    static_assert(U<true>::value == 42, "!");
    static_assert(U<false>::value == 0, "!");
}

I'm used to using structs like T, but more than once I've seen structs like U used for the same purpose (mostly traits definition).

As far as I can see, they are both resolved at compile time and they solve almost the same problem, but it seems to me that T is far more readable than U (well, I know, my personal opinion).

My question is pretty simple: is there any technical reason for which one solution is better than the other one?
Even more, is there any case for which one of them is not a viable solution?

Answer 1

Please note, answer below is not applicable for C++ 17 and later.

There will be no noticeable difference for integral constants when used like this.

However, enum is actually better, because it is a true named constant. constexpr integral constant is an object which can be, for example, ODR-used - and that would result in linking errors.

#include <iostream>

struct T {
    static constexpr int i = 42;
    enum : int {x = 42};
};

void check(const int& z) {
    std::cout << "Check: " << z << "
";
}

int main() {
    // check(T::i); // Uncommenting this will lead to link error
    check(T::x);
}

When check(T::i) is uncommented, the program can not be linked:

/tmp/ccZoETx7.o: In function `main': ccc.cpp:(.text+0x45): undefined reference to `T::i' collect2: error: ld returned 1 exit status

However, the true enum always works.