Undefined reference to static constexpr

Question

In this code snippet:

template <size_t N>
struct Foo {
   static constexpr std::array<char, N> arr{{0}};
   static const char *data() { return &arr[0]; }
};

template<>
constexpr std::array<char, 5> Foo<5>::arr;

int main()
{
   std::cout << Foo<5>::data() << std::endl;
}

with gcc 5.2 I got undefined reference to Foo<5ul>::arr, while clang 3.7 gives a compile time error:

declaration of constexpr static data member 'arr' requires an initializer

What is wrong, and how should static constexpr be defined outside class declaration?

Answer 1

The out-of-line definiton is the same as for other static (non integral) members, minus the initialization:

template<size_t N>
constexpr std::array<char, N> Foo<N>::arr;

Like other static members, this goes in a header - like the class template itself.