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?

melak47 · Accepted Answer · 2015-10-25 16:00:35Z

7

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.

edited Oct 25, 2015 at 16:00

answered Oct 24, 2015 at 20:41

melak47

4,8602 gold badges29 silver badges37 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

user1244932 Over a year ago

But where this should go? if it is in header file, will I got multiply definition if include into several files? If it is in c++ file, then how compiler handle different specilizations?

Sebastian Redl Over a year ago

Put it in the header. Since it is a template definition, it has to work.

Collectives™ on Stack Overflow

Undefined reference to static constexpr

1 Answer 1

2 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related