C++14 static constexpr member array fails at linking

Question

I'm having some difficulties with static constexpr attributes: it works with integral types, with enum class members, but when I try to do it with a statically initialized integral array it fails at linking saying undefined reference to S::a inside main.

That is with either clang 3.9 or g++ 6.3, and ld 2.27.90; and all that with -std=c++14.

Here is the quickest snippet to reproduce this:

struct S { static constexpr int a[5] = {0}; }; int main() { S s{}; [[gnu::unused]] int b = s.a[0]; // force S stuff to be emitted return 0; }

Thank you for any suggestion you may have for this situation.

You need to define your object; at namespace scope: constexpr int S::a[5]; — ildjarn
– ildjarn, Commented Feb 19, 2017 at 16:38
@ildjarn thanks ! but do you know why I do have to do this for arrays but not for other stuff like plain integral types ? — suut
– suut, Commented Feb 19, 2017 at 17:04

Employed Russian · Accepted Answer · 2017-02-19 19:44:30Z

Consider this code:

enum class E { foo, bar }; struct S { static constexpr int a[5] = {0}; static constexpr int b = 42; static constexpr E e = foo; };

All of above are declarations, and are not definitions. For each one, you must provide a definition:

int S::a[5]; int S::b; E S::e;

it works with integral types, with enum class members

This works more or less by accident. Specifically, it works because you never have a context where the address of that variable is taken (never ODR-use the variable).

Often I see people adding an innocent-looking call to std::max, and suddenly discovering that they didn't provide a definition. That is:

int main() { printf("%d\n", S::b); // works fine int x = std::max(1, S::b); // fails to link in non-optimized build. }

Collectives™ on Stack Overflow

C++14 static constexpr member array fails at linking

1 Answer 1

Comments

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Related