Undefined reference to static constexpr char[]

Question

I want to have a static const char array in my class. GCC complained and told me I should use constexpr, although now it\'s telling me

Answer 1

This is really a flaw in C++11 - as others have explained, in C++11 a static constexpr member variable, unlike every other kind of constexpr global variable, has external linkage, thus must be explicitly defined somewhere.

It's also worth noting that you can often in practice get away with static constexpr member variables without definitions when compiling with optimization, since they can end up inlined in all uses, but if you compile without optimization often your program will fail to link. This makes this a very common hidden trap - your program compiles fine with optimization, but as soon as you turn off optimization (perhaps for debugging), it fails to link.

Good news though - this flaw is fixed in C++17! The approach is a bit convoluted though: in C++17, static constexpr member variables are implicitly inline. Having inline applied to variables is a new concept in C++17, but it effectively means that they do not need an explicit definition anywhere.