In which compilation unit lives a constexpr variable?

Question

Consider this code:

struct foo
{
    static constexpr int value = 42;
};

void bar(const int* value) { std::cout << *value; }
int main() { bar(&foo::value); }

This compiles fine and without warnings under the couple of online compilers I tried. Given that there is no single .cpp file defining the constexpr value, could the value of the pointer be different if the bar method is invoked from different compilation units? Or does the standard guarantee that the value ends up allocated only one time across all compilation units (ie an implicit _declspec(selectany))?

Answer 1

It doesn't work for me---I get a linker error. http://coliru.stacked-crooked.com/a/59e2cf56122733d0

If you do not odr-use the static constexpr member, you can imagine that it's inlined wherever required and doesn't live in any compilation unit. If you do odr-use it, as you have done in your program, you must define it.

Answer 2

The other answers are correct but the situation will change in C++17, with the adoption of inline variables (p0386). constexpr will then imply inline.