"static const int" causes linking error (undefined-reference) - c++

Question

I am baffled by the linker error when using the following code:
// static_const.cpp -- complete code
#include <vector>
struct Elem {
static const int value = 0;
};
int main(int argc, char *argv[]) {
std::vector<Elem> v(1);
std::vector<Elem>::iterator it;
it = v.begin();
return it->value;
}
However, this fails when linking -- somehow it needs to have a symbol for the static const "value."
$ g++ static_const.cpp
/tmp/ccZTyfe7.o: In function `main':
static_const.cpp:(.text+0x8e): undefined reference to `Elem::value'
collect2: ld returned 1 exit status
BTW, this compiles fine with -O1 or better; but it still fails for more complicated cases. I am using gcc version 4.4.4 20100726 (Red Hat 4.4.4-13).
Any ideas what might be wrong with my code?

Answer 1

If you want to initialize it inside the struct, you can do it too:
struct Elem {
static const int value = 0;
};
const int Elem::value;

Answer 2

Try writing it as
struct Elem {
static const int value;
};
const int Elem::value = 0;
etc
.

Answer 3

static class members are generally supposed to be defined outside the class (declared inside, defined outside) in one compilation unit.
I don't remember how that interacts with inline initialization of const static integral members.

Answer 4

Also see this post: essentially, the problem is that somehow compiler ends up expanding your code into taking the address of Elem::value.

Answer 5

Why not just do this?
return Elem::value;
But the answer is that you are assigning a value in the declaration. This is supposed to work for basic types such as int, and is only required for complex types (i.e. classes, such as if you had a string instead of int). What I have found in practice is that this is hit or miss depending on what version of what compiler you are using. And, as you found out, which optimization level.

Answer 6

In most of the compilers defining in-class static const <integral type> works fine. But some compilers like Android NDK, such in-class definitions results in linker errors. For such case, we may use the typed enums:
struct X
{
enum : int64_t { VALUE = 100; }; // == static const int64_t VALUE = 100;
};