Why do you _have_ to initialize a C++ static member variable?

Question

I know that you generally initialize a static member variable from within a .cpp file. But my question is: why do you have to?

Here's an example:

#include <vector>

using namespace std;

class A {
    public:
        static vector<int> x;
};

main() {
    int sz = A::x.size();
}

This gives a compiler error: undefined reference to 'A::x'

However, this:

#include <vector>

using namespace std;

class A {
    public:
        static vector<int> x;
};

// Initialize static member
vector<int> A::x;

main() {
    int sz = A::x.size();
}

compiles and runs fine.

I can understand if I was initializing the vector using something other than the default constructor, but I'm not. I just want a vector of size 0 created. Surely, any static members will have to be allocated memory on program initialization, so why doesn't the compiler just use the default constructor?

Answer 1

That's not about initialization, it's about definition. Or more precisely : it's about knowing which compilation unit (.cpp) will hold the object (that have to be uniquely defined SOMEWHERE)

So, what's needed is simply to put the definition somewhere, in a unique place, that is a cpp, to let the compiler know that when the class's static object is called, it's defined there and nowhere else. (if you try to define your static in a header, each cpp including this header will have a definition, making impossible to know where it should be defined - and manually initialized if it's required for you use) .