Why won't my C++ program link when my class has static members?

Question

I have a little class called Stuff that I want to store things in. These things are a list of type int. Throughout my code in whatever classes I use I want to be able to a

Answer 1

Static data members have to be defined outside class declarations, much like methods.

For example:

class X {
    public:
        static int i;
};

Must also have the following:

int X::i = 0; // definition outside class declaration