Question

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What does it mean to have an undefined reference to a static member?

I don’t know why I got error “undefined reference to `TT::i'” if I implement the function outside of class declaration?
Here is the code..

class TT{
private:
    static int i;
public:
    void t();
};

void TT::t(){
    i=0;
}

Answer 1

It’s nothing to do with where you defined the function, it’s that you didn’t define the static member variable, you only declared it. You need to put its definition outside the class, e.g.

int TT::i;