c++ - unresolved external symbol structs

error LNK2001: unresolved external symbol "public: static int WrappedVector::_N" (?_N@WrappedVector@@2HA)

header.h

struct WrappedVector
{
    static int _N;

    double *_x;
};

main.cpp

const int WrappedVector::_N = 3;

i don't understand whats wrong