class A{
public:
    static void initA();
private:
    static int a;
};

void A::initA(){
    A::a = 1;
}

int main(){
    
    return 0;
}

--------------------Configuration: static — Win32 Debug--------------------
Compiling...
main.cpp
Linking...
main.obj : error LNK2001: unresolved external symbol "private: static int A::a" (?a@A@@0HA)
Debug/static.exe : fatal error LNK1120: 1 unresolved externals
Error executing link.exe.
Creating browse info file...

static.exe — 2 error(s), 0 warning(s)