Undefined reference to static variable [duplicate]

Answer 1

You have declared A::i inside your class, but you haven't defined it. You must add a definition after class A

class A {
public:
    static int i;
    ...
};

int A::i;

Answer 2

You only declared A::i, need to define A::i before using it.

class A  
{ 
public:     
  static int i;
  static void init(){
     i = 1;  
  }
 };

int A::i = 0;

int WINAPI WinMain (HINSTANCE hThisInstance,
                HINSTANCE hPrevInstance,
                LPSTR lpszArgument,
                int nFunsterStil)
{
  A::i = 0;
  A::init();

  return 0;
}

Also your init() function should return a value or set to void.