// header file
class Foo
{
   public:
   static int bar;
   int baz;
   int adder();
};

// implementation
int Foo::adder()
{
   return baz + bar;
}

int Foo::bar = you_initial_value_here;

int Foo::bar = 0;  // or whatever value you want

int Foo::bar;

class Foo {
    static int bar;
    int adder();
};

int Foo::bar=0;
int Foo::adder() { ... }