Get type of outer class

#define GET_OUTER ????
class Outer
{
    int a;
    struct Inner
    {
        size_t GetOffset()
        {
            return offsetof(GET_OUTER(Inner),a);
        }
        //or
        size_t GetOffset2()
        {
            return offsetof(GET_OUTER(this),a);
        }
    }
}

In General all i need is to get offset of variable in Outer class by inner class instance\type. Anyone have an idea ?