A class declaration can contain static object of self type, it can also have pointer to self type, but it cannot have a non-static object of self type.
If a non-static object is member then declaration of class is incomplete and compiler has no way to find out size of the objects of the class.
Static variables do not contribute to the size of objects. So no problem in calculating size with static variables of self type.
For a compiler, all pointers have a fixed size irrespective of the data type they are pointing to, so no problem with this also.
