Question

Like Java and C#, can I create object of a class within the same class?

/* State.h */
class State{
  private:
    /*...*/
    State PrevState;
};

Error:

field 'PrevState' has incomplete type

Answer 1

You cannot do this as written. When you declare a variable as some type directly in a class (Type variablename) then the memory for the variable’s allocation becomes part of its parent type’s allocation. Knowing this, it becomes clear why you can’t do this: the allocation would expand recursively — PrevState would need to allocate space for it’s PrevState member, and so on forever. Further, even if one could allocate an infinite amount of memory this way, the constructor calls would recurse infinitely.

You can, however, define a variable that is a reference or pointer to the containing type, either State & or State * (or some smart pointer type), since these types are of a fixed size (references are usually pointer-sized, and pointers will be either 4 or 8 bytes, depending on your architecture).