why can't we declare object of a class inside the same class?

Question

class A
{
  A a;//why can\'t we do this
};

Answer 1

I take it you're coming from Java or something? A a will create a full instance of type A, which, well, contains A, which contains A, which contains A.

You're probably thinking about this:

class A
{
  A *a; // A pointer to A, not a full instance
};