Question

If I use a default constructor for an iterator, how to check if it was assigned later on?

For pointers, I could do this :

int *p = NULL;
/// some code
if ( NULL == p ) {
  // do stuff
}

How do I do the above for iterators?
Is it possible at all?

#include <iostream>
#include <list>

int main ()
{
    std::list<int>::iterator it;

  if ( NULL == it ) // this fails
  {
      std::cout<<"do stuff" << std::endl;
  }
}

Answer 1

I managed to find this in the current standard (c++03 ). 24.1 p 5 tells :

Just as a regular pointer to an array guarantees that there is a
pointer value pointing past the last element of the array, so for any
iterator type there is an iterator value that points past the last
element of a corresponding container. These values are called
past-the-end values. Values of an iterator i for which the expression
*i is defined are called dereferenceable. The library never assumes
that past-the-end values are dereferenceable. Iterators can also have
singular values that are not associated with any container. [Example:
After the declaration of an uninitialized pointer x (as with int* x;),
x must always be assumed to have a singular value of a pointer. ]
Results of most expressions are undefined for singular values; the
only exception is an assignment of a non-singular value to an iterator
that holds a singular value. In this case the singular value is
overwritten the same way as any other value. Dereferenceable values
are always non- singular.

(Emphasis mine)

So the answer is : no, it is not possible.