How to check if the iterator is initialized?

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.

Most iterators don't have any global special values in the same way that all pointers can be NULL. Typically, though, you'll be working with specific containers, and if you keep one iterator per container, then you can use end() as the sentinel value:

std::list<int> mylist;
std::list<int>::iterator it = mylist.end();

/* do stuff */

if (it == mylist.end()) { ... }

I'm not sure if insertion/deletion invalidates the end() iterator, though, so if you're planning on modifying your container, maybe save a copy of the original end, too:

std::list<int>::iterator end = mylist.end(), it = end;

if (it == end) { ... }

Though again I'm actually not sure if it's well-defined to compare two invalid iterators (in the event that the two do get invalidated).