Can I check a C++ iterator against null?

Question

I'm having trouble with vector iterators. I've read in a few places that checking for null iterators isn't possible, and that the usual way to check iterators is to check it against vector.end() after a search. So for example:

vector< Animal* > animalList;

vector<Animal*>::iterator findInList(const type_info& type)
{
    // Loop through list of Animals, if Dog found, return iterator to it
}

auto it = findInList(typeid(Dog));
// With a pointer I can check if it's null, but with an iterator I have to check against animalList.end();

The problem is that the container could be empty. With an iterator I can't return null to indicate the container's empty or the search failed. I can return vector::end(), but cplusplus.com says:

If the container is empty, vector::end() function returns the same as vector::begin()

and then for vector::begin() it says:

If the container is empty, the returned iterator value shall not be dereferenced.

So if I have an empty container, vector::end() and vector::begin() point to the same place, I don't think I can dereference it, and I'm not even sure it's pointing to allocated memory.

Edit: Thanks to everyone. As you have iterated out, vector::end() or vector::begin() do not dereference the iterator, I can safely check against vector::end().

Return an iterator for an empty vector. When the caller loops over the iterator, the loop will end immediately, and they'll never dereference the iterator. — Barmar
Are you trying to avoid exposing the container you're using internally, and are thus hesitant to provide a way for the user to check the returned value against the end of your container themselves? — jaggedSpire
"So if I have an empty container...I don't think I can dereference it" The same would be true if you were returning pointers that might be null. — Drew Dormann
@jaggedSpire, Now necessarily. Well I wanted the function to handle as much as possible. I think from the answers I understand I can get the same behaviour checking against vector::end(), that is, the same as returning a pointer and checking against nullptr. — Zebrafish

paweldac paweldac · Accepted Answer · 2016-12-27T22:32:33

You don't need to check if the iterator is null, because it will never be. You need to check if the returned iterator is different from the container's end() position. If it is, you can safely dereference the iterator by *it.

If the container is empty, the returned iterator value shall not be dereferenced. So if I have an empty container, vector::end() and vector::begin() point to the same place, I don't think I can dereference it, and I'm not even sure it's pointing to allocated memory.

No, checking if(myIt != container.end()) is not dereferencing the iterator. Iterator dereferencing is done via *myIt, which means getting the value of the object that the iterator is pointing to. It's always safe to check iterators to other iterators from the same container, it's unsafe to dereference iterators not pointing to containers' elements.

Can I check a C++ iterator against null?

3 Answers