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().

Answer 1

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.

Answer 2

No, you can't check against NULL because it is not a pointer. Return and also check against animalList.end(). Only when the iterator is not equal to end() should you dereference it.

Answer 3

Just check like this

auto it = findInList( someInfo );

if ( it == animalList.end() ) std::cout << "not found";

Answer 4

Might be it's possible to use this workaround like checking an iterator against a NULL value:

#include <iostream>
#include <vector>


int main()
{
    std::vector<int> vector_int = {1, 2, 3, 4};

    std::vector<int>::iterator vector_int_iterator = --vector_int.begin();

    while(vector_int_iterator != vector_int.end())
    {
        if(vector_int_iterator == --vector_int.begin())
        {
            std::cout 
                << "Doing some stuff only once before iterate through vector's elements" 
                << std::endl;
            vector_int_iterator++;
        }

        std::cout << *vector_int_iterator++ << std::endl;
    }
}

It might be especially useful when we have to run while cycle more than once.