Comparing default-constructed iterators with operator==

Question

Does the C++ Standard say I should be able to compare two default-constructed STL iterators for equality? Are default-constructed iterators equality-comparable?

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Answer 1

This is going to change in C++14. [forward.iterators] 24.2.5p2 of N3936 says

However, value-initialized iterators may be compared and shall compare equal to other value-initialized iterators of the same type.

Answer 2

I believe you should pass a range to the function.

void fun(std::list<int>::iterator beg, std::list<int>::iterator end)
{
    while(beg != end)
    {
        // do what you want here.
        beg++;
    }
}

Answer 3

OK, I'll take a stab. The C++ Standard, Section 24.1/5:

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 excep- tion is an assignment of a non-singular value to an iterator that holds a singular value.

So, no, they can't be compared.

Answer 4

Specification says that the postcondition of default constructor is that iterator is singular. The comparison for equality are undefined, so it may be different in some implementation.