Is there a (valid) reason why the default ctors for list<...>::iterator and
map<...>::iterator do not initialize the object to a default value? The
node pointers in deque<...>::iterator do get initialized. Thus, it is
possible to do this:

    deque<...>::iterator iter;
    if(iter == deque<...>::iterator())
        ;

but not this:

    list<...>::iterator iter;
    if(iter == list<...>::iterator())
        ;

because the list node pointers are uninitialized.

The problem can be circumnavigated for lists by:

    template <...> class List<...> : public list<...>
    {
    public:
        static iterator get_nil_iterator() { return iterator(0); }
    };

since iterator::iterator(link_type) is protected. However, this fails for
maps since the iterator is a member of an enclosed class.

Does anyone know if the ANSI spec explicitly requires or disallows default
initialization for iterators? In other words, can I be guaranteed that two
default constructed iterators are equal in value (or more precicely, that
their equality can be reliably ascertained using the equality operator ==)?
If not, is there any portable way to perform this?

Thanks,
Dietmar
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