Will null iterators cause undefined behaviour?

using namespace std;
class myList
{
  public:
    mylist():_internalList(),_lastPostition(0)
  {
  }
    typedef list<string>::iterator Itr;
    bool enqueue(string);
    Itr next()
    {
      if(_lastPostition == 0)
        _lastPostition = _internalList.begin();
      if(_lastPostition == _internalList.end())
        return (_lastPostition = 0);
      return _lastPostition开发者_如何学C++;
    }
  private:
    list<string> _internalList;
    Itr _lastPostition;
}

enqueue is not push_back, it inserts based on some custom logic. I can't use std::set and overload operator <, because my insertion logic isn't transitive - ( a < b && b < c) does not imply a < c.

This works but I'm not sure if its an undefined behavior. Is it safe to assign 0 to an iterator and check for 0 ?

"Is it safe to assign 0 to an iterator and check for 0 ?" No.

You can't assign 0 to an iterator, you have to use some other special value, like end().

You should implement next as shown below, and should also provide has_next() to check whether there is next item in the list or not.

Itr next()
{
   if ( !has_next() )
       _lastPostition = _internalList.begin();
   return ++_lastPostition;
}
bool has_next() const
{
   Itr  temp = lastPostition;
   return (++temp== _internalList.end());
}

Note: the behaviour of next() is circular; that is, on reaching end, it returns begin again!