C++: list iterators vs. vector iterators

Question

I thought the idea of the iterator object was that you can apply it similarly to the C++ container classes. When I try to iterate through a list object, however, I tried using

for(list<int>::iterator it = obj.begin(); it < obj.end(); it++){ // some code }

And I got an error. Why doesn't this work? Why would it work for vector::iterator? Is it just because of the implementation of list being bi-directional linked lists? I thought the iterator object abstracts that notion of moving through containers, thereby allowing it to operationally be the same, whether for vectors or lists.

I'd really appreciate a clarification.

it < obj.end() should be it != obj.end(), Also do not use it++, use ++it(more here) and use non-member begin and end if you use c++11... and maybe you don't want end function to be called every time... — joy
– joy, Commented Feb 21, 2013 at 15:48

Sergey Kalinichenko · Accepted Answer · 2013-02-21 15:48:16Z

This does not work because, unlike std::vector iterators, std::list iterators are not random-access - they are sequential. You need to use != on them:

for(list<int>::iterator it = obj.begin(); it != obj.end(); it++)

In general, it's a good idea to use "not equals" on all iterators when you are looking to cover the entire range, even when these iterators allow comparisons for < and >. There is also an argument in favor of using != in your regular for loops, too, because it gives you the strongest postcondition.

Didier Trosset · Accepted Answer · 2013-02-21 15:49:15Z

You have to compare with != as list iterators are scattered throughout all memory in random order.

Use: for(list<int>::iterator it = obj.begin(); it != obj.end(); it++)

Andy Prowl · Accepted Answer · 2013-02-21 15:49:31Z

That's because list does not support random access iterators, but only forward iterators. Therefore, operator < is not defined for iterators of a list. You have to use operator != for inequality comparisons.

Angew is no longer proud of SO · Accepted Answer · 2013-02-21 15:49:56Z

Operator arithmetic, including ordering-comparison operators (like <), is only defined for random access iterators. If you change the code to use !=, it will work (assuming obj is a list<int>):

for(list<int>::iterator it = obj.begin(); it != obj.end(); it++){ // some code }

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