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](http://stackoverflow.com/questions/1077026/incrementing-iterators-it-more-efficient-than-it)) 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, Feb 21 '13 at 15:48

score 7 · Accepted Answer · answered Feb 21 '13 at 15:48

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.

score 1 · Answer 2 · answered Feb 21 '13 at 15:49

1

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++)

answered Feb 21 '13 at 15:49

Didier Trosset

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score 0 · Answer 3 · answered Feb 21 '13 at 15:49

0

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.

answered Feb 21 '13 at 15:49

Andy Prowl

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score 0 · Answer 4 · answered Feb 21 '13 at 15:49

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
}

C++: list iterators vs. vector iterators

4 Answers4