why does it need it!=obj.end() although every iterator should know itself when to terminate?

Question

the following question just shot through my head. For the c++ stl iterators, a common practice is to have something like:

for (iterator it=obj.begin(); it!=obj.end(); it++)

what I am wondering is actually that obj.begin() could have told "it" when to stop which would make the for loop look like:

for (iterator it=obj.begin(); !it.end(); it++)

The benefit would be to make the iterator more self contained, and one could save (iterator end()) in the container class.

Answer 1

Sometimes you want to do something other than iterate over the entire contents of a container. For example, you can create a pair of iterators that will iterate over only the first half of a container. So having a separate object to represent the end is more flexible since it allows the user more control of where to place the end of a range.

You are right that it is somewhat inconvenient for the most common case of iterating over everything. However, C++11 provides a range based for loop which makes looping over a whole container really easy, so like many things in programming, it's really just a matter of selecting the right construct to best express your intention.

Answer 2

If the iterator API would be designed like that, a pointer wouldn't be a valid iterator (since a pointer obviously wouldn't have an end()) method. So that would rule out the most straight-forward way to implement iterators for data structures with contiguous memory.