Why use rbegin() instead of end() - 1?

Question

I'm wondering what the benefits of using rbegin() rather than end() - 1 are for STL containers.

For example, why would you use something like:

vector<int> v;
v.push_back(999);
vector<int>::reverse_iterator r = v.rbegin();
vector<int>::iterator i = r.base();

Rather than:

vector<int> v;
v.push_back(999);
auto r = v.end() - 1;

Think about `` , such as `std::for_each(v.rbegin(), v.rend(), worker);`. — Nawaz, Aug 25 '15 at 16:18
@VictorBrunell So you shortened it to make it look simpler and better than `rbegin` :) — iTzVoko, Aug 01 '23 at 12:22

Paolo M · Accepted Answer · 2015-08-25T16:06:02.043

34

rbegin() return an iterator with a reverse operator++; that is, with a reverse_iterator you can iterate through a container going backward.

Example:

#include <vector>
#include <iostream>

int main()
{
    std::vector<int> v{0,1,2,3,4};
    for( auto i = v.rbegin(); i != v.rend(); ++i )
        std::cout << *i << '\n';
}

Furthermore, some standard containers like std::forward_list, return forward iterators, so you wouldn't be able to do l.end()-1.

Finally, if you have to pass your iterator to some algorithm like std::for_each that presuppose the use of the operator++, you are forced to use a reverse_iterator.

edited Aug 25 '15 at 16:06

answered Aug 25 '15 at 15:59

Paolo M

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I was thinking of iterating backwards using itr-- rather than itr++ to go in reverse, but using the built-in functionality makes more sense I guess. Thanks. – Victor Brunell Aug 25 '15 at 16:03
@VictorBrunell Updated my answer ;) – Paolo M Aug 25 '15 at 16:09
Thanks. That's very helpful. – Victor Brunell Aug 25 '15 at 16:48

score 20 · Answer 2 · answered Aug 25 '15 at 15:58

20

If the container is empty, end() - 1 will not be defined.

answered Aug 25 '15 at 15:58

Mark Ransom

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So, for a one element vector, begin() and rbegin() point to the same element? I see. Thanks. – Victor Brunell Aug 25 '15 at 16:00
4

@VictorBrunell: Yes and no. Forward iterators and reverse iterators are not compatible. You cannot do `v.begin() == v.rbegin()`. But you can compare the elements they point to, i.e. `&*(v.begin()) == &*(v.rbegin())` is defined and will be true for a 1-element vector. – Christian Hackl Aug 25 '15 at 16:15
Ah, interesting. Thanks. – Victor Brunell Aug 25 '15 at 16:45
well, for one element vector, `end() - 1` is defined though. @VictorBrunell – apple apple Jan 28 '22 at 08:11

Why use rbegin() instead of end() - 1?

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