Iterating C++ vector from the end to the beginning

Question

Is it possible to iterate a vector from the end to the begin?

for (vector<my_class>::iterator i = my_vector.end();
        i != my_vector.begin(); /* ?! */ ) {
}

Or is that only possible with something like that:

for (int i = my_vector.size() - 1; i >= 0; --i) {
}

Answer 1

The best way is:

for (vector<my_class>::reverse_iterator i = my_vector.rbegin(); 
        i != my_vector.rend(); ++i ) { 
} 

rbegin()/rend() were especially designed for that purpose. (And yes, incrementing a reverse_interator moves it backward.)

Now, in theory, your method (using begin()/end() & --i) would work, std::vector's iterator being bidirectional, but remember, end() isn't the last element — it's one beyond the last element, so you'd have to decrement first, and you are done when you reach begin() — but you still have to do your processing.

vector<my_class>::iterator i = my_vector.end();
while (i != my_vector.begin())
{
     --i;
    /*do stuff */

} 

UPDATE: I was apparently too aggressive in re-writing the for() loop into a while() loop. (The important part is that the --i is at the beginning.)