C++ Reverse Iterators


Example

If we want to iterate backwards through a list or vector we can use a reverse_iterator. A reverse iterator is made from a bidirectional, or random access iterator which it keeps as a member which can be accessed through base().

To iterate backwards use rbegin() and rend() as the iterators for the end of the collection, and the start of the collection respectively.

For instance, to iterate backwards use:

std::vector<int> v{1, 2, 3, 4, 5};
for (std::vector<int>::reverse_iterator it = v.rbegin(); it != v.rend(); ++it)
{
    cout << *it;
} // prints 54321

A reverse iterator can be converted to a forward iterator via the base() member function. The relationship is that the reverse iterator references one element past the base() iterator:

std::vector<int>::reverse_iterator r = v.rbegin();
std::vector<int>::iterator i = r.base();
assert(&*r == &*(i-1)); // always true if r, (i-1) are dereferenceable
                        // and are not proxy iterators

 +---+---+---+---+---+---+---+
 |   | 1 | 2 | 3 | 4 | 5 |   |
 +---+---+---+---+---+---+---+
   ↑   ↑               ↑   ↑
   |   |               |   |
rend() |         rbegin()  end()
       |                   rbegin().base()
     begin()
     rend().base()

In the visualization where iterators mark positions between elements, the relationship is simpler:

  +---+---+---+---+---+
  | 1 | 2 | 3 | 4 | 5 |
  +---+---+---+---+---+
  ↑                   ↑
  |                   |
  |                 end()
  |                 rbegin()
begin()             rbegin().base()
rend()
rend().base()