Get a reverse iterator from a forward iterator without knowing the value type

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The STL has std::reverse_iterator<Iterator>:

template <class RandomAccessIterator>
void mySort(RandomAccessIterator first, RandomAccessIterator last) 
{
  typedef std::reverse_iterator<RandomAccessIterator> RIter;
  RIter riter(last);
  RIter rend(first);
  for ( ; riter != rend; ++riter) {
    // Do stuff
  }
}

An important note:

Notice however that when an iterator
is reversed, the reversed version does
not point to the same element in the
range, but to the one preceding it.
This is so, in order to arrange for
the past-the-end element of a range:
An iterator pointing to a past-the-end
element in a range, when reversed, is
changed to point to the last element
(not past it) of the range (this would
be the first element of the range if
reversed). And if an iterator to the
first element in a range is reversed,
the reversed iterator points to the
element before the first element (this
would be the past-the-end element of
the range if reversed).

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