for( iterator_type one ; /*...*/)
{
       for( iterator_type two = one + 1 ; /*...*/)
       {
/*...*/
        }
}

for( iterator_type one ; /*...*/)
{
       iterator_type two = one ;
       for( ++two ; /*...*/)
       {
/*...*/
        }
}

#include <iterator>
#include <iostream>
#include <vector>
#include <list>
using namespace std ;
 
template< typename iterator_type > inline
void bar( iterator_type iterator, random_access_iterator_tag )
{ cout << "random access iterator!\n" ; }
 
template< typename iterator_type > inline
void bar( iterator_type iterator, bidirectional_iterator_tag )
{ cout << "bidirectional iterator!\n" ; }
// etc
 
template< typename iterator_type >
inline void foo( iterator_type iterator )
{
  bar( iterator,
   typename iterator_traits<iterator_type>::iterator_category()
   ) ;
}
 
int main()
{
  vector<int> vec ;
  list<int> lst ;
  cout << "vector<int>::reverse_iterator: " ; 
  foo( vec.rbegin() ) ;
  cout << "list<int>::iterator: " ; 
  foo( lst.begin() ) ;
}