[boost] bounded_iterator

From: Chad Parry (chad_at_[hidden])
Date: 2008-01-10 03:14:47

I created a "bounded_iterator" concept that seems really useful. A
bounded_iterator behaves just like a regular iterator, except that the
termination condition can be checked by just casting the iterator to a bool.
Here's an example:

 int array_range[] = { 1, 1, 2, 3, 5 };
 for (bounded_iterator<int *> i(array_range); i; ++i)
 {
   std::fill_n(std::ostream_iterator<int>(std::cout), *i, *i);
 }

I like it for the same reason that I like iterator_range: it frees the
developer from having to worry about both a begin and an end iterator. It
can be used as a wrapper for operations on a container or a sub-range. Just
like iterator_range simplifies the invocation of functions for ranges,
bounded_iterator simplifies the traversal of ranges.

In my application I was storing lots of iterators in containers, and using
bounded_iterator proved to be a better abstraction than storing iterator
pairs or ranges. The main disadvantage to using a bounded_iterator is that
it competes with the standard iterator concept, and it's usually more
confusing to support many ways to do the same thing. Here is a simple
implementation:

 template<typename base_iterator>
 class bounded_iterator :
   public boost::iterator_adaptor<bounded_iterator<base_iterator>,
base_iterator>
 {
   base_iterator const end;
   typedef boost::iterator_adaptor<bounded_iterator, base_iterator>
super_type;
   struct unspecified_bool_helper { int true_tag; };
   typedef int unspecified_bool_helper::*unspecified_bool_type;
 public:
   template<typename source_iterator>
   bounded_iterator(source_iterator begin, source_iterator end) :
     super_type(begin),
     end(end)
   { }
   template<typename source_range>
   bounded_iterator(source_range &range) :
     super_type(boost::begin(range)),
     end(boost::end(range))
   { }
   template<typename source_range>
   bounded_iterator(source_range const &range) :
     super_type(boost::begin(range)),
     end(boost::end(range))
   { }
   operator unspecified_bool_type() const
   {
     return super_type::base() == end ? 0 :
&unspecified_bool_helper::true_tag;
   }
 };

-- chad