Re: [Boost-users] [iterator] reverse_iterator from iterators that return references to data in self.

15 Jul 2006

      "Seb Martel"  writes:
...
Hello,
given something like:
class ProxyIterator
    : public boost::iterator_facade
{
public:
    ProxyIterator(FactoryOfCostly* pF, int i ) : m_pFactory( pF ), m_index( i )
    {}
private:
    friend class boost::iterator_core_access;
    void decrement() {--m_index;}
    // ... and others
Costly& dereference() const
    {   
        m_value = m_pFactory->MakeCostlyAtIndex( m_index );
        return m_value;
    }
private:
    FactoryOfCostly* m_pFactory;
    int m_index;
    mutable Costly m_value;
};
You might need to do something to prevent this from being reported as
a valid random access (or even forward) iterator.  24.1.3:

  --- If a and b are both dereferenceable, then a == b if and only if *a
      and *b are the same object.
...
I cannot use this ProxyIterator with a boost::reverse_iterator adaptor because the implementation of reverse_iterator's dereference dereferences a temporary:
// from reverse_iterator.hpp
typename super_t::reference dereference() const { return *boost::prior(this->base()); }
...
and in ProxyIterator, this dereference returns a reference to an
internal member, m_value, that will immediately vanish out of
existence. Bummer.
Yeah, bummer!

reverse_iterator is designed to be a better version of
std::reverse_iterator, but it does not solve this problem.
...
What if reverse_iterator carried as member the decremented iterator
instead of decrementing to a temporary on a dereference?
The problem then is that it either becomes less efficient, or
impossible to represent an end reverse iterator, because you can't
necessarily decrement the underly iterator you're passed:

         vector<int> v;
         make_reverse_iterator( v.begin() );  // undefined behavior
...
In the current reverse_iterator implementation, decrement must
behave properly on an end iterator, but not on a begin iterator. Can
a begin iterator be decremented and still used for comparison
purposes?
No.

You could build a reverse_iterator like the one you propose by storing
the inner iterator within a boost::optional wrapper, and only
initializing it upon dereference.

HTH,

-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com

Re: [Boost-users] [iterator] reverse_iterator from iterators that return references to data in self.

David Abrahams