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

From: Seb Martel (smartel_at_[hidden])
Date: 2006-07-15 12:50:41

"David Abrahams" <dave_at_[hidden]> wrote in message
news:<uwtafgdi0.fsf_at_[hidden]>...
> "Seb Martel" <smartel_at_[hidden]> writes:
> 
> > Hello,
> >
> > given something like:
> >  
> > class ProxyIterator
> >     : public boost::iterator_facade<ProxyIterator, Costly,
boost::random_access_traversal_tag>
> > {
> > 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. 
> 

As in &(*a) == &(*b) ? That's just cruel. But then again, vector<bool> 
comes to mind... Which reminds me: 

Is http://boost.org/libs/iterator/doc/new-iter-concepts.html making 
any progress ?

> 
> 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.
> 

I might just name it the DamnYou2413ReverseIterator.

Tx!

-seb