"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