[boost] [iterator_facade] postfix ++/--

From: David Abrahams (dave_at_[hidden])
Date: 2004-07-10 16:22:48

I'm getting ready to fix the issues we've discussed with postfix ++
(and by extension, --) in iterator_facade.  Basically, the current
implementation works for forward iterators but not for some input
iterators:

       {
            self x = *this;
            ++*this;   // <== here
            return x;
       }

The problem is that for many input iterators, all copies reference the
same element.  The marked line increments the iterator, which causes
the current element of the sequence to be skipped, so

    *a++

yields the /next/ element.  To fix that, a++ needs to return a proxy
type.

When considering this stuff, please look at n1640 and not C++98 or
TC1, as n1640 contains fixes for outright bugs that survived to TC1 as
well.  In particular, *a++ was required to return an input iterator's
value_type until recently.

If we don't know for certain that the iterator is a forward iterator
and if *a returns something convertible to T const& (where T is the
iterator's value type) it seems to me that we *must* assume that all
copies of the iterator reference the same element, so the result of
postfix ++ must be a proxy that contains a copy of *a.

The new iterator framework is intended to support iterators with
bidirectional traversal that are not conforming forward iterators
(e.g. their operator* may return by-value).  If an iterator is
incrementable it is also dereferenceable, but the same doesn't hold
for decrementability, so I'm not sure what to do about the result of
postfix --.  

The best solution I can think of is that the actual decrement is
delayed, and the iterator being decremented contains a flag that says
"do an extra decrement before the next operation other than postfix
decrement".  That's a really bad solution, though: you can't really do
a decrement in the destructor because it might throw, and such an
iterator could easily have side-effects, so it'd be easy to get the
iterator out-of-synch with expectations.

Thoughts?

-- 
Dave Abrahams
Boost Consulting
http://www.boost-consulting.com