[boost] Re: Proxy --- an indirect container adaptor

From: David Abrahams (dave_at_[hidden])
Date: 2004-02-24 20:06:13

Christian Engström <christian.engstrom_at_[hidden]> writes:

> The reason why
> they are derived, in turn, is that we want the proxy iterators and the
> underlying "normal" iterators to be implicitly convertable to each
> other in both directions.  If both conversions were user defined, as
> they would have to be if the proxy iterators could not utilize the
> implicit derived-to-base conversion, this would lead to a host of
> ambiguity problems, since the compiler has no reason to prefer one
> conversion over the other.
>
> The classic example is the expression  a == b, which becomes ambiguous
> because the compiler is just as happy to carry out the comparison as
> either  b_type(a) == b  or  a == a_type(b)  if both conversions are
> available as implicit user defined conversions, and there is no ==
> operator defined for the mixed types case.

A classic example of a problem that can bve solved more elegantly by
the iterator adaptors library, *without* the problematic two-way
implicit conversion -- and it *is* problematic, even if one direction
is derived-to-base.  You asked for feedback; I say look at the work
that those came before you already did in this area.  I don't know why
you won't, but I've stopped trying now.

Cheers,

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