Re: What is the best way to iterate over a map? (or any paired container)
"kanze" <kanze@gabi-soft.fr> writes:
Earl Purple wrote:
kanze wrote:
Been there, done that. I'm actually pretty good at writing
non-conformant iterators... that work most of the time (and
pass all of my own tests).
What I've found useful in such cases is to compiler with a
concept checking version of the library. A recent version of
g++, with -D_GLIBCXX_CONCEPT_CHECKS -D_GLIBCXX_DEBUG
-D_GLIBCXX_DEBUG_PEDANTIC can find a lot of errors.
How does it know though if the class is intended to be an iterator? Is
it because you try to pass it to any standard algorithm? Or does merely
calling it iterator or returning it from a begin() function suddenly
turn it into one?
It checks arguments to the standard algorithms, to ensure that
they meet the requirements for those arguments. Some standard
algorithms have a requirement that the argument meet the
requirements of e.g. forward iterator. G++ verifies that any
type you pass as a parameter to those algorithms meets *all* of
the stated requirements
Just a nit: not quite. It only (and can only) check the structural
requirements, but not the semantic ones. For example, such
requirements as
a == b implies b == a
go unchecked.
Although not particularly important for just checking concepts, this
point is actually more important than it looks in general. A type may
model concept A but exhibit all the structural requirements of a
refined concept, B. If you start dispatching to different algorithm
implementations based on structure you can cause yourself no end of
grief.
(and thus, all of the requirements of a
forward iterator, if that's what the algorithm says it
requires), even if some of the characteristics aren't used in
the actual implementation.
Although the problems it detected in my code all had to do with
iterators, I presume that it also verifies other types of
arguments, e.g. Predicate, etc. I also suppose that it isn't
perfect; I'd be very surprised, for example, if it detected that
your ordering predicate didn't specify a complete ordering.
Exactly. No semantic checks.
Still, it's definitly worth using.
Definitely.
And I should point out that this concept checking work originated in
Boost and has recently undergone several improvements:
http://lists.boost.org/Archives/boost/2006/08/109383.php
Cheers,
--
Dave Abrahams
Boost Consulting
www.boost-consulting.com
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