From: Ben Hutchings (ben.hutchings_at_[hidden])
Date: 2004-12-14 14:29:14
Jim Lear wrote:
<snip>
> I'm a little overwhelmed by these discussions, so forgive me for my
> ignorance or incapacity to communicate. My intention isn't to create
> container-independent code.
Ah, good.
> My intention is to create code for maps
> (and vectors) that looks like what I think code for maps (and vectors)
> would look like. Iterators look to me like pointers to linked lists for
> vectors, and like pairs of pointers for maps.
Iterators are kind of like pointers into arrays, but possibly with some
limitations on how you can use them; random-access iterators have no
such limitations, whereas input and output iterators only allow you to
go through the sequence once, forward, one step at a time.
Iterators over maps are bidirectional iterators, which are somewhere
in-between: they can then be moved forward and backward over the
sequence however many times you want, but only one step at a time. They
do not behave like pairs of pointers but like pointers to pairs.
(The standard iterator categories unfortunately combine requirements on
traversal operations with requirements on access operations; if you want
to know exactly what the iterator categories mean, look elsewhere.)
> There is nothing wrong
> with these semantics, except they seem odd compared to some other
> languages semantics for associative arrays (e.g. awk). However,
> overloading the operator[] in the map class to take iterators as a
> parameter would allow iterators to be treated like keys, if one desires,
> which to me seems more natural. This would still allow one to iterate
> over keys, mapped values, or pairs.
>
> So in the absolute most abhorently poor code example (let's just call it
> "meta-code"), the map operator would behave like:
>
> data_type &operator[](const iterator &i) { return i->second; }
I think that code should actually work if you only replace "data_type"
to "mapped_type", but I suggest you don't go editing your <map> header
just yet!
> Am I just too ignorant to make any sense? :-) Maybe I'll crawl back
> into my hole. :-)
No, but I think your concept of what an iterator is may be faulty.
Since an iterator "points" to a sequence element (or to the end of a
sequence) by itself; there is no need to combine it with a container to
access that element. You seem to want a regularity which doesn't really
make sense, hence my earlier guess that you wanted to write container-
independent code.
Ben.