Re: [Boost-users] Re: Iterators and maps [was Re: Where should STL suggestionsgo?]

From: Jim Lear (jim.lear_at_[hidden])
Date: 2004-12-13 17:42:19

David Abrahams wrote:
> Ben Hutchings wrote:
> > Jim Lear wrote:
> > > The map iterators are really what tripped me up as an STL newbie.  One 
> > > must understand the iterators are implemented as pairs for maps, and 
> > > pointers for vectors and other things.  By providing an "T 
> > > operator[](iterator)" function for random access and associative 
> > > containers (and for sequences maybe), one could achieve a consistant 
> > > interface.
> > >     
> > Container-independent code is an impossible and pointless goal.  maps 
> > have different semantics from other containers.  You may want to iterate 
> > over just the mapped values, but other people may want to iterate over 
> > the keys, or over the pairs (and indeed, so may you, in other part of 
> > your program).  Who's to say which of these is right?  What you can do 
> > is to write iterator-independent code, then use an iterator adapter to 
> > convert the iterators over pairs into iterators over the mapped values:
> >   
>
> Have you seen http://www.zib.de/weiser/vtl/ and
> http://www.boost.org/libs/range/index.html?
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.  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.  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;
}

Am I just too ignorant to make any sense? :-)  Maybe I'll crawl back into
my hole. :-)
-- 
Jim Lear