Re: [Boost-users] [ptr_container] ptr_map predicates

From: Thorsten Ottosen (tottosen_at_[hidden])
Date: 2006-02-11 05:47:12

Bill Buklis wrote:
> The design of the normal iterator is beautiful. I much prefer it this way,
> i.e. using it as if it were a pointer to the object. The downside, of
> course, was just when you do need a key/value iterator. Since it can be done
> using the pair<key,void*> and base() iterator, perhaps that is sufficient
> and no changes need to be made.
> 
> However, from a purely design perspective (as opposed to usage), the only
> problem I see with that is that it exposes the internal workings of the
> class a little (I try to keep my classes as "black box" as possible. It's
> not always feasible, but at least that's the initial goal). The only way
> around that I can see at the moment is to have two types of iterators
> offered by the class - the normal iterator (same as it is now) and a
> "map_iterator" which acts the same as a std::map iterator would.

you mean iterating over pair<key,T*> or pair<key,T&> ?

> I'm not sure how feasible that would be to implement. I haven't studied the
> inner workings of the ptr_map class that much (although I'm quickly learning
> more and more about it).

it's certainly doable, probably with a little overhead to create a proxy.

>>From a user's perspective all you care about is that it is relatively easy
> to use, that you can get it do what you want, and it's not too difficult to
> maintain.

right.

>>From an aesthetics standpoint, it would be nice to have the two types of
> iterators depending on what you need. The code would be more readable. It
> would also shut those up who complain about the iterators not acting like
> std::map. "Look, use map_iterator if you want that!" Also in those cases
> where you do need both the key and the value, you wouldn't have to cast the
> void* back to the correct object type.

yeah, that is bad.

> Unless I'm mistaken this whole argument only pertains to ptr_map and
> ptr_multimap. The other classes only have a single value, so the iterators
> already act like the standard versions.

true.

> What do you think? 

I'm leaning towards a new iterator type that iterators over pair<key,T*>.

-Thorsten