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From: "Nick Roberts" <nickroberts@adaos.worldonline.co.uk>
Newsgroups: comp.lang.ada
Subject: Re: List Container Strawman 1.4
Date: Fri, 28 Dec 2001 19:12:25 -0000
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Date: 2001-12-28T19:12:25+00:00
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"Hyman Rosen" <hyrosen@mail.com> wrote in message
news:3C2CA143.6030006@mail.com...

> Ted Dennison wrote:
>
> > Actually, I was thinking along the lines of an Containers.Lists.Fixed
>
>  > package that works with arrays.
>
> I think this kind of thing is where C++'s iterators and algorithms
> approach shows its superiority. Why should a List have any notion
> of other data structures to which its elements can be copied?
>
> In the STL, creating or copying one structure from another can be
> done just using the std::copy algorithm, a pair of iterators giving
> the source range, and a target iterator which receives the copies.
> If you want to copy to a fixed-size array, the target iterator can
> simply be a pointer to the first element. If you are creating a more
> complicated target (such as another list, or a vector), then it is
> the iterator that holds the semantics of where the data goes, for
> example std::inserter or std::back_inserter.

Well, speaking for what my own library (which I'm calling Tenet) will
provide, I am supplying a set of container-wide iterator types, which come
with a Copy_All procedure. So if I have, say, a lookup and a list, I can
copy from one to the other thus:


   package Widget_Lookups is new Tenet.Lookups.Unbounded(Widget);
   package Widget_Lists is new Tenet.Lists.Unbounded(Widget);

   M: Widget_Lookups.Lookup;
   L: Widget_Lists.List;

   package Widget_Iteration is Tenet.Iteration(Widget);
   package Widget_Lookup_Iteration is new
Widget_Lookups.Iteration(Widget_Iteration);
   package Widget_List_Iteration is new
Widget_Lists.Iteration(Widget_Iteration);

   R: Widget_Lookup_Iteration.Reader;
   W: Widget_List_Iteration.Writer;

   ...

   Open(R,M);
   Open(W,L);
   Copy_All(R,W);
   Close(W);
   Close(R);


It is all very wordy. The C++ equivalent is no doubt more concise. But, of
course, this is only a tiny example. Realler software would show (IMHO ;-)
the expressiveness of this design. In a hurry; must go.

--
Best wishes,
Nick Roberts