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From: Simon Wright <simon@pushface.org>
Newsgroups: comp.lang.ada
Subject: Re: Grace and Maps (was Re: Development process in the Ada community)
Date: 27 Apr 2002 06:29:42 +0100
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"Marin David Condic" <dont.bother.mcondic.auntie.spam@[acm.org> writes:

> O.K. Sounds like we agree. Package 1 defines the interface needed
> for an in-memory map with facilities to get them in and out of
> streams. Package 2 defines the interface for a disk-based map that
> probably doesn't need streams but would need certain file
> manipulation capabilities. For the garden variety add/change/delete
> and scan kinds of stuff, there doesn't need to be any difference in
> the interface presented. But you can't just unplug the one and
> substitute the other because of some different requirements.

The BCs already do 'Input and 'Output for Maps (and indeed many other
container kinds; provided you use GNAT 3.14 or later!). The problem
it seems to me with disk-based containers is that you need to think
very hard about whether they're limited or not; what does assignment
mean for disk-based maps? overwrite, I suppose?

Creation is interesting, too. If you say

  M : Disk_Based_Map;

how do you make sure it's associated with a file? ... well, I guess
that's easy:

  procedure Associate (M : in out Disk_Based_Map;
                       With_File_Named : String);

and raise an exception (No_Associated_File) if the punter tries to use
it before association.

I certainly would expect a Disk_Based_Map to be in Abstract_Map'Class
(perhaps just because that's the way the BCs do it).