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From: Simon Wright <simon@pushface.org>
Newsgroups: comp.lang.ada
Subject: Re: Getting the index for an element in mutually referencing
 containers
Date: Sat, 11 Mar 2017 08:40:15 +0000
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Date: 2017-03-11T08:40:15+00:00
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"Randy Brukardt" <randy@rrsoftware.com> writes:

> "Mart van de Wege" <mvdwege@gmail.com> wrote in message 
> news:86wpbxneuz.fsf@gaheris.avalon.lan...
>> Simon Wright <simon@pushface.org> writes:
> ...
>>> But, I really think you're going to need to use access types here. A
>>> Person is a unique object, and should only exist once in your data
>>> structures. A Child should hold references to its Parent(s) and vice
>>> versa; if a Child holds *copies* of its Parents, what happens when
>>> you change the Parent's data in one place? how are you going to find
>>> all the copies and change all of them?
>>>
>> That was what I was afraid of. I like the way Ada usually takes care
>> of this for you, but I've been trying to work it out during downtime
>> today at work, and I'm afraid it's indeed time to grab the access
>> types out of the toolbox.
>
> The trick in such cases is to have a global "person" vector, which
> contains all of the actual Person objects. Then use the cursors into
> that array in your other data structures. That's gives the needed
> level of indirection without involving any actual access types
> (meaning that you let Ada do the storage management, and depending on
> your container implementation, also reduce/eliminate the danger of
> dangling access types).

In the general case, you'd need to cope with deleting (the equivalent
of) Person objects.

And wouldn't even appending a new Person risk invalidating any existing
cursors? AARM A18(5.k) says "Cursors *generally* remain valid as long as
the container exists and the element referenced is not deleted",
emphasis added. AdaCore's Vector implementation would be OK with
inserting elements _after_ the Cursor concerned, since the Cursor is
based on an index into an array. See discussion at AARM A18.2(240) ff.

I quite like the idea of using an indefinite container to hold the
Person objects, but I think I'd use a Map with the key an integer
(Natural?), with the next key held in a private global, incremented as
each new Person is created. The reference to a Person would be the
associated key.

One disadvantage to this design is that Containers hold non-limited
types. Your users run the risk of writing code that gets hold of a copy
of the object and modifies that, rather than modifying the actual
object.