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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: Getting the index for an element in mutually referencing
 containers
Date: Sun, 12 Mar 2017 12:55:05 +0100
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Date: 2017-03-12T12:55:05+01:00
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On 2017-03-12 12:30, Simon Wright wrote:
> "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> writes:
>
>> On 2017-03-11 22:46, Simon Wright wrote:
>
>>> Basically, I (Simon, now) am having trouble thinking of an application
>>> where reference counting would be an appropriate solution.
>>
>> As I said, in practice it is all cases when you would have a container
>> of mutable elements.
>
> I had some trouble understanding your point the first time, and skipped
> over it (sorry).
>
> What's the opposite of "mutable"? constant? because, if so, why wouldn't
> I use Update_Element?

Yes, But it is almost never element update. Usually what you want is to 
call some mutable operations on the element. The language does not offer 
user-defined by-reference or copy-out-copy-in methods to element access.

Thus it becomes an access type which opens the whole can of worms. 
Reference counting is merely a method to deal with access types and to 
prevent excessive copying, e.g. when inserting elements.

> (aside from it being very clumsy; more likely to
> use a reference, Person (P).Health := Unwell; )

An access type, you mean. References are only for built-in containers: 
records and arrays.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de