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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: Tue, 14 Mar 2017 09:25:21 +0100
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Date: 2017-03-14T09:25:21+01:00
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On 14/03/2017 00:25, Simon Wright wrote:
> "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> writes:
>
>> On 2017-03-12 17:44, Simon Wright wrote:
>>> "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> writes:
>>>
>>>> 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.
>>>
>>> with Ada.Containers.Vectors;
>>> package Mutable_Elements is
>>>    type Element is record
>>
>> [...]
>>
>> Your example uses access type in Reference_Type which was the point.
>
> There may be pointers under the hood,

Under a straw hat with huge holes meant for ventilation alas exposing 
ugly nasty lack of hair and warts. Pointers are right in the public part 
of Ada.Containers.Vectors.

Even if Reference_Type were an opaque type with no visible access 
discriminant, it would still be a pointer in its semantics with all 
strings attached. Thus if it must be there [*], it must better be a 
smart pointer, e.g. to a reference-counted object.

BTW, reference-counted design gives means to resolve problems with 
at-cursor object deletion and aliased updates in one or another manner.

------------------------
* There should be no helper types needed to access container elements 
"in place".

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