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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: Carbon
Date: Fri, 2 Sep 2022 19:23:21 -0500
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"Niklas Holsti" <niklas.holsti@tidorum.invalid> wrote in message 
news:jneucmFn478U1@mid.individual.net...
> On 2022-09-02 17:59, J-P. Rosen wrote:
>> Le 02/09/2022 à 16:18, G.B. a écrit :
>>> On 01.09.22 08:46, J-P. Rosen wrote:
>>>> Le 28/08/2022 à 19:34, G.B. a écrit :
>>>>> Also the convenience of being able to compare two objects for
>>>>> being the same object, using "===" in some languages---out of the box!
>>>>>
>>>>> procedure same_object (A : T; B in out T) is
>>>>> begin
>>>>> what_do_i_put_here; -- ?
>>>> A'Has_Same_Storage (B);
>>>>> end same_object;
>>>>
>>>> Of course, this procedure should be a function...
>>>
>>> Cool, new attributes.

Not very new, these were added in Ada 2012. They were considered inportant 
for some preconditions.

>>>
>>> Is the expectation that A and B occupy the same
>>> bits if I pass the same X as an actual for both?
>> Yes.
>
>
> Surely only if their type (T, above) is passed by-reference?
>
> G.B.'s post had a later example where T is "range 1 .. 10", which is a 
> by-copy type, so there A and B would be separate copies of X.

Agreed. Parameters of a by-copy type are separate objects from the actual 
objects. Parameters of a by-reference type (like tagged types) are the same 
objects. Parameters of types which are not specified as either may or may 
not be new objects. And of course, Has_Same_Storage only if true when an 
object is not copied. So the technique shown only certainly works for 
by-reference types. The compiler can copy other types when it wants, so it 
isn't meaningful to compare objects (only values).

                                 Randy.