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From: ytomino <aghia05@gmail.com>
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Subject: Re: Problem in "X (1).Re := X (1).Re + 1"
Date: Mon, 7 May 2012 11:53:58 -0700 (PDT)
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Date: 2012-05-07T11:53:58-07:00
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On Tuesday, May 8, 2012 12:37:44 AM UTC+9, Adam Beneschan wrote:
> 
> No.  But if you change the 1 to 1.0 then I think it will be OK.

Oops!
1.0 is correct.

> I'm not yet really familiar with the ins and outs of user-defined references
> and user-defined indexing.  But my guess is that the phrase in A.18.2(147.17)
> that says "Reference returns an object whose discriminant is an access value
> that designates the element designated by Position" means that Reference
> can't return an object whose discriminant designates a *copy* of the element.
> If I understand your question correctly, this means that the scenario you're
> concerned about can't happen.

Imagine an implementation of an reference-counted container.
Probably, it's like below:

function Reference (Container : Vector; Position : Cursor) return Ref_Type is
begin
   --  if data is shared, do copy-on-write to prepare for change
   --  (memory management and exclusive control is omitted in this pseudo-code)
   if Container.Data_Block.Ref_Count > 1 then
      Container.Data_Block.Ref_Count := Container.Data_Block.Ref_Count - 1;
      Container.Data_Block := new Data_Block_Type'(
         Length => Container.Data_Block.Length,
         Elements => (1 .. Length => Container.Data_Block.Elements (1 .. Length)),
         Ref_Count => 1);
   end if;
   -- return access value designated by Position
   return (Element => Container.Data_Block.Elements (To_Index (Cursor))'Access);
end Reference;

And, similar copy-on-write is inserted into Replace_Element and Update_Element.

In this container, Reference copies its elements as your saying. But the container comes to own copied elements, and Reference returns an access value that designates the element in same area. I hope that this is allowed for effective implementations.
Besides, if this is disallowed in Ada.Containers.Vectors, please assume my original container.
Surely you don't think that all reference-counted containers written by user are forbidden to implement user-defined indexing.