Re: Getting the index for an element in mutually referencing containers

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

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.

Issues to consider are:

1. Two useless Reference_Type type where there should be none, if Ada 
had references.

2. The case of long living multiple references for which Reference_Type 
is no solution.

The case #1 is compiler-managed implicit references or copies. The case 
#2 is user-managed referential helper types. Reference-counted objects 
serve both #1 and #2, being really necessary only for #2.

P.S. It is a pity that Implicit_Dereference type does not have proper 
interface to work with *any* type instead of discriminated records. This 
is design fault when the use case #2 was taken for #1.

P.P.S. Issues with inheritance upon derivation from the container type, 
from the element type, and from both not even mentioned.

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