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From: Lawrence =?iso-8859-13?q?D=FFOliveiro?= <ldo@nz.invalid>
Newsgroups: comp.lang.ada
Subject: Re: In precision typing we trust
Date: Fri, 29 Aug 2025 03:53:41 -0000 (UTC)
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On Thu, 28 Aug 2025 10:24:54 +0200, Dmitry A. Kazakov wrote:

> On 2025-08-28 03:45, Lawrence D’Oliveiro wrote:
> 
>> But the function has to return the base class type, it cannot have
>> different return types dynamically.
> 
> Of course you can. That is the meaning of the term "class" and "class
> type" as a type which values are the values of the class members.

Assigning the result to a variable of type subclass involves dynamic type 
checking. Which you have been insisting so far is “untyped”.

>> Note how you have a switch-expression that uses the value of the
>> event-type field to return the corresponding type object, and I cast
>> the info pointer to point to that type. And then the “additional”
>> variable-length array can contain pointers to additional data,
>> depending on the event type.
> 
> That is not much OO as you have all alternatives static.

No, they are dynamic, not static. See below.

> But for the OO approach:
> 
>     function F return Abstract_Packet'Class is begin
>        case Kind_Of is
>           when Foo =>
>              return Foo_Packet; -- Derived from Abstract_Packet
>           when Bar =>
>              return Bar_Packet; -- Derived from Abstract_Packet
>        ...
> 
> Note also that one can return instances not knowing the specific type at
> all.

But they are still of that same “Abstract_Packet'Class”, which is the base 
class. You cannot specify “Foo_Packet'Class” as the return type on one 
call, “Bar_Packet'Class” on another call, etc.