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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: Ada Annoyances
Date: Sun, 17 Dec 2017 18:27:07 +0100
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Xref: reader02.eternal-september.org comp.lang.ada:49508
Date: 2017-12-17T18:27:07+01:00
List-Id: <comp.lang.ada>

On 2017-12-17 17:24, Mehdi Saada wrote:
> So, some of you think the stupidity is on the side of certain
> customers, to ban tagged types as a whole, as objetively there is no
> added cost when we don't use 'Class types ? Rather than on the language
> designers', for tagging controlled type and cursor etc by default ?
> Can someone answer simply, if there is or not, really no difference
> (save for some more bits for the tag) between regular records types and
> tagged types, when not using class types ?

There is a huge difference even without class-wide objects. Some things 
from the top:

1. You can derive a new type from a tagged type, which you cannot do 
from a non-tagged type

2. You can clone a non-tagged type, which you cannot do with a tagged type.

3. The meaning of abstract operations like

    procedure Foo (X : T) is abstract;

for tagged and non-tagged types is totally different

4. Type polymorphism is still available for tagged types without 
class-wide objects as you can have generic formal parameters:

    generic
       type X is new Y with private;

5. You can inherit operations of tagged types.

6. You cannot declare certain operation signatures with tagged types due 
to multiple-dispatch issues.

7. Declaration of tagged types are subject of freezing rules.

8. Visibility of primitive operations is radically different. You can 
reach primitive operation even if they are not visible.

9. Certain signatures behave differently if arguments are tagged, e.g. 
when several arguments are controlled the tags must be same.

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