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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: Redispatching
Date: Sat, 19 Apr 2014 12:58:05 +0200
Organization: cbb software GmbH
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Xref: number.nntp.dca.giganews.com comp.lang.ada:185846
Date: 2014-04-19T12:58:05+02:00
List-Id: <comp.lang.ada>

On Sat, 19 Apr 2014 11:17:22 +0200, Georg Bauhaus wrote:

> On 18/04/14 22:51, Dmitry A. Kazakov wrote:
> 
>> Consider this set of declarations:
>>
>>     type T is ...;
>>     procedure Bar (X : T);
>>
>>     type S is new T ...;
>>     procedure Foo (X : S);
>>
>> Now the following is illegal:
>>
>>     procedure Bar (X : T) is
>>     begin
>>        X.Foo;  -- Compile error
>>     end Bar;
>>
>> X in Bar is of the type T and of no other type.
>>
>> You can convert X using so-called "view-conversion" to an object of another
>> type, e.g. S or T'Class. This conversion is a type conversion. Yes, it has
>> certain properties when X is tagged (and thus by-reference), such as
>> reusing the old object for the new one. These properties are irrelevant to
>> the type semantics though. You get an object of a different type.
> 
> Actually, in Ada, you will predictably get the object that is passed
> for X, which "is" of its type. (As usual, the discussion hinges on
> the meaning of "is".) This provides the programmer with more semantic
> possibilities:
> 
>     procedure Bar (X : T) is
>     begin
>        S(T'Class(X)).Foo;  -- No compile error
>     end Bar;
> 
> This type conversion (downwards) is legal,

So would an instantiation of Unchecked_Conversion.

> If you think this is improper design,

Yes I do. You should abstain design of this kind as much as you would ones
using Unchecked_Conversion.

> why not outline an algorithm that transfers any
> such program into one that demonstrably dispenses with downward conversions.

Easily. The algorithm is. Type:

$ rm -f bar.adb

then take a cup of coffee  and review your design.

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