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From: Maciej Sobczak <no.spam@no.spam.com>
Newsgroups: comp.lang.ada
Subject: Re: Dynamically tagged expression required
Date: Fri, 09 Dec 2005 10:09:04 +0100
Organization: CERN - European Laboratory for Particle Physics
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 <2852224.m2vuDFxfOX@linux1.krischik.com>
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Xref: g2news1.google.com comp.lang.ada:6791
Date: 2005-12-09T10:09:04+01:00
List-Id: <comp.lang.ada>

Martin Krischik wrote:

>>    A : Shape;
>>    B : Shape'Class := A;             -- (1)
>>    C : Triangle := (SideLen => 7);

>>begin
>>
>>    A := C;                           -- (2)
>>    B := C;                           -- (3)
>>
>>end Hello;

>>Why is (1) allowed?
> 
> because it is an initialization.

OK, so that explains why it's possible to use Shape'Class as a parameter 
in subroutine. Every time the subroutine is called, the parameter is 
*initialized* with the value, and therefore it can be different for each 
call:

procedure Draw(S : Shape'Class);

T : Triangle;
R : Rectangle;

and later:

Draw(T);   -- S initialized with T
Draw(R);   -- S initialized with R

Right?

>>Why is (3) not allowed?
> 
> because it is an assignment.

And this is where I want to dig deeper.

> http://en.wikibooks.org/wiki/Ada_Programming/Object_Orientation

This page says:

"The Class type is a real data type. You can declare variables of the 
class-wide type, assign values to them [...]"

"Assign values to them" is a problem here.
As the subject of my original post says (and what compiler told me), 
"dynamically tagged expression required".

What can I provide as a dynamically tagged expression?

It looks like some ShapeClassPtr.all is OK there, except that I get the 
CONSTRAINT_ERROR, if the new value has a different tag. This observation 
agrees with what Dmitry said - the tag of the class-wide object cannot 
be changed.

What other dynamically tagged expression can be provided as a right-hand 
side of the (default) assignment to the object of class-wide type?
Is this useful in practice?

Regards,

-- 
Maciej Sobczak : http://www.msobczak.com/
Programming    : http://www.msobczak.com/prog/