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From: Matthew Heaney <matthew_heaney@acm.org>
Subject: Re: run-time type identification
Date: 1998/09/03
Message-ID: <m33ea965l0.fsf@mheaney.ni.net>#1/1
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Sender: matt@mheaney.ni.net
References: <01bdd72e$49ee66b0$f330ea9e@ukp03332>
 <m367f58j5g.fsf@zaphod.enst.fr> <01bdd73c$082f0230$f330ea9e@ukp03332>
NNTP-Posting-Date: Thu, 03 Sep 1998 06:57:51 PDT
Newsgroups: comp.lang.ada
Date: 1998-09-03T00:00:00+00:00
List-Id: <comp.lang.ada>

"Bob Fletcher" <bob@radge.globalnet.co.uk> writes:

> Yeah, this is the problem, there's no way of ensuring the safety of the
> operation.

No.  If the actual type isn't in A2'Class, then you'll get
Constraint_Error.

> Also, in the code you give, are you certain that any record fields specific
> to the class A2 would be copied by the assignment to the constant? It seems
> likely to me that they would be lost. 

No.

> I imagine that invoking Z.All, in your example, will return only the
> class A part of the object, regardless of whether it is in fact an
> object of class A2.

No.  The type cast to A2 gives you an A2 view of the object.  If O
really is of type A2 (or its descendent), then you'll get all the
fields.

The thing I don't like about Samuel's example is that it makes a copy of
the object:

>    Y : constant A2 := A2 (Z.all);

There is no need for a copy.  Just do a "view conversion":

   Y : A2 renames A2 (Z.all);

or better

   Y : A2'Class renames A2'Class (Z.all);


> As you rightly say, there will be problems if you try to do this in a
> situation where you don't know whether the object in question is of the
> base class or one of it's derived classes, which is what would really be
> useful. The C++ dynamic_cast operator returns a zero (null) pointer if the
> object being pointed to is not of the correct class.

No, there will be _no_ problems.  This is Ada.  If the actual type of
Z.all isn't in A2'Class, then you'll get Constraint_Error.