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From: Matthew Heaney <matthew_heaney@acm.org>
Subject: Re: generic formal object of class-wide type
Date: 1999/04/29
Message-ID: <m390bbs908.fsf@mheaney.ni.net>#1/1
X-Deja-AN: 472341381
References: <m3d80nsu54.fsf@mheaney.ni.net> <7g9kga$dl1@hobbes.crc.com>
NNTP-Posting-Date: Thu, 29 Apr 1999 09:05:28 PDT
Newsgroups: comp.lang.ada
Date: 1999-04-29T00:00:00+00:00
List-Id: <comp.lang.ada>

"David C. Hoos, Sr." <david.c.hoos.sr@ada95.com> writes:

> Matthew Heaney wrote in message ...
> >When I try to compile package Q (see below), I get this error:
> >
> >q.ads:3:25: type of actual does not match type of "O"
> >
> >Why?
> >
> The actual needs to be explicitly converted to the type of the
> formal, like so:
> 
> package Q is new GQ (P.T'Class (P.C.O));

Yes, but why?

If I have a subprogram with a parameter of a class-wide type, like this:

procedure Op (O : T'Class);

and NT is a type in T'Class, then I can do this:

declare
  O : NT;
begin
  Op (O);
end;


No conversion to type T'Class is required in order to call Op.

It is inconsistent that a type conversion is required in order pass the
object as a generic actual.  I'd like to know the specific rule, and a
rationale for the inconsistency.