From: "Matthew Heaney" <matthew_heaney@acm.org>
Subject: Re: Array of Variant Records Question...
Date: 1999/09/13
Date: 1999-09-13T00:00:00+00:00 [thread overview]
Message-ID: <37dcf193@news1.prserv.net> (raw)
In-Reply-To: Pine.BSF.4.10.9909091910250.12989-100000@shell5.ba.best.com
In article <Pine.BSF.4.10.9909091910250.12989-100000@shell5.ba.best.com> ,
Brian Rogoff <bpr@shell5.ba.best.com> wrote:
> Matt, can you give examples where you find that explicit conversions are
> painful, or damage readability?
Consider the Interpreter pattern. An And_Exp is a Bool_Exp object with two
components, a left exp and a right exp.
The spec looks like this:
package Bool_Exps.And_Exps is
type And_Exp is new Bool_Exp with private;
function New_And
(L, R : access Bool_Exp'Class)
return Bool_Exp_Access;
...
private
type And_Exp is
new Bool_Exp with record
L, R : Bool_Exp_Access;
end record;
end Bool_Exps.And_Exps;
As you can see above, the constructor for the type accepts two access
parameters, and returns a pointer to the class-wide root type, which is
defined as
type Bool_Exp_Access is access all Bool_Exp'Class;
Here's the implementation of the constructor:
function New_And
(L, R : access Bool_Exp'Class)
return Bool_Exp_Access is
use And_Exp_Storage;
Exp : constant Exp_Access := New_Exp;
-- type Exp_Access is access all And_Exp;
begin
Exp.L := Bool_Exp_Access (L); -- (1)
Exp.R := Bool_Exp_Access (R); -- (2)
return Bool_Exp_Access (Exp); -- (3)
end New_And;
Observe the following:
(1) L is an access parameter. I have to manually convert
L : access Bool_Exp'Class
to an access value of type
type Bool_Exp_Access is access all Bool_Exp'Class;
What's the point?
(2) Same for R as for L in (1).
(3) Exp is an access object designating an And_Exp value. I have to
manually convert an access value from
type Exp_Access is access all And_Exp;
to an access value of
type Bool_Exp_Access is access all Bool_Exp'Class;
But what's the point? An And_Exp value is in And_Exp, but it's also in
Bool_Exp'Class.
The issue is that a tagged object kinda sorta has potentially many types:
it's own specific type, and the class-wide type of each of its ancestors.
I would prefer that all these manually conversions be done implicitly, so I
could just write:
function New_And
(L, R : access Bool_Exp'Class)
return Bool_Exp_Access is
use And_Exp_Storage;
Exp : constant Exp_Access := New_Exp;
begin
Exp.L := L;
Exp.R := R;
return Exp;
end New_And;
It's not a show-stopper though...
next prev parent reply other threads:[~1999-09-13 0:00 UTC|newest]
Thread overview: 69+ messages / expand[flat|nested] mbox.gz Atom feed top
1999-09-08 0:00 Array of Variant Records Question Bruce Detter
1999-09-08 0:00 ` Martin C. Carlisle
1999-09-08 0:00 ` Ted Dennison
1999-09-08 0:00 ` Thank you Bruce Detter
1999-09-08 0:00 ` Martin C. Carlisle
1999-09-08 0:00 ` Array of Variant Records Question Matthew Heaney
1999-09-08 0:00 ` Mike Silva
1999-09-08 0:00 ` Matthew Heaney
1999-09-09 0:00 ` Robert Dewar
1999-09-09 0:00 ` Matthew Heaney
1999-09-09 0:00 ` Matthew Heaney
1999-09-09 0:00 ` Mark Lundquist
1999-09-09 0:00 ` Robert Dewar
1999-09-09 0:00 ` Robert Dewar
1999-09-09 0:00 ` Brian Rogoff
1999-09-13 0:00 ` Matthew Heaney [this message]
1999-09-13 0:00 ` Robert A Duff
1999-09-13 0:00 ` Matthew Heaney
1999-09-13 0:00 ` Brian Rogoff
1999-09-14 0:00 ` Robert Dewar
1999-09-14 0:00 ` Brian Rogoff
1999-09-14 0:00 ` Robert Dewar
1999-09-09 0:00 ` Matthew Heaney
1999-09-10 0:00 ` Mark Lundquist
1999-09-10 0:00 ` Matthew Heaney
1999-09-11 0:00 ` Robert Dewar
1999-09-10 0:00 ` Robert Dewar
1999-09-10 0:00 ` Mark Lundquist
1999-09-10 0:00 ` Matthew Heaney
1999-09-11 0:00 ` Jean-Pierre Rosen
1999-09-14 0:00 ` "cast away const" (was Re: Array of Variant Records Question...) Mark Lundquist
[not found] ` <wccd7viiv59.fsf@world.std.com>
1999-09-22 0:00 ` Array of Variant Records Question Robert I. Eachus
[not found] ` <7rrmqd$l89@drn.newsguy.com>
[not found] ` <wcciu59n2uf.fsf@world.std.com>
1999-09-22 0:00 ` Robert I. Eachus
1999-09-23 0:00 ` Robert Dewar
1999-09-23 0:00 ` Robert I. Eachus
1999-09-11 0:00 ` Richard D Riehle
1999-09-13 0:00 ` Hyman Rosen
1999-09-14 0:00 ` Mark Lundquist
[not found] ` <7roohh$s6r@dfw-ixnews7.ix.netcom.com>
[not found] ` <37e01168@news1.prserv.net>
[not found] ` <7rp86o$c6h@dfw-ixnews3.ix.netcom.com>
[not found] ` <37E18CC6.C8D431B@rational.com>
[not found] ` <7rs8bn$s6@dfw-ixnews4.ix.netcom.com>
[not found] ` <wccemfxn15s.fsf@world.std.com>
1999-09-22 0:00 ` 'constant functions' and access constant params (was Re: Array of Variant Records Question...) Richard D Riehle
[not found] ` <37e2e58c@news1.prserv.net>
1999-09-22 0:00 ` Richard D Riehle
1999-09-22 0:00 ` Mark Lundquist
1999-09-22 0:00 ` Mark Lundquist
1999-09-22 0:00 ` Matthew Heaney
1999-09-22 0:00 ` Richard D Riehle
1999-09-22 0:00 ` Matthew Heaney
1999-09-22 0:00 ` Matthew Heaney
1999-09-23 0:00 ` Vincent Marciante
1999-09-23 0:00 ` Matthew Heaney
1999-09-24 0:00 ` Robert A Duff
1999-09-25 0:00 ` Matthew Heaney
1999-09-27 0:00 ` Richard D Riehle
1999-09-27 0:00 ` David Kristola
1999-09-27 0:00 ` Richard D Riehle
1999-09-23 0:00 ` Robert Dewar
1999-09-27 0:00 ` Richard D Riehle
1999-09-28 0:00 ` Robert Dewar
1999-09-28 0:00 ` "Competence" (was: 'constant functions' and access constant params) Ted Dennison
1999-09-28 0:00 ` Robert Dewar
1999-09-28 0:00 ` 'constant functions' and access constant params (was Re: Array of Variant Records Question...) Robert Dewar
1999-09-28 0:00 ` Richard D Riehle
1999-09-29 0:00 ` Robert Dewar
1999-09-29 0:00 ` Robert A Duff
1999-09-10 0:00 ` Proposed Ada features " Mark Lundquist
1999-09-10 0:00 ` Matthew Heaney
1999-09-10 0:00 ` tmoran
1999-09-09 0:00 ` Array of Variant Records Question Nick Roberts
1999-09-09 0:00 ` Robert Dewar
1999-09-09 0:00 ` Tucker Taft
1999-09-10 0:00 ` Nick Roberts
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