From: "Peter C. Chapin" <pchapin@sover.net>
Subject: Subtype conformance... not what I was expecting.
Date: Tue, 25 Jul 2006 23:02:56 -0400
Date: 2006-07-25T23:02:56-04:00 [thread overview]
Message-ID: <44c6db66$0$2928$4d3efbfe@news.sover.net> (raw)
I'm experimenting with Ada's handling of access to subprogram types. I
was surprised to discover that the following example does not work (I'm
using GNAT GPL 2006):
procedure Check is
subtype Narrow is Integer range -10..10;
type Function_Ptr is access function(X : Narrow) return Integer;
function F(Para : Integer) return Narrow is
begin
return Para;
end F;
G : Function_Ptr := F'Access;
Result : Integer;
begin
Result := G(0);
end Check;
The compiler complains about the initialization of G with F'Access
saying that it is not "subtype conformant." However, I believe this
initialization would be type safe. Since F's argument types are super
types of G's argument types, there is no context where G can be called
that would violate the constraints on the underlying arguments of F.
Similarly since F's return type is a subtype of G's return type,
anything F might return would be acceptable as a return from G. I
assumed that this was what subtype conformance was about, but apparently
not.
In fact, GNAT appears to require the argument and return subtypes to
match exactly. However, this seems overly restrictive. I'm curious about
the rationale for this restriction.
Peter
next reply other threads:[~2006-07-26 3:02 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2006-07-26 3:02 Peter C. Chapin [this message]
2006-07-26 6:58 ` Subtype conformance... not what I was expecting Craig Carey
2006-07-26 7:54 ` Dmitry A. Kazakov
2006-07-26 14:32 ` Peter C. Chapin
2006-07-26 15:38 ` Georg Bauhaus
2006-07-27 0:02 ` Peter C. Chapin
2006-07-26 18:40 ` Dmitry A. Kazakov
2006-07-27 0:00 ` Peter C. Chapin
2006-07-26 8:06 ` Georg Bauhaus
2006-07-26 14:15 ` Peter C. Chapin
2006-07-26 21:23 ` Adam Beneschan
replies disabled
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox