From: "Stanley R. Allen" <s_allen@hso.link.com>
Subject: Derivation, discriminants, and views.
Date: 1999/07/12
Date: 1999-07-12T00:00:00+00:00 [thread overview]
Message-ID: <378A5C0B.EFD41854@hso.link.com> (raw)
Language lawyers and philosophers:
The packages Pack2 and Pack2.Child given below are treated differently
by two Ada 95 compilers. I am unsure about which one is correct. From
the partial view, type Basic is unconstrained. The full view of Basic
is constrained. The spec of Pack2.Child derives a new type from Basic
in the partial view (visible part) and adds a discriminant. This seems
like it should be illegal according to RM95 3.7(13) -- and that is
the interpretation of the compiler I am calling COMPILER A. COMPILER B
has a different interpretation; see the comments in the spec of package
Pack2.Child.
So, I have three questions: which compiler is correct? If COMPILER B
is correct, which rules make it so? And if COMPILER B is correct,
doesn't this represent a language anomoly, because the 'clients' of
package heirarchy Pack2 'see' a violation of RM95 3.7(13)?
Stanley Allen
mailto:s_allen@hso.link.com
------------------------------------------------------------
package Pack2 is
type Basic (<>) is abstract tagged limited private;
procedure Increment (B : in out Basic'Class);
procedure Operation (B : in out Basic) is abstract;
private
type Basic is abstract tagged limited
record
Item : Integer;
end record;
end Pack2;
package body Pack2 is
procedure Increment (B : in out Basic'Class) is
begin
B.Item := B.Item + 1;
end Increment;
end Pack2;
package Pack2.Child is
type Fancy (N : access Integer) is new Basic with private;
-------------------------------------------^
-- Fancy is derived from an unconstrained type, in this view
--
-- COMPILER A complains, referencing RM95 3.7(13)
-- COMPILER B accepts, here is a comment from vendor B:
--
-- "COMPILER B is correct here, the declaration of derived type Fancy
-- in your example is legal. The full type is derived from a
-- constrained view of the parent type and doesn't violate the
-- stated rule."
--
-- COMPILER B reports an error if Fancy is not a private type, but is
-- declared instead as this:
--
-- type Fancy (N : access Integer) is new Basic with null record;
--
type Fancy_Ptr is access all Fancy'Class;
function New_Fancy (Init : access Integer) return Fancy_Ptr;
procedure Operation (F : in out Fancy); -- override
private
type Fancy (N : access Integer) is new Basic with null record;
-------------------------------------------^
-- Fancy is derived from a constrained type, in this view
--
end Pack2.Child;
package body Pack2.Child is
function New_Fancy (Init : access Integer) return Fancy_Ptr is
Temp : Fancy_Ptr;
begin
Temp := new Fancy (N => Init);
Basic (Temp.all).Item := Init.all;
return Temp;
end New_Fancy;
procedure Operation (F : in out Fancy) is
begin
null;
end Operation;
end Pack2.Child;
---------------------------------------------------------------
next reply other threads:[~1999-07-12 0:00 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
1999-07-12 0:00 Stanley R. Allen [this message]
1999-07-14 0:00 ` Derivation, discriminants, and views Tucker Taft
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