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From: Stephen Leake <Stephen.Leake@gsfc.nasa.gov>
Subject: Re: Generic children, instances and visibility...
Date: 1997/04/09
Message-ID: <334BA5F2.4111@gsfc.nasa.gov>#1/1
X-Deja-AN: 231815619
References: <JSA.97Apr8212412@alexandria>
Organization: NASA Goddard Space Flight Center -- Greenbelt, Maryland USA
Reply-To: Stephen.Leake@gsfc.nasa.gov
Newsgroups: comp.lang.ada
Date: 1997-04-09T00:00:00+00:00
List-Id: <comp.lang.ada>


Jon asks about visibility into a private type within a generic body. I
constructed a compilable example:

package Parent is
   type T is abstract tagged limited private;
private
   type T is abstract tagged limited record
      F : Integer;
   end record;
end Parent;

generic
   type Nt is new T with private;
package Parent.Child is

   procedure Foo (Object : in out Nt);

end Parent.Child;

package body Parent.Child is
   procedure Foo (Object : in out Nt)
   is begin
      Object.F := 1;
   end Foo;
end Parent.Child;

Jon then tries to instantiate this in two ways:

with Parent.Child;
with Inst_Types;
procedure test is
   package PC_Inst is new Parent.Child (Inst_Types.It);
begin
   null;
end test;

with Parent.Child;
with Inst_Types;
package Parent.PC_Inst is new Parent.Child (Inst_Types.It);

gnat says neither case allows visibility:

gnatmake test.adb
gcc -c test.adb
test.adb:4:04: instantiation error at parent-child.adb:4
test.adb:4:04: undefined selector for type "It" defined at
inst_types.ads:3
gnatmake: "test.adb" compilation error


gnatmake parent-pc_inst.ads
gcc -c parent-pc_inst.ads
parent-pc_inst.ads:3:01: instantiation error at parent-child.adb:4
parent-pc_inst.ads:3:01: undefined selector for type "It" defined at
inst_types.ads:3
gnatmake: "parent-pc_inst.ads" compilation error

So, hoping that a private child would have visibility, I tried a third
variation:

with Parent.Child;
with Inst_Types;
private package Parent.Test is
   type It is new Parent.T with null record;

   package PC_Inst is new Parent.Child (Inst_Types.It);

end Parent.Test;

It would seem that no generic can see inside type It; it is a limited
private type, period.
-- 
- Stephe