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From: anon@att.net
Newsgroups: comp.lang.ada
Subject: Re: Ada2012 Invariants and obaque types
Date: Thu, 23 Jun 2011 16:21:03 +0000 (UTC)
Organization: Aioe.org NNTP Server
Message-ID: <itvp5e$apu$1@speranza.aioe.org>
References: 
 <239a78ad-0937-4a7a-8163-231430fd5ffe@k27g2000yqn.googlegroups.com>
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Date: 2011-06-23T16:21:03+00:00
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package P1 is

   type T1 is tagged private
      with Invariant => Is_Valid ( T1 ) ;

   function Create               return T1 ;
   function Is_Valid ( This : T1 ) return Boolean ;

private
   type Imp ; --  ???? Where is the completed type definition of "Imp"

              --  Another thing the syntax for Aspect Types may require
              --  that "Imp" be defined before T1, so "Imp" may be 
              --  required in this example to be public.

   type T1 is tagged record -- ???? You forgot the keyword "tagged" 
                       I : Imp ;
                     end record ;
end P1 ;


An Example of one that might work is from sem_ch13.ads

      package AspectVis is
        R_Size : constant Integer := 32;
   
        package Inner is

          type R is new Integer with
              Size => R_Size;

             F : R; -- freezes
             R_Size : constant Integer := 64;
             S : constant Integer := R'Size; -- 32 not 64
          end Inner;

       end AspectVis;

Check file Aspects.ads for a list of Aspects like "Invariant"


In <239a78ad-0937-4a7a-8163-231430fd5ffe@k27g2000yqn.googlegroups.com>, Martin <martin.dowie@btopenworld.com> writes:
>A fairly common Ada idiom is to define the full view of a private type
>using an incomplete declaration. Thus leaving the actual
>implementation to the package spec. Trying this out with the public
>view defined with an invariant lead to a compiler error - is this:
>
>a) expected?
>b) an unexpected consequence? or
>c) a compiler bug?
>
>Example:
>package P1 is
>   type T1 is tagged private
>      with Invariant => Is_Valid (T1);
>   function Create               return T1;
>   function Is_Valid (This : T1) return Boolean;
>private
>   type Imp;
>   type T1 is
>      record
>         I : Imp;
>      end record;
>end P1;
>
>gnatmake -ws -c -u -PH:\Ada\test_invariants\test_invariants.gpr p1.ads
>gcc -c -g -g -gnatE -gnatVn -gnato -fstack-check -gnat12 -gnatf -I- -
>gnatA H:\Ada\test_invariants\src\p1.ads
>p1.ads:11:04: type "Imp" is frozen at line 3 before its full
>declaration
>p1.ads:13:09: full declaration of private type "T1" defined at line 3
>must be a tagged type
>p1.ads:15:14: invalid use of type before its full declaration
>gnatmake: "H:\Ada\test_invariants\src\p1.ads" compilation error
>
>[2011-06-21 09:46:55] process exited with status 4 (elapsed time:
>00.26s)