Re: Ada2012 Invariants and obaque types

[Martin <martin.dowie@btopenworld.com>]

On Jun 21, 9:53 am, Martin <martin.do...@btopenworld.com> wrote:
> 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)

A more correct version is:

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 Imp_Ref is not null access Imp;
   type T1 is tagged
      record
         I : Imp_Ref;
      end record;
end P1;

Resulting in:

gnatmake -d -PH:\Ada\test_invarients\test_invarients.gpr p1.ads
gcc -c -g -g -gnatE -fstack-check -gnato -gnatf -fcallgraph-info=su,da
-gnat12 -I- -gnatA H:\Ada\test_invarients\src\p1.ads
p1.ads:7:04: type "Imp" is frozen at line 2 before its full
declaration
gnatmake: "H:\Ada\test_invarients\src\p1.ads" compilation error

[2011-06-21 11:50:40] process exited with status 4 (elapsed time:
00.42s)