Re: Elaboration circularity with generics

[AdaMagica <christ-usch.grein@t-online.de>]

On 16 Jan., 16:09, Simon Wright <si...@pushface.org> wrote:
> AdaMagica <christ-usch.gr...@t-online.de> writes:
> > On 14 Jan., 23:46, Simon Wright <si...@pushface.org> wrote:
> >> AdaMagica <christ-usch.gr...@t-online.de> writes:
> >> > A unit can only be instantiated if it is fully elaborated.
> >> > I guess GNAT chose the elaboration order P'Spec, P.Q'Spec, P'Body
> >> > (crash since P.Q'body is not elaborated).
> >> > This dependence is not present for nongeneric units.
>
> >> > Perhaps addition of pragma Elaborate_Body to P.Q helps.
>
> >>    with P.Q;
> >>    pragma Elaborate (P.Q);          <<<<<<< does the trick for me
> >>    package body P is
>
> > Of course this works, but in my opinion, Elaborate_Body is better
> > because it has to be applied just once to P.Q, whereas Elaborate has
> > to be applied on every unit withing P.Q.
>
> > As a general rule of thumb I think Elaborate_Body should be applied
> > whenever a unit provides functions that are used in the spec of other
> > units providing initial values or constants, such like:
>
> > package P is
> >   pragma Elaborate_Body;  -- prevents elaboration error when F is
> > called
> >   function F (...) return T;
> > end P;
>
> > with P;  -- no need for "pragma Elaborate (P);"
> > package Q is
> >   V: [constant] T := P.F (...);
> > end Q;
>
> That may be true in general, but *in this case* it does not solve the
> problem;
>
>    generic
>       type T is private;
>    package P.Q is
>       pragma Elaborate_Body;
>       procedure Proc;
>    end P.Q;
>
> then
>
>    $ gnatmake test
>    gcc -c test.adb
>    gcc -c p.adb
>    gcc -c p-q.adb
>    gnatbind -x test.ali
>    error: elaboration circularity detected
>    info:    "p (body)" must be elaborated before "p (body)"
>    info:       reason: implicit Elaborate_All in unit "p (body)"
>    info:       recompile "p (body)" with -gnatwl for full details
>    info:          "p (body)"
>    info:             must be elaborated along with its spec:
>    info:          "p (spec)"
>    info:             which is withed by:
>    info:          "p.q (spec)"
>    info:             which is withed by:
>    info:          "p (body)"
>
>    gnatmake: *** bind failed.
>
> -gnatE still succeeds (provided you remember to rebuild the world!)

Ha, yes, because of the implicit Elaborate_All implied by the
nonstandard behaviour of GNAT (I didn't think of this connection).
Obviously with standard Ada behaviour (-gnatE), GNAT can find a good
order without the pragma (there is no Elaborate_All in this case).

But I think the rule of thumb should be followed nevertheless. (It's
only a rule of thumb because Elaborate_Body is sometimes impossible.)