Re: Run-Time Type Assignment

[SIMON Claude <claude.simon@equipement.gouv.fr>]

Does a "limited access procedure" could be THE solution ?

Claude Simon

Ben Brosgol wrote:
> 
> Bob Duff wrote:
> 
> > > > GNAT supports downward closures via the 'Unrestricted_Access
> attribute,
> > > > but that's not Ada (unfortunately).
> > >
> > > What about using just the plain old 'Access attribute to pass a pointer
> to
> > > the procedure as a parameter? That is supported by all Ada 95 compilers.
> >
> > No, 'Access doesn't work.  Typically, the loop body (the procedure to be
> > passed as a parameter) is more nested than the iterator, which makes
> > 'Access illegal.
>  ... snip ...
> > GNAT's 'Unrestricted_Access attribute works in this case, but it is
> > nonportable and dangerous.  The question was whether Pascal supports
> > features not supported by Ada: in this case, it does; the feature is
> > portable and safe (compared to 'Unrestricted_Access).
> 
> The reason it is safe to pass nested subprograms as parameters in Pascal is
> that Pascal does not allow subprograms to be used as values for variables or
> placed in data structures.  GNAT's 'Unrestricted_Access is just as safe,
> provided that you abide by the Pascal restrictions.  (I.e.,
> Unrestricted_Access still entails the other checks required by the language;
> e.g. static subtype conformance for corresponding formals, matching
> conventions.)  For example:
> 
> procedure Test_Unrestricted_Access is
>   type Acc is access procedure;
>   Ref : Acc;
>   procedure P1(N : Integer) is begin null; end P1;
>   procedure P2;
>   pragma Convention(C, P2);
>   procedure P2 is begin null; end P2;
> begin
>   Ref := P1'Unrestricted_Access; -- error
>   Ref := P2'Unrestricted_Access; -- error
>   declare
>     procedure Q is begin null; end Q;
>   begin
>     Ref := Q'Unrestricted_Access; -- OK
>     Ref := Q'Access;  --error
>   end;
> end Test_Unrestricted_Access;
> 
> The lines indicated by the "-- error" comments were diagnosed as
> compile-time errors by GNAT.
> 
> As for portability, if you are using GNAT why would you ever want to move to
> some other compiler? :-)
> 
> Ben Brosgol
> Ada Core Technologies