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From: "Ben Brosgol" <brosgol@world.std.com>
Subject: Re: Run-Time Type Assignment
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Date: 2002-08-29T13:32:22+00:00
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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