Re: 'Read for pointers

["Pat Rogers" <progers@NOclasswideSPAM.com>]

"Stephen Leake" <stephen.a.leake.1@gsfc.nasa.gov> wrote in message
news:uem4awakr.fsf@gsfc.nasa.gov...
> "Pat Rogers" <progers@NOclasswideSPAM.com> writes:
>
> > "Stephen Leake" <stephen.a.leake.1@gsfc.nasa.gov> wrote in message
> > news:uaef1lxlw.fsf@gsfc.nasa.gov...
> > > Suppose the user writes:
> > >
> > > My_Handle : Handle;
> > >
> > > (somehow, My_Handle is given some value)
> > >
> > > Read (Stream, My_Handle);
> > >
> > > The value of My_Handle is _not_ copied to Item at the procedure
> > call.
> > > That only happens for "in" or "in out" parameters, not "out"
> > > parameters.
> >
> > Have a look at RM 6.4.1{12,13} for a surprise.
>
> I stand corrected. I guess the Ada designers were trying to prevent
> garbage pointers.

That's what the AARM indicates.

> I think I'd prefer the standard "initialized to
> null" behavior here, for consistency.
>
> I guess it's easy enough to remember that "out" is the same as "in
> out" for access types, and composite types with discriminants.
>
> On further thought, I can't think of a time when this behavior is
> actively bad; it's just not what I thought the rules were.

There is, however, an interesting side issue from the part of RM
6.4.1{13} that says no constraint check is done on the incoming value.
Keith Thompson (no, the other Keith:) submitted a question and this
bit of code that led to AI95-00110:

   with Ada.Text_IO; use Ada.Text_IO;
   procedure Foo is
      type Pointer is access String;
      P: Pointer := new String'("hello");
      subtype Pointer_10 is Pointer(1 .. 10);

      procedure Proc(Param: out Pointer_10) is
      begin
         Put_Line("Param'Length = " & Integer'Image(Param'Length));
      end Proc;

   begin
      Proc(P);
   end Foo;

Param'Length is clearly 5, which violates the constraint but is not
checked.  (Evaluation of the attribute is another matter.)  The AI is
pretty interesting reading (but then I thought the Implementer's Guide
was interesting too:).

--
Pat Rogers                            Consulting and Training in:
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Adam ... does not deserve all the credit; much is due to Eve, the
first woman, and Satan, the first consultant.
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