Re: 'Read for pointers

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

"Stephen Leake" <stephen.a.leake.1@gsfc.nasa.gov> wrote in message
news:uaef1lxlw.fsf@gsfc.nasa.gov...
> "Pat Rogers" <progers@NOclasswideSPAM.com> writes:
>
> > "Ted Dennison" <dennison@telepath.com> wrote in message
> > news:8lndgv$1om$1@nnrp1.deja.com...
>
> > > package Example is
> > >    type Handle is private;
> > >
> > >    ...
> > > private
> > >
> > >    type Instance;
> > >
> > >    type Handle is access all Instance;
> > >
> > > end Example;
> > >
> > > But there's a problem. Following is the profile for 'Read:
> > >
> > >    procedure Read
> > >      (Stream : access Ada.Streams.Root_Stream_Type'Class;
> > >       Item   : out    Handle
> > >      );
> > >    for Handle'Read use Read;
> > >
> > > The problem is that Item is an *out* parameter. That means I
won't
> > have
> > > acces to the pointer's old value inside Read.
> >
> > You can read mode out parameters in Ada 95 (unlike Ada 83).  (Am I
> > missing something?)
>
> Yes, you are missing something.
>
> 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.

--
Pat Rogers                            Consulting and Training in:
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