Re: Hiding a type

["Frank J. Lhota" <NOSPAM.FrankLho@rcn.com>]

"Simon Wright" <simon@pushface.org> wrote in message
news:x7vd6tziy2c.fsf@pushface.org...
> "Frank J. Lhota" <NOSPAM.FrankLho@rcn.com> writes:
>
> > package Test is
> >      type Public_Name is limited private;
> >      function Initialize( File_Name : String ) return Public_Name;
> >      procedure Process( Input : Public_Name );
> > private
> >      type Private_Type is record ... end record;
> >      type Public_Name is access all Private_Type;
> > end Test;
>
> I don't know if this will compile, but it's certainly not possible to
> use it; if Public_Name is _limited_ private, clients have no
> assignment operation.

You've got a good point, but what you say is true of any implementation of
Public_Name. One fix would be to make Initialize a procedure, as follows:

package Test is
     type Public_Name is limited private;
     procedure Initialize( Item : in out Public_Name; File_Name : in
String );
     procedure Process( Input : Public_Name );
private
     ...
     type Public_Name is ...;
end Test;

This change is probably necessary irrespective of the implementation of
Public_Name.

Given further clarification by Ryan, I would recommend the following:

package Test is
     type Public_Name is limited private;
     procedure Initialize( Item : in out Public_Name; File_Name : in
String );
     procedure Process( Input : Public_Name );
private
     -- May want to declare the following type as abstract.
     type Private_Root_Type is tagged record ... end record;
     type Public_Name is access all Private_Root_Type'Class;
end Test;