Re: How to hide type internals

["Alex R. Mosteo" <devnull@mailinator.com>]

Gerd wrote:

> Hi,
> 
> I would like to write a package that exports functions using a special
> type, without showing the details in the spec (even not in the private
> part).
> In Modula-2 one can declare opaque types (which mostly are pointers but
> also can be any type of word size).

I've done this in occasions where I want to have a multi-platform package,
so switching bodies is enough, keeping the same spec file. Like this:

package Blah is

   type External is private;

   -- Operations on external

private

   type Internal; -- forward declaration

   type Internal_Acess is access all Internal;

   type External is record
      Ptr : Internal_Access;
   end record;

end;

And then you define Internal in the body.

With Ada05 you can even get rid of Internal_Access and simply have

type External is record 
   Ptr : access Internal;
end record;

Of course you can have reference counting, deep copying or whatever making
External a Controlled type, so you can have a proper abstraction (This will
be probably a requisite for initialization anyways...)

Hope this helps.