Re: Visibility issue

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On 11/09/2020 12:37, Daniel wrote:

> I want to use a tagged type as a link to communicate users of a library, in the way to make one part visible to them and also to hide some content that is only needed for the implementing of the library.
> 
> Lets say i have the "Library" packages to expose things to users, and Implementing_Library packages to deal with the implementation:
> 
> package Library.Users is
>     type User_type is tagged with private;
>     procedure foo_public (S: User_Type);
> private
>     type User_Type is tagged with record
>        hide_content_to_user : Natural;
>     end record;
> function Access_to_hide_content return User_type;

Why is this a primitive operation? I would consider rather

    function Access_to_hide_content return User_type'Class;

In presence of potential overriding things like these are usually 
class-wide.

> So,  are there more ways to see the hided content of this tagged type without extending the type itself in a child package? I needed to see this content outside of Library package.

You can also use aggregation:

    type User_type is tagged with private;
private
    type User_type is tagged with record
       Implementation : Private_Stuff;
    end record;

Private_Stuff may have its own hierarchy. This pattern works well with 
limited types. You want the interface being non-limited, but the 
implementation of limited in order to control/prevent excessive copying 
and support in-place updates. So you make the interface a holder or a 
smart pointer to the reference-counted implementation.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de