Re: Generic default parameters

[Stephen Leake <stephen.a.leake.1@gsfc.nasa.gov>]

Dmitry A. Kazakov <mailbox@dmitry-kazakov.de> writes:

> A concrete example. Let we want to implement smart pointers / handles.
> It could go as follows:
> 
> package Object is
>    type Entity is new
>       Ada.Finalization.Limited_Controlled with
>    record
>       Use_Count : Natural := 0; -- Reference count
>    end record;
> ...
> end Object;
> 
> generic
>    type Object_Type (<>) is abstract new Entity with private;
>    type Object_Type_Ptr is access all Object_Type'Class;
> package Object.Handle is
>    type Handle is tagged private;
> ...
> private
>    type Handle is new Ada.Finalization.Controlled with record
>       Ptr : Object_Type_Ptr := null;
>    end record;
> end Object.Handle;
> 
> Now the problem, how to instantiate Object.Handle having Object_Type
> private? Let we have:
> 
> package IO is
> ...
> private
>    type File_Descriptor is new Entity with ...;
>    type File_Descriptor_Ptr is access all File_Descriptor'Class;
> end IO;
> 
> package IO.Handles is
>    package Handles is
>        new Object.Handle (File_Descriptor, File_Descriptor_Ptr);
>            -- Illegal, File_Descriptor is not visible here
>    subtype IO_Handle is Handles.Handle;
>    ...
> end IO.Handles;

Simple. Make IO.Handles a private package:

private package IO.Handles is

Now the only places IO.Handles.IO_Handles can be seen are the same
places IO.File_Descriptor can be seen.

Hmm, perhaps you _wanted_ IO.Handles.IO_Handle to be public, so
clients could do stuff. Simple :

with Object.Handle;
package IO.Handles is
   type IO_Handle is private;
private
   package Handles is new Object.Handle (File_Descriptor, File_Descriptor_Ptr);
           -- Illegal, File_Descriptor is not visible here
   type IO_Handle is new Handles.Handle with null record;

end IO.Handles;


-- 
-- Stephe