Re: Passing an instance of a generic child as a generic parameter

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

On Sat, 14 Jun 2008 22:54:46 +0200, Dennis Hoppe wrote:

> I'm trying to pass an instance of a generic child unit to another 
> generic package, but the only syntax, I could found specifies a concrete 
> generic package.

Ada does not have interfaces of packages. A generic child package is not an
implementation of the parent's package interface, which could then be
referenced as a formal parameter of some other generic package. So the
answer is no.

But it seems to me that your example does not require that stuff:

> Let's first have a look at my pathological example:
> 
> -- GENERIC PARENT 'other' ADS
> generic
>    B : Integer;
> package Other is
>    type Object is tagged null record;
>    procedure Solve (Obj : Object);	
> end Other;
> 
> 
> -- GENERIC PARENT 'other' ADB
> with Ada.Text_IO;
> 
> package body Other is
>    procedure Solve (Obj : Object) is
>    begin
>      Ada.Text_IO.Put_Line ("solved");
>    end;
> end Other;
>  
> -- GENERIC CHILD of 'other' ADS
> with Other;
> 
> generic
>    -- omitted
> package Other.Child is
>    type Instance is new Other.Object with null record;	
> 	
>    overriding
>    procedure Solve (Obj : in Instance);
> end Other.Child;
> 
> -- GENERIC CHILD of 'other' ADB
> with Ada.Text_IO;
> 
> package body Other.Child is
>    procedure Solve (Obj : in Instance) is
>    begin
>      Ada.Text_IO.Put ("other child");
>    end;
> end Other.Child;
> 
> -- GENERIC PARENT 'parent' ADS
> with Other;
> 
> generic
>    A: Integer;
>    with package XX is new Other (A);

    type Object is new XX.Object with null record;

> parent Parent is
>    type Object is abstract tagged null record;

   remove this

>    procedure Print (Obj: in Object; X : in XX.Object'Class);
> end Parent;

Now Parent takes an instance of Other, which determines the class of
Objects to use. The class is parametrized by A. The formal type Object is
an instance from the class to deal with.

BTW, you could also remove the parameter A:

generic
   with package XX is new Other (<>); -- Brings some A with it
   type Object is new XX.Object with null record;

> parent Parent is

[...]

> procedure Test is
>    A : Integer := 2;
>    package O is new Other (A);
>    package E is new O.Child;
>    package P is new Parent (A, E);  -- O is replaced by the child E
>    ..                               -- compiler error expected

   package P is new Parent (A, O, E.Instance);

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