Dispatching and Child Packages

Dispatching and Child Packages

[Klatt, Nathan]

Is it possible to dispatch from within a child package?  The below code does
not work.  If I collapse the child packages into their respective parents,
it dispatches as expected.  (Ndk is an empty package.)

Thanks,

Nathan Klatt

----------------------------------------------------------------------------
package Ndk.Base is

    type Base is abstract tagged null record;

end Ndk.Base;
----------------------------------------------------------------------------
package Ndk.Base.Child is

    procedure Do_Something (This : Base'Class);
    procedure Real_Do_Something (This : Base) is abstract;

end Ndk.Base.Child;
----------------------------------------------------------------------------
package body Ndk.Base.Child is

    procedure Do_Something (This : Base'Class) is
    begin
        Real_Do_Something (This);
    end Do_Something;

end Ndk.Base.Child;
----------------------------------------------------------------------------
with Ndk.Base;

package Ndk.Sub is

    type Sub is new
       Ndk.Base.Base with null record;

end Ndk.Sub;
----------------------------------------------------------------------------
package Ndk.Sub.Child is

    procedure Real_Do_Something (This : Sub);

end Ndk.Sub.Child;
----------------------------------------------------------------------------
with Ada.Text_Io;

package body Ndk.Sub.Child is

    procedure Real_Do_Something (This : Sub) is
    begin
        Ada.Text_Io.Put_Line ("Sub doing something...");
    end Real_To_String;

end Ndk.Sub.Child;
----------------------------------------------------------------------------

Compiler error:
Real_Do_Something (This); is a call on an abstract subprogram which should
be a dispatching call [RM_95 3.9.3(7)]

RM_95 3.9.3(7) says...
A call on an abstract subprogram shall be a dispatching call;
nondispatching calls to an abstract subprogram are not allowed.

Re: Dispatching and Child Packages

[Simon Wright]

"Klatt, Nathan" <nklatt@excite.com> writes:

> Is it possible to dispatch from within a child package?  The below code does
> not work.  If I collapse the child packages into their respective parents,
> it dispatches as expected.  (Ndk is an empty package.)

So why didn't you include it :-)

> ----------------------------------------------------------------------------
> package Ndk.Base is
> 
>     type Base is abstract tagged null record;
> 
> end Ndk.Base;
> ----------------------------------------------------------------------------
> package Ndk.Base.Child is
> 
>     procedure Do_Something (This : Base'Class);
>     procedure Real_Do_Something (This : Base) is abstract;
>
> end Ndk.Base.Child;
> ----------------------------------------------------------------------------
> package body Ndk.Base.Child is
> 
>     procedure Do_Something (This : Base'Class) is
>     begin
>         Real_Do_Something (This);
>     end Do_Something;

GNAT says:

ndk-base-child.adb:6:09: cannot call abstract subprogram "Real_Do_Something"
ndk-base-child.adb:6:28: class-wide argument not allowed here
ndk-base-child.adb:6:28: "Real_Do_Something" is not a primitive operation of "Base"

and the last remark is your problem. You need to declare
Real_Do_Something in Ndk.Base.

> package body Ndk.Sub.Child is
> 
>     procedure Real_Do_Something (This : Sub) is
>     begin
>         Ada.Text_Io.Put_Line ("Sub doing something...");
>     end Real_To_String;
               ^^^^^^^^^   ahem! I don't think you compiled this before
                           posting!

> end Ndk.Sub.Child;

Re: Dispatching and Child Packages

[Peter Hend�n]

[-- Warning: decoded text below may be mangled, UTF-8 assumed --]
[-- Attachment #1: Type: text/plain, Size: 471 bytes --]

"Klatt, Nathan" wrote:
> Is it possible to dispatch from within a child package?  The below code
does
> not work.  If I collapse the child packages into their respective parents,
> it dispatches as expected.  (Ndk is an empty package.)

No, dispatching operations must be declared in the package where the type is
declared.

Regards,
Peter
--
Peter Hend�n           http://www.algonet.se/~phenden
ICQ: 14672398
Teknisk Dokumentation AB          http://www.tdab.com