Re: Access to function returning class-wide type

[Georg Bauhaus <rm.dash-bauhaus@futureapps.de>]

Paweł 'Nivertius' Płazieński schrieb:

> package B is 
>         type Derived is new A.Abstracted with null record;
>         function Proc (N : Natural) return Derived;
> 
>         Failing_Object : A.The_Access_Type := B.Proc'Access;
>         --  will not compile, error: 'expected type The_Access_Type / found type
> access function Proc defined at ...'

(Compiler is right because the profile of B.Proc is different
from that of The_Access_Type (whose functions return Abstracted'Class,
not Derived)

In general, public access types lead to unneccessary complication;
tagged types are by reference already, so you could simply use
dispatching and a factory instead. Do you have spefific needs
for pointers to functions returning by-reference objects?

Or follow Dmitry's advice.


   package A is
      type Abstracted is abstract tagged null record;
      function Proc (N : Natural) return Abstracted is abstract;

      package Constructor is
         No_Suitable_Type: exception;

         function Proc_Wrapper(N : Natural) return Abstracted'Class;
         -- create objects of types in Abstracted'Class; the
         -- specific type depends on N and other things that are
         -- in scope in Constructor's body
      end Constructor;

   end A;

   with A;
   package B is
      type Derived is new A.Abstracted with null record;
      overriding
      function Proc (N : Natural) return Derived;
   end B;



   package body A is
      package body Constructor is

         function Proc_Wrapper(N : Natural) return Abstracted'Class is
         begin
            if N > 666 then  -- and possibly Some_Other_Condition ...
               declare
                  use B;
               begin
                  return Derived'(Proc(N));
               end;
            else
               --
               -- other cases TBD
               --
               raise No_Suitable_Type;
            end if;
         end Proc_Wrapper;

      end Constructor;
   end A;

   package body B is
      function Proc (N : Natural) return Derived is
      begin
         return Derived'(null record);
      end Proc;
   end B;


HTH
--
Georg Bauhaus
Y A Time Drain  http://www.9toX.de