Implement an interface with class wide operations in Ada 95 - oop

Question

I'm creating a program with Ada 95 and I have a problem. Specifically, I'm trying to implement a class which executes functors given as parameters.
The behavior I want to achieve is:
Declare an interface IF with a procedure Execute.
Derive from IF with a class C and implement Execute.
Create a class D which has a field that is an array of IF. Since IF cannot be instantiated, I use an array of access IF.
Instantiate an object of class D giving it several instances of C as parameters.
Call Execute on every instance of C contained in the array of D.
I've been able to implement the above and compile it, but when I execute it I obtain the error accessibility check failed when trying to assign an object of class C to a component of the array in D.
I know that the error I obtain is because the assignment I'm doing can lead to a danging pointer error according to the Ada policies, so my question is what is the proper way to implement this in Ada 95?
The source code is below. The error is raised in the file elevators.adb, in the procedure Add_Event_Handler, I have commented the statement that causes it.
Functors.ads
package Functors is
type IFunctor is abstract tagged null record;
procedure Execute(Self : in out IFunctor) is abstract;
end Functors;
Elevators.ads
with Functors; use Functors;
package Elevators is
NOT_A_FLOOR : constant := -1;
MAX_EVENT_HANDLERS : constant := 255;
type Floor is new Integer range NOT_A_FLOOR .. 4;
type Elevator is private;
subtype Event_Handler is IFunctor'Class; --'
type Event_Handler_Index is new Integer range 0 .. MAX_EVENT_HANDLERS;
type Event_Handers is array(Event_Handler_Index) of access Event_Handler;
function Create_Elevator return Elevator;
procedure Add_Stop_Handler(Self : in out Elevator; Handler : access Event_Handler);
procedure Add_Moving_Handler(Self : in out Elevator; Handler : access Event_Handler);
procedure Add_Called_Handler(Self : in out Elevator; Handler : access Event_Handler);
procedure Add_Button_Pressed_Handler(Self : in out Elevator; Handler : access Event_Handler);
procedure Run_Simulation(Self : in out Elevator);
private
type Elevator is
record
Current_Floor : Floor := 0;
Is_Moving : Boolean := False;
Next_Floor : Floor := NOT_A_FLOOR;
Stop : Event_Handers := (others => null);
Moving : Event_Handers := (others => null);
Called : Event_Handers := (others => null);
Button_Pressed : Event_Handers := (others => null);
end record;
procedure On_Stop(Self : in out Elevator);
procedure On_Moving(Self : in out Elevator);
procedure On_Called(Self : in out Elevator);
procedure On_Button_Pressed(Self : in out Elevator);
procedure Add_Event_Handler(Self : out Event_Handers; Handler : access Event_Handler);
procedure Exec_All_Events(Self : in out Elevator; EH : in Event_Handers);
end Elevators;
Elevators.adb
with Ada.Text_IO; use Ada.Text_IO;
package body Elevators is
function Create_Elevator return Elevator is
elev : Elevator;
begin
return elev;
end;
procedure Add_Stop_Handler(self : in out Elevator; Handler : access Event_Handler) is
begin
Add_Event_Handler(self.Stop, Handler);
end;
procedure Add_Moving_Handler(self : in out Elevator; Handler : access Event_Handler) is
begin
Add_Event_Handler(self.Moving, Handler);
end;
procedure Add_Called_Handler(self : in out Elevator; Handler : access Event_Handler) is
begin
Add_Event_Handler(self.Called, Handler);
end;
procedure Add_Button_Pressed_Handler(self : in out Elevator; Handler : access Event_Handler) is
begin
Add_Event_Handler(self.Button_Pressed, Handler);
end;
procedure Run_Simulation(self : in out Elevator) is
begin
Put_Line("Floor: " & Floor'Image(self.Current_Floor)); --'
self.Next_Floor := 3;
On_Called(self);
On_Moving(self);
On_Stop(self);
end;
procedure On_Stop(self : in out Elevator) is
begin
self.Current_Floor := self.Next_Floor;
self.Is_Moving := False;
self.Next_Floor := NOT_A_FLOOR;
Put_Line("Stopped. Current floor = " & Floor'Image(self.Current_Floor)); --'
Exec_All_Events(self, self.Stop);
end;
procedure On_Moving(self : in out Elevator) is
begin
self.Is_Moving := True;
self.Current_Floor := NOT_A_FLOOR;
Put_Line("Moving to floor " & Floor'Image(self.Next_Floor)); --'
Exec_All_Events(self, self.Moving);
end;
procedure On_Called(self : in out Elevator) is
begin
Put_Line("Calling button pressed (" & Floor'Image(self.Next_Floor) & ")..."); --'
Exec_All_Events(self, self.Moving);
end;
procedure On_Button_Pressed(self : in out Elevator) is
begin
null;
end;
procedure Add_Event_Handler(Self : out Event_Handers; Handler : access Event_Handler) is
I : Event_Handler_Index := Event_Handler_Index'First; --'
begin
while I < Event_Handler_Index'Last loop --'
if Self(I) = null then
Self(I) := Handler; -- ======> The error is raised here <======
exit;
end if;
I := I + 1;
end loop;
end;
procedure Exec_All_Events(self : in out Elevator; EH : in Event_Handers) is
I : Event_Handler_Index := Event_Handler_Index'First; --'
begin
while I < Event_Handler_Index'Last loop --'
if EH(I) /= null then
EH(I).Execute;
end if;
I := I + 1;
end loop;
end;
end Elevators;
main.adb
with Ada.Text_IO; use Ada.Text_IO;
with Functors; use Functors;
with Elevators; use Elevators;
procedure Main is
type My_Functor is new IFunctor with
record
I : Integer := 0;
end record;
overriding
procedure Execute(Self : in out My_Functor) is
begin
Put_Line("Executing functor, I is " & Integer'Image(Self.I)); --'
Self.I := Self.I + 1;
end;
Generic_Functor : aliased My_Functor;
Elev : Elevator := Create_Elevator;
begin
Add_Stop_Handler(elev, Generic_Functor'Access); --'
Add_Moving_Handler(elev, Generic_Functor'Access); --'
Add_Called_Handler(elev, Generic_Functor'Access); --'
Run_Simulation(Elev);
end;
EDIT
I have done the following changes in order to fix the mentioned runtime error, but I still obtain the accessibility check failed.
elevators.ads
...
type Event_Handler_Generic_Ptr is access all Event_Handler;
type Event_Handers is array(Event_Handler_Index) of Event_Handler_Generic_Ptr;
...
elevators.adb
procedure Add_Event_Handler(Self : out Event_Handers; Handler : access Event_Handler) is
I : Event_Handler_Index := Event_Handler_Index'First; --'
begin
while I < Event_Handler_Index'Last loop --'
if Self(I) = null then
-- Notice the casting here
Self(I) := Event_Handler_Generic_Ptr(Handler); -- ======> The error is raised here <======
exit;
end if;
I := I + 1;
end loop;
end;

Answer 1

Since you store a pointer generated with 'Access in Event_Handlers, you must declare it with access all, so that it is a general access type:
type Event_Handers is array(Event_Handler_Index) of access all Event_Handler;
If you miss all, it is a pool-specific access type. See Ada 95 RM, 3.10 Access Types, (8) and (10). pool-specific access types may only hold pointers to objects allocated in a storage pool, which your object is not.