Re: Calling a record type's methods (functions or procedure) when record is in an array

[Optikos <ZUERCHER_Andreas@outlook.com>]

On Thursday, January 23, 2020 at 9:00:18 AM UTC-6, joak...@kth.se wrote:
> > And now, you could change the type of the components to Point to Integer, remove the type-casts,
> > and recompile the package Example **without having to recompile  dependent packages** because
> > they are dependent only on the visible portion of the package.
> 
> Using Taft types introduced in Ada95 it is possible to add new components to a record type without
> recompilation of dependent packages. Here is an Ada 2005 example:
> 
> private with System.Storage_Elements;
> 
> package Cars is
> 
>    type Any_Car (<>) is limited private;
>    --  This type is defined with unknown discriminant in order to make
>    --  sure instances are properly initialized by allocator function.
> 
>    function Allocate_Car return Any_Car;
>    --  The allocator function.
> 
>    type Car_Passenger_Count is range 0 .. 5;
>    
>    function Passenger_Count (Car : Any_Car) return Car_Passenger_Count;
> 
>    procedure Set_Passenger_Count
>      (Car   : in out Any_Car;
>       Value : Car_Passenger_Count);
> 
> private
> 
>    type Taft_Car;
> 
>    type Taft_Car_Ptr is access all Taft_Car;
> 
>    type Any_Car
>      (Offset : System.Storage_Elements.Storage_Offset)
>    is limited record
>       Allocated_Memory : aliased
>         System.Storage_Elements.Storage_Array (1 .. Offset);
>       Reference : Taft_Car_Ptr;
>    end record;
> 
> end Cars;
> 
> 
> 
> with System.Address_To_Access_Conversions;
> 
> package body Cars is
> 
>    type Taft_Car is record
>       Passenger_Count : Car_Passenger_Count;
>    end record;
> 
>    package Conversions is new System.Address_To_Access_Conversions
>      (Object => Taft_Car);
> 
>    function Allocate_Car return Any_Car is
>       Default : constant Any_Car
>         := (Offset           => Taft_Car'Max_Size_In_Storage_Elements,
>             Allocated_Memory => (others => 0),
>             Reference        => null);
>    begin
>       return Car : Any_Car (Taft_Car'Max_Size_In_Storage_Elements) do
>          declare
>             First_Index : constant System.Storage_Elements.Storage_Offset
>               := Car.Allocated_Memory'First;
>          begin
>             Car.Reference := Conversions.To_Pointer
>               (Car.Allocated_Memory (First_Index)'Address).all'Access;
>          end;
>       end return;
>    end Allocate_Car;
> 
>    function Passenger_Count (Car : Any_Car) return Car_Passenger_Count is
>    begin
>       return Car.Reference.Passenger_Count;
>   end Passenger_Count;
> 
>    procedure Set_Passenger_Count
>      (Car   : in out Any_Car;
>       Value : Car_Passenger_Count) is
>    begin
>       Car.Reference.all.Passenger_Count := Value;
>    end Set_Passenger_Count;
>    
> end Cars;
> 
> 
> with Ada.Text_IO;
> 
> with Cars;
> use  Cars;
> 
> procedure Main is
>    Car : Cars.Any_Car := Cars.Allocate_Car;
> begin
>    Set_Passenger_Count (Car, 3);
>    Ada.Text_IO.Put_Line (Passenger_Count (Car)'Image);
> end Main;
> 
> Does somebody have a better implementation of Taft types where heap allocations are not used?
> 
> Perhaps this example is something to add to documentation on Ada on the internet like for example:
> https://en.wikibooks.org/wiki/Ada_Programming/Tips
> 
> Anything wrong or could be improved with the implementation above?
> 
> Best regards,
> Joakim

Background on "Taft types" (a.k.a. Ichbiah's name "Taft-amendment types" or the generic/language-independent name "opaque pointers" or the C++ jargon "pimpl").
https://groups.google.com/forum/#!msg/comp.lang.ada/KelH3y5WzEA/z3pyInm_hqgJ