Ada: Understanding private types and understanding packaging

Question

I am trying to learn how to use private declarations in Ada and also to understand packaging. I have tried to keep my codes as short as possible.

We start with the main file rectangular_Form.ads

with Rectangular_Method_1;
package Rectangular_Form renames Rectangular_Method_1;
-- with Rectangular_Method_2;
-- package Rectangular_Form renames Rectangular_Method_2;

This tells us that we can have two implementations and that for now Rectangular_Method_1 has been chosen.

Then we have the specifications file Rectangular_Method_1.ads:

package Rectangular_Method_1 is
   type Rectangular is private;

   procedure Vector_Basis_r (A : in Long_Float; D : out Rectangular);
   procedure Set_Horz (R : in out Rectangular; H : Long_Float);
   function Get_Horz (R : Rectangular) return Long_Float;

 private
   type Rectangular is
        record
             Horz, Vert: Long_Float;
        end record;
end Rectangular_Method_1;

and its body Rectangular_Method_1.adb:

with Ada.Numerics.Long_Elementary_Functions;
use  Ada.Numerics.Long_Elementary_Functions;

package body Rectangular_Method_1 is
procedure Vector_Basis_r (A : in Long_Float; D : out Rectangular) is
begin
  D.Horz := Cos (A, Cycle => 360.0);
  D.Vert := Sin (A, Cycle => 360.0);
end Vector_Basis_r;

procedure Set_Horz (R : in out Rectangular; H : Long_Float) is
begin
  R.Horz := H;
end Set_Horz;

function Get_Horz (R : Rectangular) return Long_Float is
begin
  return R.Horz;
end Get_Horz;

end Rectangular_Method_1;

and finally the test script: test_rectangular_form.adb:

with Ada.Long_Float_Text_IO;
with Ada.Text_IO; use Ada.Text_IO;
with Rectangular_Form;
use type Rectangular_Form.Rectangular;

procedure Test_Rectangular_Form is
Component_Horz, Component_Vert, Theta  : Long_Float;
Basis_r                                : Rectangular_Form.Rectangular;

begin
  Ada.Text_IO.Put("Enter the angle ");
  Ada.Long_Float_Text_IO.Get (Item => theta);

  --Vector basis
  Rectangular_Form.Vector_Basis_r (A => Theta, D => Basis_r);
  Ada.Text_IO.New_Line;

  Ada.Text_IO.Put("rhat_min_theta = ");
  Ada.Long_Float_Text_IO.Put (Item => Rectangular_Form.Get_Horz (Basis_r), Fore => 3, Aft  => 5, Exp  => 0);
  Ada.Text_IO.Put(" ihat + ");
  Ada.Long_Float_Text_IO.Put (Item => Rectangular_Form.Get_Vert (Basis_r), Fore => 3, Aft  => 5, Exp  => 0);
  Ada.Text_IO.Put (" jhat ");
end Test_Rectangular_Form;

Question (applied to test_rectangular_form.adb):

I am getting the components A.Horz and A.Vert directly using

    Rectangular_Form.Vector_Basis_r (A => Theta, D => Basis_r);

and accessing the horizontal and vertical components later on by just using

    Rectangular_Form.Get_Horz (Basis_r)

and

    Rectangular_Form.Get_Vert (Basis_r)

This seems OK as I am using the methods Get_Horz and Get_Vert to access the private Rectangular horizontal and vertical components. However I am reading the variable Theta directly from the command line and finding its horizontal and vertical components using

    Rectangular_Form.Vector_Basis_r (A => Theta, D => Basis_r);

If the Rectangular components A.Horz and A.Vert are private as I have defined them, why do not I have to use the methods set_Horz(A) and set_Vert(A) prior to issuing the command

    Rectangular_Form.Vector_Basis_r (A => Theta, D => Basis_r);

PS: The functions for Set_Vert and Get_Vert were omitted to keep the codes short.

Thanks a lot...

Answer 1

why do not I have to use the methods set_Horz(A) and set_Vert(A) prior to issuing the command

Because you're setting the fields of your private record by executing:

Rectangular_Form.Vector_Basis_r (A => Theta, D => Basis_r);

(Review the implementation of Vector_Basis_r.)

-- EXPANDED Answer

When you declare the Basis_r variable in your Test_Rectangular_Form procedure, the memory for that "Rectangular" variable is allocated. At this point the Horz and Vert fields in the private type are present, but unset (they could contain anything).

When you call Rectangular_Form.Vector_Basis_r, its implementation sets the values of those private fields. Maybe this is what's confusing you: the body of the package in which a private type is declared has direct access to those fields. This is exactly the aspect that Vector_Basis_r is exploiting when setting their values (as well as Set_Vert and Set_Horz).