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Newsgroups: comp.lang.ada
Date: Tue, 21 Jan 2020 12:51:45 -0800 (PST)
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Subject: Re: Calling a record type's methods (functions or procedure) when
 record is in an array
From: Shark8 <onewingedshark@gmail.com>
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Xref: reader01.eternal-september.org comp.lang.ada:57910
Date: 2020-01-21T12:51:45-08:00
List-Id: <comp.lang.ada>

On Saturday, January 18, 2020 at 11:02:32 AM UTC-7, Mace Ayres wrote:
>=20
> In your code below with my mark up =E2=80=9C * ?=E2=80=9D,  are the sub p=
rograms private in any way since they are below Type Thing is private, or t=
he privacy is applied to Thing?

Ada has a concept called private types, "TYPE X IS PRIVATE;", where the typ=
e's implementation is hidden from the clients of the package. Thus you coul=
d have the following:

Package Example is
   Type Point is private;
 =20
   -- Getters:
   Function  X( Object : in     Point ) return Integer;
   Function  Y( Object : in     Point ) return Integer;
 =20
   -- Setters:
   Procedure X( Object : in out Point; Value : in     Integer );
   Procedure Y( Object : in out Point; Value : in     Integer );
Private
 =20
   Type Point is record
     X_Value, Y_Value : -- Integer;
                         Real;
   End record;
End Example;

Package Body Example is
   Function  X( Object : in     Point ) return Integer is
   Begin
    Return Integer(Object.X_Value);
   End X;
 =20
   Procedure X( Object : in out Point; Value : in     Integer )
   Begin
    Object.X_Value:=3D Real( Value );
   End X;

   Function  Y( Object : in     Point ) return Integer is
   Begin
    Return Integer(Object.Y_Value);
   End Y;
 =20
   Procedure Y( Object : in out Point; Value : in     Integer ) is
   Begin
    Object.Y_Value:=3D Real( Value );
   End Y;

End Example;

And now, you could change the type of the components to Point to Integer, r=
emove the type-casts, and recompile the package Example **without having to=
 recompile dependent packages** because they are dependent only on the visi=
ble portion of the package.

>=20
> That is, what is the full scope of privacy in declaring type thing as pri=
vate, to type thing only,  or the sub programs too. Or is this a confused q=
uestion anyway?

See above.
Private types are Ada's method of hiding implementation-details. For exampl=
e, you could have a Password type that, internally, is a string but none of=
 the package-clients can see that:

Package Other_Example is
  Min_Length : Constant Natural;
  Function Valid_Characters( Input : String ) return Boolean;
 =20
  Type Password(<>) is private;
 =20
 =20
  Function Create( Input : String ) return Password
   with Pre =3D> Valid_Characters(Input) and Input'Length >=3D Min_Length;
  Function "=3D"( Left, Right : Password ) return Boolean;
  Function "=3D"( Object : Password; Value : String ) return Boolean;
 =20
Private
 =20
  -- A Password must be at least 5 characters,
  -- and may contain only alphanumeric characters.
  Min_Length : Constant Natural:=3D 5;
  Function Valid_Characters( Input : String ) return Boolean is
  (for all C of Password =3D> C in 'a'..'z'|'A'..'Z'|'0'..'9');
 =20
  Type Password is new String
    with Dynamic_Predicate =3D> Password'Length >=3D Min_Length
        and then Valid_Characters( String(Password) );
 =20
    Function Create( Input : String ) return Password is
    ( Password(Input) );
End Other_Example;