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Newsgroups: comp.lang.ada
Date: Tue, 3 Apr 2018 19:18:55 -0700 (PDT)
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Subject: Re: limited agregate and limited components default initialization
From: Jere <jhb.chat@gmail.com>
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Xref: reader02.eternal-september.org comp.lang.ada:51329
Date: 2018-04-03T19:18:55-07:00
List-Id: <comp.lang.ada>

On Tuesday, April 3, 2018 at 2:18:18 PM UTC-4, Jean-Claude Rostaing wrote:
> Talking about limited aggregates, how to make the exemple below with prot=
ected types work ? It should be pretty much the same as what I already did,=
 yet...
> package P is
>    type Essai (<>) is limited  private;
>    function Constructor (v: Natural) return  ESSAI;
>    procedure CHANGE (Obj: Essai; V: Natural);
> private
>    protected type ESSAI (D1: Natural) is=20
>       procedure CHANGE (V: Natural);
>    private
>       Private_var: Natural :=3D D1;=20
>    end Essai;
> end P;
>=20
> package BODY P is
>    -- procedure change(Obj: Essai; V: natural) renames ESSAI.CHANGE;
>    procedure CHANGE (Obj: ESSAI; V: NATURAL) is
>    begin
>       OBJ.CHANGE(V); -- 5
>    end CHANGE;
>    function CONSTRUCTOR ( V : NATURAL) return ESSAI is
>      (Essai(D1 =3D> V)); -- 9
>=20
>    protected body Essai is
>       procedure CHANGE (V: natural) is
>       begin
>          PRIVATE_VAR :=3D V;
>       end change;
>    end Essai;
> end P;
>=20
> I get
> p.adb:5:10: prefix of protected procedure or entry call must be variable
> p.adb:9:14: invalid prefix in call
>=20
> Actually, I wanted CHANGE to be a renaming of Change, but since Essai her=
e behave more like a package, the protected operation doesn't its protected=
 object as an argument, so the profile wouldn't be the same, so a renaming =
can only point to a specific task/protect object's entry.
>=20
> I want to force people to use a constructor on any object of ESSAI type t=
hrough setting its discriminants. Discriminants who would serve to set the =
default value with the constructor, so they must be hidden from the user.
>=20
> I must confess I never used anything protected types before... I thought =
the use would be more intuitive ?

As an alternative.  If you end up needing to handle protected types without
discriminants, you can use a private implementation type and composition:

    package P is
       type Essai (<>) is limited private;
       function Constructor (v: Natural) return ESSAI;
       procedure CHANGE (Obj: in out Essai; V: Natural) with Inline;
    private
   =20
       -- Internal implementation type
       protected type ESSAI_Impl  is
          procedure CHANGE (V: Natural);
       private
          Private_var: Natural :=3D 0;
       end Essai_Impl;
      =20
       -- new wrapper type
       type Essai is limited record
          Impl : Essai_Impl;
       end record;
    end P;
   =20
    package BODY P is
   =20
       procedure CHANGE (Obj: in out ESSAI; V: NATURAL) is
       begin
          OBJ.Impl.CHANGE(V); -- 5
       end CHANGE;
      =20
       function CONSTRUCTOR ( V : NATURAL) return ESSAI is
       begin
          -- Can use aggregate to create the wrapper type
          return Result : Essai :=3D (others =3D> <>) do
             Result.Impl.CHANGE(V);  --now initialize the value
          end return;
       end CONSTRUCTOR;
   =20
       protected body Essai_Impl is
      =20
          procedure CHANGE (V: natural) is
          begin
             PRIVATE_VAR :=3D V;
          end change;
         =20
       end Essai_Impl;
      =20
    end P;=20