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Newsgroups: comp.lang.ada
Date: Sun, 5 Jun 2016 20:12:11 -0700 (PDT)
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Subject: Re: Avoiding dispatching in procedure's with classwide types
From: rieachus@comcast.net
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Xref: news.eternal-september.org comp.lang.ada:30626
Date: 2016-06-05T20:12:11-07:00
List-Id: <comp.lang.ada>

On Saturday, May 28, 2016 at 3:01:37 PM UTC-4, Jeremiah wrote:
> I have a procedure that uses one tagged type (the primitive type) and a c=
lasswide type.

1) I have no clue as to what you are trying to do here.
2) I have no idea why you think that this would improve performance.

If you have a subroutine with a parameter of a class wide type (Foo'Class),=
 dispatching happens.  If you have a parameter of the base type (Foo), ther=
e is no dispatching, however there may be inheritance. If a subprogram of a=
 type is declared in the same package spec as the type, you get inheritance=
.  So:

  package P is
    type Foo is private;
    procedure Put(F: in out Foo);
    function Get return Foo;
  ...
  end P;
=20
  with P;
  package Q is=20
    type Bar is new P.Foo with ...;
    -- Bar has a procedure Put and a function Get.
  end Q;


   Temp_Foo: P.Foo :=3D P.Get;
   Barf: Q.Bar :=3D Temp_Foo;
 begin=20
   Put(Foo(Barf));

The only way you could do something like what you are proposing is to inste=
ad of using an operation of a class wide type, do a type conversion to the =
parent type, and call its operation:

  Fooc: Foo'Class :=3D Get;
begin
  Put(Fooc);  -- normal
  Put(Foo(Fooc));

If the descendants of Foo add no new fields, the two calls will be equivale=
nt.  If Bar or other descendants add state, that state will be stripped off=
 in the bottom call to Put, and you will get the Put of Foo not of Bar.  Wh=
y you would want to bake such trouble in a program is beyond me.

Notice the extra effort to be able to call Get and assign the result to a v=
ariable of type Foo.  Also notice that you may have to use declare blocks t=
o create objects of indefinite sizes.