Re: How to use read-only variables?

[Shark8 <OneWingedShark@gmail.com>]

On 03-Jan-15 17:58, hreba wrote:
>
>     type R is abstract tagged limited record
>        n:    access constant Integer;
>     end record;
>
>     type Rv is new R with private;
>
> private
>
>     type Rv is new R with record
>        nv:    aliased Integer;
>     end record;

Try it like this:

With
Ada.Text_IO.Text_Streams;

procedure Test is


    package tmp is

       type R is abstract tagged limited record
          n:    access constant Integer;
       end record;

       type Rv is new R with private;

       function Make( I : Integer ) return Rv;
       function Val( Item : Rv ) return Integer;

    private

       type Rv is new R with record
          -- The following referenced "this" as a particular
          -- instance for the default value by using the type-name
          -- (and 'access) as a self-reference. we have to convert
          -- the de-reference to the parent class, and then access
          -- the appropriate value.
          nv:    aliased Integer:= R(Rv'access.all).n.all;
       end record;

       -- Note that we let the default values take care of ensuring
       -- that the values are properly in-sync.
       function Make( I : Integer ) return Rv is
         ( n => new Integer'(I), others => <> );

       function Val( Item : Rv ) return Integer is
         ( Item.nv );

    end tmp;

    use Ada.Text_IO;
begin

    declare
       K : tmp.Rv:= tmp.make( 129 );
    begin
       Put_Line( "K.n:" & K.Val'Img );
    end;

    Put_Line( "Done." );
end Test;