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From: Georg Bauhaus <sb463ba@l1-hrz.uni-duisburg.de>
Newsgroups: comp.lang.ada
Subject: Re: ADA Popularity Discussion Request
Date: Tue, 31 Aug 2004 16:02:07 +0000 (UTC)
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Date: 2004-08-31T16:02:07+00:00
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Hyman Rosen <hyrosen@mail.com> wrote:
: Hyman Rosen wrote:
:>             restorer(int &var, int newval) : sg(var), oldval(var) { }
: 
: Oops. That should be { var = newval; } of course.

I think there is still an interesting difference. If, for the sake
of an example, I write

     void some_func(int some_para)
     {
         extern int some_global_a, some_global_b;
         struct restorer {
             int &sg, oldval, def;
             restorer(int &var, int newval) : sg(var), oldval(var) {
	       var = newval;
	       def = some_para;  // <--
	     }
             ~restorer() { sg = oldval; }
     };

the compiler tells me I can't do this, because I am using a parameter
from the containing function. In Ada, this works:


procedure nc is

   type Int_Ref is access all Integer;

   some_global_a, some_global_b: aliased Integer;
   --   global to `some_func` anyway


   procedure some_func(some_para: Integer) is

      type Restorer is record  -- "new Controlled with" for the real thing ...
         sg: Int_Ref;
         oldval: Integer;
         def: Integer;   --  only for this example
      end record;
      function make_restorer(var: Int_Ref; newval: Integer) return restorer is
         result: constant restorer :=
           (sg => var, oldval => var.all, def => some_para);
      begin
         var.all := newval;
         return result;
      end make_restorer;

   begin
      declare
         rsga: restorer := make_restorer(some_global_a'access, 53);
         rsgb: restorer := make_restorer(some_global_b'access, -1);
      begin
         null; -- ...
      end;
   end some_func;

begin -- nc
   some_func(5);
end nc;