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From: "Matthew Heaney" <matthew_heaney@acm.org>
Subject: Re: Function name problem
Date: 2000/01/17
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In article <B4A7F12A.45EC%Harald.Schmidt@tomcat.de> , Harald Schmidt 
<Harald.Schmidt@tomcat.de>  wrote:

> ...my intenstion was not to implement a functional operator
> equivalent to C/C++. My primary business is Smalltalk, and
> I was trying to implement the equal method "=", means
> equality of any attribute of two given objects. The second
> method "==" (basis for this discussion) is a identity
> method comparing the identity of two given objects.

OK.

Smalltalk has reference semantics.  To implement a by-reference type in
Ada(95), use access types.  "Comparing identity" in Ada95 means
comparing access objects.

package P is

  type T (<>) is abstract tagged limited null record;

  type TA is access all T'Class;

  function "=" (L, R : T) return Boolean is abstract;

  procedure Op (O : access T) is abstract;

end P;

package P.C is

  type NT is new T with private;

  function New_NT return TA;

  procedure Op (O : access NT);

  function "=" (L, R : NT) return Boolean;

private

  type NT is new T with record ...;

  ...

end P.C;


A client of a by-reference abstraction declares an access object, and
then calls the constructor for the type:

declare
  O1, O2 : TA := New_NT;
begin
  ...
  if O1.all = O2.all then   -- compare values

  if O1 = O2 then           -- compare identities


In neither case do you require an "==" operator.


--
"I do not feel obliged to believe that the same God who has endowed us
with sense, reason, and intellect has intended us to forgo their use."

Galileo Galilei