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From: eachus@spectre.mitre.org (Robert I. Eachus)
Subject: Re: "use" clauses and Ada 95 OOP
Date: 1996/07/23
Message-ID: <EACHUS.96Jul23175154@spectre.mitre.org>#1/1
X-Deja-AN: 169772275
references: <Dup54L.2q2.0.-s@inmet.camb.inmet.com>
 <EACHUS.96Jul19102232@spectre.mitre.org>
organization: The Mitre Corp., Bedford, MA.
newsgroups: comp.lang.ada
Date: 1996-07-23T00:00:00+00:00
List-Id: <comp.lang.ada>



   In article <31F170FC.672C@csehp3.mdc.com> "James A. Squire" <m193884@CSEHP3.MDC.COM> wrote:

   > Very interesting!  Let me see if I'm reading you right:

   > 1.  func would have to be a primitive operation in order to be a
   > dispatching call.  Right?

   > 2.  Therefore, func would be covered by the "use type" clause on
   > <type_of_x>, right?

   > I liked the idea of "use type" when I thought it only referred to
   > implicit operations like "&", "+", "=" (operations that I wish the Ada
   > designers had made a part of the language proper so we wouldn't have any
   > need for the use clause).  Now that it seems "use type" applies to all
   > primitive operations (some of which are user-written, like "Handle",

In article <JSA.96Jul22154610@alexandria> jsa@alexandria (Jon S Anthony) writes:

   > A use_type clause only gives "direct" visibility to primitive _operators_
   > of the type: RM 8.4(8).

   The reality is somewhere in the middle.

   RM 3.2.3(8): "A primitive subprogram whose designator is an
operator_symbol is called a _primitive_operator_."

   RM 3.2.3(1):  "The _primitive_operations of a type are the
predefined operations of the type, plus any user-defined primitive
subprograms."

   So yes, this "trick" only works for 21 specific names.  But there
are lots of cases where using these symbols (including and, or, xor,
"&", mod, rem, abs, not) make lots of sense for abstract data types.

   For example, take a set abstraction.  All but two operations
(Is_Empty and Empty) are best defined as Ada operators.  (Note also
that function "+"(L: Set; R: Element) return Set; can be a primitive
operator of either Set or Element depending on where it is defined.)

--

					Robert I. Eachus

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