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From: donh@syd.csa.com.au (Don Harrison)
Subject: Re: Real OO
Date: 1996/05/07
Message-ID: <Dr0rp1.1Ks@assip.csasyd.oz>#1/1
X-Deja-AN: 153450458
sender: news@assip.csasyd.oz
references: <4mls4h$sau@watnews1.watson.ibm.com>
organization: CSC Australia
reply-to: donh@syd.csa.com.au
newsgroups: comp.lang.eiffel,comp.lang.ada,comp.object
Date: 1996-05-07T00:00:00+00:00
List-Id: <comp.lang.ada>


Norman H. Cohen lectures Joachim (but in doing so provides a good summary of 
classwide operations):

:In article <6850x6pV3RB@herold.franken.de>,
:jhd@herold.franken.de (Joachim Durchholz) writes: 
:
:|> As I understand it, you mean the multiple dispatching mechanisms
:|> (a.k.a. classwide operations).
:
:No, that's not what classwide operatins are.  Your post reflects a number
:of misconceptions.

Just one: that classwide operations are multiply dispatching.

[...]

Translating the examples into equivalent Eiffel (my additions in brackets),

:If you write a procedure [in the same interface as A]
:
:   procedure P(X: in A);

  class A feature
    p is ...             -- X = Current

:... If you write a procedure [anywhere]
:
:   procedure Q(X: in A'Class);

  class A feature
    frozen q is ...             -- X = Current, or

  class SOME_OTHER_CLASS feature
    q (x: A) is ...

:   procedure R(X: in A; Y: in B); [where A, B and R are in same interface]

(It's syntactically impossible in Eiffel for an operation to be declared in two 
different classes).

:--which would appear to be a procedure that dispatches based on both the
:tag of X (identifying a type in the class rooted at A) and the tag of Y
:(identifying a type in the class rooted at B).  THERE IS NO CLOS-LIKE
:MULTIPLE DISPATCH IN ADA.  You CAN declare
:
:   procedure S(X: in A'Class; Y: in B'Class); [anywhere]

  class A feature
    frozen q (y: B) is ...             -- X = Current, or

  class B feature
    frozen q (x: X) is ...             -- Y = Current, or

  class SOME_OTHER_CLASS feature
    frozen q (x: X; y: B) is ...

[...]

:It is also possible to write [in the same interface as A]
:
:   procedure T(X:in A; Y: in B'Class);

The addition of 'in' protects X from update.

  ...

  class SOME_OTHER_CLASS feature
    frozen q (x: X; y: B) is
    do
      ...
    ensure
      x = old x
      y = old y
    end

[...]



As the equivalent Eiffel shows, the term 'classwide operation' is misleading.
An operation may be dispatching WRT one parameter but classwide WRT another.
It is more accurate to speak of classwide operands rather than classwide 
operations.
 
:--
:Norman H. Cohen    ncohen@watson.ibm.com



-- 
                    ///
Don.               (o o)
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