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Date: 21 Oct 91 18:36:43 GMT
From: netnews.upenn.edu!uofs!guinness.cs.uofs.edu!beidler@rutgers.edu  (Jack Be
idler)
Subject: Re: Hopefully simple question regarding generics
Message-ID: <10285@platypus.uofs.uofs.edu>
List-Id: <comp.lang.ada>

In article <1991Oct16.085901@lglsun.epfl.ch>, gauthier@lglsun.epfl.ch (Michel G
authier) writes:
|> 
|> Stephen J Bevan writes:
|> 
|> >I read (and think I understand) the following in "Software Engineering
|> >with Ada" by Booch :- 
|> >[ generic array sorter's interface deleted ]
|> >
|> >What I'd like to know is how can I change it so that it doesn't assume
|> >the items to sort are in an array i.e. how to I parameterize the above
|> >over the implementation of the items to be sorted.  For example, say I
|> >want to sort a list as opposed to an array.  With a parameterized
|> >version I could then do something like :-
|> >
|> >  procedure STRING_LIST_SORT is new 
|> >    SORT(ITEM => CHARACTER,
|> >         INDEX => POSITIVE,
|> >         ITEMS => STRING,
|> >         COLLECTION => LIST  -- or maybe LIST(INDEX, ITEM) ???
|> >         "<" => "<");
|>

 
Laverman and Gauthier's separate replies gave excellent advise.

Let me take a slightly different approach -- does Bevan need to change the gene
ric sort at all?
Specifically, if the generic sort is something like

generic
    type Object_Type is private
    type Array_range is (<>) ;
    type array_type is array (Array_range) of Object_Type ;
    with function "<" (left, right: Object_Type return boolean ;
procedure Sort (An_Array : in out AArray_Type) ;

if the objects are pointers to the values and the sort orders the pointers acco
rding to the 
values of the objects being represented by the strings, lists, or whateverthe p
ointers are
pointing to, he can use this generic Sort directly simply by defining a special
 purpose

    function less_than (left, right : Object_Type) return boolean ;

that does the comparisons he want between the objects he is trying to manipulat
e.  Then
use this function to instantiate the "<" generic formal parameter to Sort and h
e's is business.
Further, since he is only moving the locations of the pointers and not trying t
o physically
move the objects being referenced by indirection, he is as safde an he can be, 
given that
he is using private types.  It might be wise to create a "limited private" vers
ion of this
Sort,

generic
    type Object_Type is limited private
    type Array_range is (<>) ;
    type array_type is array (Array_range) of Object_Type ;
    with function "<" (left, right: Object_Type return boolean ;
    with procedure SWAP (left, right : in out OBJECT_TYPE) ;
procedure Sort (An_Array : in out AArray_Type) ;

which would need a procedure, like SWAP, to flip the object references around.

-- 
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|  John (Jack) Beidler				                              |
|  Prof. of Computer Science  Internet (VAX/VMS)  BEIDLER@JAGUAR.UOFS.ED      |
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