Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!cornell!uw-beaver!apollo!vinoski
From: vinoski@apollo.HP.COM (Stephen Vinoski)
Newsgroups: comp.object
Subject: Pre-computing objects
Message-ID: <4671be53.20b6d@apollo.HP.COM>
Date: 25 Oct 89 20:23:00 GMT
Sender: root@apollo.HP.COM
Organization: Hewlett-Packard Apollo Division - Chelmsford, MA
Lines: 38


  Some time ago a discussion arose in comp.lang.c++ about reading objects in
from, say, a disk file.  There were several answers, but none that seemed all
that great, so I hope to start a discussion of the subject here.

  The subject line of this article comes from Bertrand Meyer's "Object-oriented
Software Construction" book, page 290.  The problem he is solving on that page
is how to create the appropriate object to fulfill a command within an editor
program; the user enters the command, and the program must construct a command
object to carry out the desired operation.  He advocates associating an integer
identifier with each command, pre-computing an array of command elements, and
initializing the nth array element with a command object having integer
identifier n.  When the user enters an editor command, the integer identifier
associated with that command is used to index into the array to obtain a
command object which is copied and used to carry out the command.

  This problem seems pervasive; other examples are reading objects from disk
storage and receiving objects from another process.  Basically, how does one
know what type of object to construct to receive the object unless the type of
the blob coming in from the external source is already known?  This appears to
be one area of OOP that requires something like a switch or case statement to
work properly.

  Doesn't Meyer's method trade the maintenance woes of a switch or case
statement for having to maintain the initialization function for the command
object array?  It's a good tradeoff, but isn't there anything better?  It also
seems that his method is appropriate only for collections of objects which are
somehow related.

  Does anyone out there know of any good solutions for this problem?  Pointers
to literature would be appreciated (I'm working in C++).


-steve

| Steve Vinoski       | Hewlett-Packard Apollo Div. | ARPA: vinoski@apollo.com |
| (508)256-6600 x5904 | Chelmsford, MA    01824     | UUCP: ...!apollo!vinoski |
| "Yikes, not another OOP book using the tired old 'stack' class example!!!"   |