| What about real polymorphism ?? [message #41991] |
Thu, 09 December 2004 06:12  |
Antonio Santiago
Messages: 201
Registered: February 2004
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Senior Member |
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Hi group,
my problem is with IDL's polymorphism, i think it's a half-polymorphims
instead real polymorphism.
Supose we have a class Person and two subclasses Man and Woman. Class
Person has a class called GetInfo() that is overriden in both Man and
Woman. The question is:
How can i create an array of Person's (that is Man or Woman objects) and
call the method GetInfo() in the way that depends on the subtype of
every object it invoques the Man GetInfo() or the Woman GetInfo()??
One possible solution is using OBJ_ISA funtion for every object in the
array but i want to know if it is possible only with polymorfism in IDL
like in Java or other OO languages.
Thanks.
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| Re: What about real polymorphism ?? [message #42018 is a reply to message #41991] |
Tue, 14 December 2004 13:58  |
David Fanning
Messages: 11724
Registered: August 2001
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Senior Member |
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Michael Wallace writes:
> Will wonders never cease?
No, never. I suppose it's even possible I'll find a reason
for an iTool. :-)
Cheers,
David
P.S. I notice that two of the four iTool examples on
the RSI code site I couldn't get to work (following
the author's notes). I'm not sure what this says about
iTools, but I'm afraid it doesn't make me hopeful. The
box plot just doesn't even want to try, giving a cryptic
error about "Internal Error" and no traceback. I wouldn't
know where to begin to look, but maybe that is the point. :-(
--
David W. Fanning, Ph.D.
Fanning Software Consulting, Inc.
Coyote's Guide to IDL Programming: http:/www.dfanning.com/
Phone: 970-221-0438, IDL Book Orders: 1-888-461-0155
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| Re: What about real polymorphism ?? [message #42020 is a reply to message #41991] |
Tue, 14 December 2004 10:56 |
Michael Wallace
Messages: 409
Registered: December 2003
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Senior Member |
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> My favorite quote:
>
> Singleton is a social disease. Because Singleton is so easy to
> understand, it is the single pattern that almost anybody who merely
> thumbed through GoF at the bookstore can remember and explain. That
> leads to unwarranted exuberance for the pattern among those who can
> least afford such exuberence.
That's a pretty good quote. Really, you could say that about any of the
patterns. One thing that novice programmers will do is discover the
Singleton, Decorator, Visitor, Factory or some other pattern and then
try to apply the pattern to every possible task usually with great
exuberance. What you wind up with are programs that are 10 times as big
as what they need to be (if not more) and layer upon layer of needless
complexity. I've suffered that same exuberance before, so I know
exactly what it feels like and what it feels like to look at the code a
year later. :-p
> You *can* implement a singleton in IDL (yes, using a common block or
> system variable). Try a search:
>
> http://groups.google.com/groups?as_q=singleton&as_ugroup =*idl-pvwave*
That's interesting. I had no clue that you could create your own system
variables. While not totally ideal, the read only system variable
allows you to do what I thought that IDL couldn't do -- be able to
enforce the rule of having one and only one class in existence. The
previous example just said to have a global variable, which wouldn't be
a singleton at all because you could instantiate as many of those
variables as you want. I sure didn't know of any way to limit IDL so
that you could only create a single instance. I must admit that I am
VERY surprised that IDL has this feature and happy to see it. Will
wonders never cease?
Mike
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| Re: What about real polymorphism ?? [message #42021 is a reply to message #41991] |
Tue, 14 December 2004 09:06 |
JD Smith
Messages: 850
Registered: December 1999
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Senior Member |
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On Tue, 14 Dec 2004 02:29:57 -0600, Michael Wallace wrote:
>> A singleton is very possible with IDL:
>> Just use a global variable (also as your access point) and
>> check its existance in the constructor.
>
> You know you drive all of us OO guys crazy with comments like "just use
> a global variable"?
>
> There is a big difference between a global variable and a singleton.
> You're only thinking of the access point, not the rest of the pattern.
> I'd be VERY surprised if it's possible to implement a true singleton
> pattern in IDL. I'd love to be proved wrong, but I don't think it's
> possible with IDL's so-called "objects."
>
> Mike
Good Wiki article with description and plusses and minuses on
singletons: http://c2.com/cgi/wiki?SingletonPattern
My favorite quote:
Singleton is a social disease. Because Singleton is so easy to
understand, it is the single pattern that almost anybody who merely
thumbed through GoF at the bookstore can remember and explain. That
leads to unwarranted exuberance for the pattern among those who can
least afford such exuberence.
You *can* implement a singleton in IDL (yes, using a common block or
system variable). Try a search:
http://groups.google.com/groups?as_q=singleton&as_ugroup =*idl-pvwave*
The chief drawback is, since there are no class methods in IDL, either
you can't override or extend the code which enforces "only one
instance" (without modifying it directly), or you have to use a
regular method as if it were a class method, discarding a "dummy"
instance. A bit ugly, but it gets the job done. Note that IDL>6
won't let you delete the self variable in Init, so my old Singleton
class would need changing. I prefer the singleton generator function
technique.
JD
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| Re: What about real polymorphism ?? [message #42026 is a reply to message #41991] |
Tue, 14 December 2004 00:29 |
Michael Wallace
Messages: 409
Registered: December 2003
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Senior Member |
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> A singleton is very possible with IDL:
> Just use a global variable (also as your access point) and
> check its existance in the constructor.
You know you drive all of us OO guys crazy with comments like "just use
a global variable"?
There is a big difference between a global variable and a singleton.
You're only thinking of the access point, not the rest of the pattern.
I'd be VERY surprised if it's possible to implement a true singleton
pattern in IDL. I'd love to be proved wrong, but I don't think it's
possible with IDL's so-called "objects."
Mike
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