| Re: IDL Object for creating a Singleton [message #11984 is a reply to message #11979] |
Mon, 22 June 1998 00:00   |
davidf
Messages: 2866
Registered: September 1996
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Senior Member |
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Phillip David (pdavid@earthling.net) writes:
> I've been doing a fair amount of reading on OO design and methodology. One of
> the books I've had pointed out to me was 'Design Patterns', which deals with
> OO algorithm design quite nicely. One of the patterns the authors describe is
> a Singleton -- A class for which only one instance exists. If the sole
> instance exists, the constructor function (object's init function in IDL) will
> return a reference to the existing object. If none exists, a new object is
> created and returned.
>
> This structure is particularly useful when there is no reason to have multiple
> objects around. I am trying to create a user preferences object that fits
> this criteria very nicely. I'd like to be able to store user preferences for
> screen size in an object. If that object exists, other screens can size
> themselves appropriately. The book gives sample C++ code that relies on class
> variables and class methods (i.e., not associated with a specific instance of
> the object, but with the object itself). IDL doesn't seem to have any way to
> do something similar. Does anyone know of a way to do this?
>
> My first attempt uses a common block called preferencesclassvars to store an
> object reference. This common block is called from a function that, while not
> part of the object, is stored in the same file. If the reference contained in
> the common block is not an object reference, then I know the object hasn't
> been initialized, and I set it to a new instance of the object. Otherwise, I
> return the object reference from the common block.
>
> Is there a better way to achieve the techniques of class variables and class methods?
Humm. Objects are turning out to be not only to be fun and powerful to
program, but synchronistic as well. This is the second time in two
days the _Design Patterns_ book (which I have not seen or read) was
mentioned to me and in an almost identical context. I was talking
to a friend about writing a "lurker" object, a singleton object like
Philip's screen preferences object, that lurks in the background
waiting for something to happen. In this case, I wanted to
update all the open windows when a color table was changed on a
24-bit display.
This wonderful idea suffers from other unsolvable (so far)
problems, but I did manage to get it started up. Like Philip,
I had to resort to a common block to initialize it properly.
Since common blocks are never my preferred solution, and since
this thing reeks of a hack, I'm also looking for something more
elegant. I'm afraid I am swimming in programming space that
is a little out of my depth. Perhaps someone like JD can throw
both Philip and me a life-line. :-)
Cheers,
David
--
David Fanning, Ph.D.
Fanning Software Consulting
E-Mail: davidf@dfanning.com
Phone: 970-221-0438
Coyote's Guide to IDL Programming: http://www.dfanning.com/
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