| Re: singleton usage [message #47604] |
Tue, 21 February 2006 10:28 |
JD Smith
Messages: 850
Registered: December 1999
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Senior Member |
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On Tue, 21 Feb 2006 15:47:26 +0100, Antonio Santiago wrote:
> vcarlos wrote:
>> Hi Antonio,
>>
>> I saw the system variable singleton implementation, but what I am in
>> doubt is about the usage of the pattern itself. Since everytime I need a
>> reference to the singleton object in my objects I need to call a
>> myObject = Singleton() statement, or initialize an attribute in every
>> object needing a reference to the singleton. But what if I just create a
>> read only system variable and use it over my whole program? (this
>> variable would be an instance of my "singleton" class)
>>
> Ok sorry, I don't understand you in the first email. Yes, I think you can
> do it, but this is a convention that you use in your program. Then you
> need to document very well that variable XXX is a "reference" to a
> singleton object in your whole application.
Using a singleton generator is easy (see various threads dating back
several years). As far as preventing rogue "obj_new()" calls, you can
also check the common block or system variable explicitly in the Init
function and generate an error if anyone uses it directly (except for
the first call).
JD
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| Re: singleton usage [message #47609 is a reply to message #47604] |
Tue, 21 February 2006 06:47  |
Antonio Santiago
Messages: 201
Registered: February 2004
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Senior Member |
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vcarlos wrote:
> Hi Antonio,
>
> I saw the system variable singleton implementation, but what I am in
> doubt is about the usage of the pattern itself. Since everytime I need
> a reference to the singleton object in my objects I need to call a
> myObject = Singleton() statement, or initialize an attribute in every
> object needing a reference to the singleton. But what if I just create
> a read only system variable and use it over my whole program? (this
> variable would be an instance of my "singleton" class)
>
> Thanks
>
> Vinicius
>
Ok sorry, I don't understand you in the first email.
Yes, I think you can do it, but this is a convention that you use in
your program. Then you need to document very well that variable XXX is a
"reference" to a singleton object in your whole application.
--
-----------------------------------------------------
Antonio Santiago P�rez
( email: santiago<<at>>grahi.upc.edu )
( www: http://www.grahi.upc.edu/santiago )
( www: http://asantiago.blogsite.org )
-----------------------------------------------------
GRAHI - Grup de Recerca Aplicada en Hidrometeorologia
Universitat Polit�cnica de Catalunya
-----------------------------------------------------
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| Re: singleton usage [message #47610 is a reply to message #47609] |
Tue, 21 February 2006 06:06 |
vcarlos
Messages: 21
Registered: February 2006
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Junior Member |
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Hi Antonio,
I saw the system variable singleton implementation, but what I am in
doubt is about the usage of the pattern itself. Since everytime I need
a reference to the singleton object in my objects I need to call a
myObject = Singleton() statement, or initialize an attribute in every
object needing a reference to the singleton. But what if I just create
a read only system variable and use it over my whole program? (this
variable would be an instance of my "singleton" class)
Thanks
Vinicius
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| Re: singleton usage [message #47611 is a reply to message #47610] |
Tue, 21 February 2006 05:58 |
Antonio Santiago
Messages: 201
Registered: February 2004
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Senior Member |
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vcarlos wrote:
> Hi all,
>
> maybe it is the dumbiest question of all time about singleton, but
> since I am in doubt, is better ask thank keep it :)
>
> When you define a singleton object, everywhere in the program you use
> it you must call something like this: myObject = Obj_New('Singleton'),
> and then myObject will point to the same instance created before. That
> is fine to me, but what I have doubt is about the usage of this feature
> in my program, since everytime I want access that object I will need to
> write the same statement above, or set an attribute in a class to point
> to the singleton instance, right?
>
> What if, instead of doing that, I create an read only system variable
> holding the object that I want use over the program?
>
One of posible implementations of a Singleton in IDL uses a system
variable to hold the object reference. Other solution uses a common block.
> I know I lost some features, but is not it more useful?
>
> Well, that is it. Thanks
>
> (sorry about my english)
>
> Vinicius
>
--
-----------------------------------------------------
Antonio Santiago P�rez
( email: santiago<<at>>grahi.upc.edu )
( www: http://www.grahi.upc.edu/santiago )
( www: http://asantiago.blogsite.org )
-----------------------------------------------------
GRAHI - Grup de Recerca Aplicada en Hidrometeorologia
Universitat Polit�cnica de Catalunya
-----------------------------------------------------
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