What is the difference between a class variable like Current in Locale class:
Object subclass: #Locale instanceVariableNames: 'id shortDate longDate time decimalSymbol digitGrouping currencySymbol currencyNotation measurement offsetLocalToUTC offsetVMToUTC dstActive' classVariableNames: 'Current CurrentPlatform KnownLocales LanguageSymbols LocaleChangeListeners PlatformEncodings' poolDictionaries: '' category: 'System-Localization'
that is accessed as (in class side):
current "Current := nil" Current ifNil: [ Current := self determineCurrentLocale. "Transcript show: 'Current locale: ' , Current localeID asString; cr"]. ^Current
and current in SmalltalkImage class side:
SmalltalkImage class instanceVariableNames: 'current'
that is accessed as: current "Note that this could be implemented differently to avoid the test"
current isNil ifTrue: [current := self basicNew]. ^ current
I'm trying to create a singleton but I have some doubts in the correct implementation.
I have also seen the explanation on
http://coweb.cc.gatech.edu/cs2340/3872
also:
http://stackoverflow.com/questions/438729/smalltalk-singleton-pattern-how-do...
that corresponds to the first type, using (I suppose) a class variable and
http://wiki.squeak.org/squeak/939
that corresponds to the second type.
Are there guidelines or pros/cons about singletons on Smalltalk?
Thanks, Miguel Cobá
Hi Miguel, A simple rule you can use is:
Use a class variable if your Singleton will not have subclasses. Use a instance class instance variable if your Singleton will have one or many subclasses, i.e. each class in the hierarchy will need its own Singleton copy.
A good practice is to implement a #reset method, very useful when cleaning or shutting down images for a release.
Best regards
Hernán
2009/8/4 Miguel Enrique Cobá Martinez miguel.coba@gmail.com
What is the difference between a class variable like Current in Locale class:
Object subclass: #Locale instanceVariableNames: 'id shortDate longDate time decimalSymbol digitGrouping currencySymbol currencyNotation measurement offsetLocalToUTC offsetVMToUTC dstActive' classVariableNames: 'Current CurrentPlatform KnownLocales LanguageSymbols LocaleChangeListeners PlatformEncodings' poolDictionaries: '' category: 'System-Localization'
that is accessed as (in class side):
current "Current := nil" Current ifNil: [ Current := self determineCurrentLocale. "Transcript show: 'Current locale: ' , Current localeID asString; cr"]. ^Current
and current in SmalltalkImage class side:
SmalltalkImage class instanceVariableNames: 'current'
that is accessed as: current "Note that this could be implemented differently to avoid the test"
current isNil ifTrue: [current := self basicNew]. ^ currentI'm trying to create a singleton but I have some doubts in the correct implementation.
I have also seen the explanation on
http://coweb.cc.gatech.edu/cs2340/3872
also:
http://stackoverflow.com/questions/438729/smalltalk-singleton-pattern-how-do...
that corresponds to the first type, using (I suppose) a class variable and
http://wiki.squeak.org/squeak/939
that corresponds to the second type.
Are there guidelines or pros/cons about singletons on Smalltalk?
Thanks, Miguel Cobá
Beginners mailing list Beginners@lists.squeakfoundation.org http://lists.squeakfoundation.org/mailman/listinfo/beginners
On 04.08.2009, at 17:28, Miguel Enrique Cobá Martinez wrote:
What is the difference between a class variable like Current in Locale class:
A class variable exists once per image. A class instance variable exists once per subclass.
If your singleton class has no subclasses, it doesn't matter. If it has subclasses, then each subclass will have a different singleton if you use a class instance variable. With a class variable, you only have one singleton for all subclasses.
- Bert -
El mar, 04-08-2009 a las 18:46 +0200, Bert Freudenberg escribió:
On 04.08.2009, at 17:28, Miguel Enrique Cobá Martinez wrote:
What is the difference between a class variable like Current in Locale class:
A class variable exists once per image. A class instance variable exists once per subclass.
If your singleton class has no subclasses, it doesn't matter. If it has subclasses, then each subclass will have a different singleton if you use a class instance variable. With a class variable, you only have one singleton for all subclasses.
Woa, thanks Bert and Hernán, that was a really clear explanation. Now I understand the issues here.
Cheers, Miguel Cobá
- Bert -
Beginners mailing list Beginners@lists.squeakfoundation.org http://lists.squeakfoundation.org/mailman/listinfo/beginners
Hi!
Or to rephrase it once more:
A class variable is like a global, but only accessible from the class, its instances and from subclasses and their instances. Kinda like a "scoped global". A bit similar to a "static" variable in say Java - "There Can Be Only One" :)
A class instance variable is a much more "natural" thing really. It is simply an instance variable in the class *itself*. Remember that classes are real objects in Smalltalk and thus can have instance variables as well as behavior.
Also, this means we naturally have "inheritance on the class side" which means that just like class side methods are inherited by subclasses (unlike static methods in Java which really are just like C functions) these class instance variables are also inherited by subclasses. But remember - inheritance doesn't mean that the subclasses will "see" the variable of the superclass - it means that the subclasses will have their *own* such instance variable, which again is natural since each class is a different object.
And also, naturally, class instance variables are not accessible from the instances of the class!
So class instance variables behave just like regular instance variables - but *in the class itself*. Btw, if you type a class name, select it, and press alt-i you get an inspector on the class object and you can see all its instance variables, as you can see there is a whole bunch.
Yes, this was a much longer explanation but might have cleared up some question marks still lingering. :)
regards, Göran
El mié, 05-08-2009 a las 09:15 +0200, Göran Krampe escribió:
Hi!
Or to rephrase it once more:
A class variable is like a global, but only accessible from the class, its instances and from subclasses and their instances. Kinda like a "scoped global". A bit similar to a "static" variable in say Java - "There Can Be Only One" :)
A class instance variable is a much more "natural" thing really. It is simply an instance variable in the class *itself*. Remember that classes are real objects in Smalltalk and thus can have instance variables as well as behavior.
Also, this means we naturally have "inheritance on the class side" which means that just like class side methods are inherited by subclasses (unlike static methods in Java which really are just like C functions) these class instance variables are also inherited by subclasses. But remember - inheritance doesn't mean that the subclasses will "see" the variable of the superclass - it means that the subclasses will have their *own* such instance variable, which again is natural since each class is a different object.
And also, naturally, class instance variables are not accessible from the instances of the class!
So class instance variables behave just like regular instance variables
- but *in the class itself*. Btw, if you type a class name, select it,
and press alt-i you get an inspector on the class object and you can see all its instance variables, as you can see there is a whole bunch.
Yes, this was a much longer explanation but might have cleared up some question marks still lingering. :)
regards, Göran
Thanks Göran for the alternative explanation. Indeed it fills some gaps in my understanding and use of those features of squeak.
Miguel Cobá
Beginners mailing list Beginners@lists.squeakfoundation.org http://lists.squeakfoundation.org/mailman/listinfo/beginners
beginners@lists.squeakfoundation.org
-
Bert Freudenberg -
Göran Krampe -
Hernán Morales Durand -
Miguel Enrique Cobá Martinez