[squeak-dev] Class variables

Hi list,

can anyone please explain the difference between class variables at the
instance side in the class browser and instance variables at the class
side (thus of the metaclass) to me? And which of them is the preferred one?

Regards,
Martin

Class variables are accessible within the instance and class sides.
class instance variables are accessible only within the class instance  
side.

I essentially use class instance variables to keep track of created  
instances (e.g., singleton pattern).

Cheers,
Alexandre

On 14 Jul 2008, at 15:27, Martin Beck wrote:

--
_,.;:~^~:;._,.;:~^~:;._,.;:~^~:;._,.;:~^~:;._,.;:
Alexandre Bergel  http://www.bergel.eu
^~:;._,.;:~^~:;._,.;:~^~:;._,.;:~^~:;._,.;:~^~:;.

Martin Beck wrote: hi martin,
it makes a difference if you consider subclasses:
a class variable is there only once, like a global
variable, and accessable to the class and all its subclasses.

a class instance variable is there for the class and
each of its subclasses, and can have a different value
for each subclass.

i hope that makes sense
cheers,
wolfgang

>>>>> "Alexandre" == Bergel, Alexandre <[hidden email]> writes:

Alexandre> Class variables are accessible within the instance and class sides.
Alexandre> class instance variables are accessible only within the class
Alexandre> instance side.

Continuing on that, suppose you have Parent and Child classes, with the Child
class being a subclass of Parent.

If you define 'Home' as a class variable of Parent, you can use Home in both
Parent and Child class *and* instance methods, and it'll refer to the single
object defined in Parent class.  (Class variables are always capitalized.)

If you define 'office' as a class-instance variable of the Parent class, you
can use it in Parent class methods only, not Parent instance methods.
And, the Child class methods can also "use it", but they're referring to
their own independent class-instance variable unique to the Child class.

In other words, a class-instance variable is to a class, what an instance
variable is to an instance: unique to the object.  A class variable, on the
other hand, is a bit like a global, but visible only to a class and its
subclasses in all class and instance methods.  As with any broad-scoped
variable, use class variables only when needed, since it makes subclassing
trickier and scope pollution and "action at a distance" more likely.

--
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On Monday 14 Jul 2008 6:57:10 pm Martin Beck wrote:
> Hi list,
>
> can anyone please explain the difference between class variables at the
> instance side in the class browser and instance variables at the class
> side (thus of the metaclass) to me? And which of them is the preferred one?
Read 'class variables' as 'shared variables'. On any given side,
classVariableNames: defines variables that are shared across all instances,
while instanceVariableNames: defines variables that are unique to each
instance. Both these sets of variables are accessible only to the methods
defined on that side and its sub-classes.

Classes are instances of a Metaclass but they are special in that there will
be only one instance of a given class (stored in a global variable whose name
is same as the class name), so there is no need for a separate
classVariableNames: on the class side.

 Metaclass allInstances select: [ :c | c instVarNames size > 0 ]

They do come in handy to track global state. For instance, SmalltalkImage
class uses 'current' to track its singleton instance. ProjectLoading class
uses it to track current world being loaded. Beeper uses one for the default
sound to be played for system beep.

Classes are global variables and their instance variables are also global.
Unintentional modifications can result in weird side-effects (see comment in
Player class>>scripts). So they are sparingly used.

HTH,
Subbu