Class variables

Hi,

I'm reading "On To Smalltalk" and trying some of its examples. On
Chapter 18 is about class variables, but Dolphin seem to have
something else called classInstanceVariableNames.

What's the difference between class variables and class instance
variables? O:-)

Class Variables are globals (or unique) for the entire hierarchy.
Animal (class variable BestFriend)
    Dog
    Cat
    ...
The class variable "BestFriend" is the same for all classes.
Dog setBestFriend: Dog new.
Cat bestFriend is identical to Dog bestFriend (or other class of the
hierarchy).
The class variable is the same for all of them.


Instances Class Variables are locals for each class.
Animal (instance class variable bestFriend)
    Dog
    Cat

Here each class has his unique bestFriend variable.
Dog setBestFriend: Dog new.
Cat setBestFriend: Cat new.

Cat bestFriend is diffent to Dog bestFriend, because are different
variables.

Regards Bruno



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Just to add a couple of extra bits to Bruno's answer.

ClassInstanceVariables are not just a Dolphin "thing", most (perhaps all?)
modern Smalltalks implement it.  They are not in the Blue Book but, from a
quick check in my library, have been around since the mid 90's at least -
FWIW I first saw them in ST/V for Win32.

It should be noted that the scope of ClassVariables and
ClassInstanceVariables is slightly different.
- ClassVariables are directly accessible to both class and instance methods.
- ClassInstanceVariables are only directly accessible to class methods.

--
Ian

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On Sun, 28 Nov 2004 15:49:11 -0200, "Bruno" <[hidden email]>
wrote:


>Instances Class Variables are locals for each class.
>Animal (instance class variable bestFriend)
>    Dog
>    Cat

Then what's the difference between them an regular instance variables?
Why the need for both? O:-)

On Sun, 28 Nov 2004 20:20:54 +0100,
Fernando <[hidden email]> wrote:
I'm not sure if you absolutely need them, but:

Vehicle
   CV: Wheels

   Car
   Bicycle

would lead to serious problems when setting Wheels := 2 in Bicycle,
unless you are James Bond.

CIVs still allow you sharing *one* object between many instances,
but they are firmly encapsulated in the owning class.

Taking another point of view:

        CIV : Class :: IV : Object

i.e. every class has its own CIVs just as every object has its own IVs.
So actually globally available IV's are the special construct.

HTH
s.

Fernando,

> Then what's the difference between them an regular instance variables?
> Why the need for both? O:-)

I'm going to regret this :-(

OK.  InstanceVariables and ClassInstanceVariables are really the same thing.

When you create an instance of a class then you can create an
InstanceVariable associated with the class.  You can create any number of
instances of a class and each one will have it's own individual
InstanceVariables.

Classes in Smalltalk are objects in their own right - each one is a
Singleton instance of its metaclass.  As a class is an instance of a parent
class (metaclass) then it can have InstanceVariables.  Because these
variable are associated with the class object they have to be accessed in a
different way to "normal" InstanceVariables and this is done by defining
them as ClassInstanceVariables (InstanceVariables associated with a classes
metaclass).  As a class is a Singleton then there is only ever one
ClassInstanceVariable associated with a class.

So a ClassInstanceVariable is an InstanceVariable defined in the Metaclass
of a Class and only accessible through class methods.

That's about the fifth edit of that and I'm still not happy with it - but
I've got to go out now :-)

--
Ian

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