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Class vars and inst vars of a class

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Uko2

Mar 19, 2014; 10:26pm

Class vars and inst vars of a class

Can anybody tell me what is the difference between the class variable on the "instance side” definition and instance variable on the “class side” definition?

Thanks in advance.
Uko

Eliot Miranda-2

Mar 19, 2014; 10:33pm

Re: Class vars and inst vars of a class

Hi Uko,

On Wed, Mar 19, 2014 at 3:26 PM, Yuriy Tymchuk <[hidden email]> wrote:

Can anybody tell me what is the difference between the class variable on the "instance side” definition and instance variable on the “class side” definition?

Thanks in advance.
Uko

Class variables, as in classVariableNames: 'AClassVariable' are global variables in scope in all methods of the class and its subclass, instannce methods of the class and instance methods if the subclass. In the example, there is only one AClassVariable in the class.

Class instance variables are instance variables in the class object. The class object inherits from Behavior, ClassDescription and Class, and hence already has the superclass, methodDict, format, instanceVariables, organization, subclasses, name, classPool, sharedPools instance variables (details depending on dialect). The instanceVariableNames: on the class side add to these, but they are per-instance, and so the class and each of its subclasses has one of these instance variables and hence each class can have its own value for it, just as the class and each of its subclasses have their own method dictionaries. Further, the class instance variable is only in scope in class methods.

HTH,

Eliot

Nicolas Cellier

Mar 19, 2014; 10:34pm

Re: Class vars and inst vars of a class

In reply to this post by Uko2

A class variable is shared between all the subclasses.

A class side instance variable is like instance variable, a slot at class side: every subclass (class side) will have its own slot.

2014-03-19 23:26 GMT+01:00 Yuriy Tymchuk <[hidden email]>:

Can anybody tell me what is the difference between the class variable on the "instance side” definition and instance variable on the “class side” definition?

Thanks in advance.
Uko

Ben Coman

Mar 19, 2014; 10:53pm

Re: Class vars and inst vars of a class

In reply to this post by Uko2

Yuriy Tymchuk wrote:
> Can anybody tell me what is the difference between the class variable on the "instance side” definition and instance variable on the “class side” definition?
>
> Thanks in advance.
> Uko
>
>
instance-variable: A variable that holds the private state of an object.
In Smalltalk these can only be accessed by instance-methods.

class-variable: An attribute of a class that is shared by that class,
its subclasses, and all instances of those classes. Can be
accessed by both instance-methods and class-methods.

class-instance-variable: An attribute of a class that is not shared
outside that specific class. Only the definition is inherited by
subclasses, not the value . Each subclass has its own private value.
Accessible only by class-methods.

You will understand it best by experimenting. Here is one I made for
myself...

ivCounter - instance variable counter
CVCounter - class variable counter
CIVCounter - class instance variable counter

"----------"
Object subclass: #TestCounter
instanceVariableNames: 'ivCounter iName'
classVariableNames: 'CVCounter'
poolDictionaries: ''
category: 'MyExamples'

TestCounter class "accessed by clicking the <Class> button"
instanceVariableNames: 'CIVCounter'

TestCounter subclass: #TestSubCounter
instanceVariableNames: ''
classVariableNames: ''
poolDictionaries: ''
category: 'MyExamples'

"----------"
TestCounter name: myInstanceName
iName := myInstanceName.

TestCounter >> next
ivCounter ifNil: [ ivCounter := 0 ].
CVCounter ifNil: [ CVCounter := 0 ].
ivCounter := ivCounter + 1.
CVCounter := CVCounter + 1.

TestCounter >> printOn: aStream
super printOn: aStream.
aStream nextPutAll: ' ['.
aStream
print: iName.
aStream nextPutAll: '] ( '.
aStream
nextPutAll: ' ivCounter=';
print: ivCounter.
aStream
nextPutAll: ', CIVCounter=n/a'.
aStream
nextPutAll: ', CVCounter=';
print: CVCounter.
aStream nextPutAll: ' )'.

"----------"
TestCounter class >> next2
CIVCounter ifNil: [ CIVCounter := 0 ].
CVCounter ifNil: [ CVCounter := 0 ].

CIVCounter := CIVCounter + 1.
CVCounter := CVCounter + 1.

TestCounter class >> printOn: aStream
super printOn: aStream.
aStream nextPutAll: ' ( '.
aStream
nextPutAll: 'ivCounter=n/a'.
aStream
nextPutAll: ', CIVCounter=';
print: CIVCounter.
aStream
nextPutAll: ', CVCounter=';
print: CVCounter.
aStream nextPutAll: ' )'.

TestCounter class >> reset
CVCounter := nil.
CIVCounter := nil.

"---------"
Then from Workspace evaluate these...

tc1 := TestCounter new.
tc2 := TestCounter new.

tsc1 := TestSubCounter new.
tsc2 := TestSubCounter new.

TestCounter reset.
TestSubCounter reset.

tc1 next.
tc2 next.
tc1 next.
tc2 next.

tsc1 next.
tsc2 next.
tsc1 next.
tsc2 next.

TestCounter next2.
TestCounter next2.
TestCounter next2.

TestSubCounter next2.
TestSubCounter next2.
TestSubCounter next2.

cheers -ben

Uko2

Mar 20, 2014; 7:32am

Re: Class vars and inst vars of a class

Thank you all for your replies.

I would say, that “Class Variable” name is a bit misleading, but now I know what is going on behind it. :)

Cheers!
Uko

On 19 Mar 2014, at 23:53, Ben Coman <[hidden email]> wrote:

> Yuriy Tymchuk wrote:
>> Can anybody tell me what is the difference between the class variable on the "instance side” definition and instance variable on the “class side” definition?
>>
>> Thanks in advance.
>> Uko
>>
>>
> instance-variable: A variable that holds the private state of an object. In Smalltalk these can only be accessed by instance-methods.
>
> class-variable: An attribute of a class that is shared by that class, its subclasses, and all instances of those classes. Can be accessed by both instance-methods and class-methods.
>
> class-instance-variable: An attribute of a class that is not shared outside that specific class. Only the definition is inherited by subclasses, not the value . Each subclass has its own private value. Accessible only by class-methods.
>
>
> You will understand it best by experimenting. Here is one I made for myself...
>
> ivCounter - instance variable counter
> CVCounter - class variable counter
> CIVCounter - class instance variable counter
>
> "----------"
> Object subclass: #TestCounter
> instanceVariableNames: 'ivCounter iName'
> classVariableNames: 'CVCounter'
> poolDictionaries: ''
> category: 'MyExamples'
>
> TestCounter class "accessed by clicking the <Class> button"
> instanceVariableNames: 'CIVCounter'
>
> TestCounter subclass: #TestSubCounter
> instanceVariableNames: ''
> classVariableNames: ''
> poolDictionaries: ''
> category: 'MyExamples'
> "----------"
> TestCounter name: myInstanceName
> iName := myInstanceName.
> TestCounter >> next
> ivCounter ifNil: [ ivCounter := 0 ].
> CVCounter ifNil: [ CVCounter := 0 ].
> ivCounter := ivCounter + 1.
> CVCounter := CVCounter + 1.
> TestCounter >> printOn: aStream
> super printOn: aStream.
> aStream nextPutAll: ' ['. aStream
> print: iName.
> aStream nextPutAll: '] ( '. aStream
> nextPutAll: ' ivCounter=';
> print: ivCounter.
> aStream
> nextPutAll: ', CIVCounter=n/a'.
> aStream
> nextPutAll: ', CVCounter=';
> print: CVCounter.
> aStream nextPutAll: ' )'. "----------" TestCounter class >> next2
> CIVCounter ifNil: [ CIVCounter := 0 ].
> CVCounter ifNil: [ CVCounter := 0 ].
>
> CIVCounter := CIVCounter + 1.
> CVCounter := CVCounter + 1.
> TestCounter class >> printOn: aStream
> super printOn: aStream.
> aStream nextPutAll: ' ( '. aStream
> nextPutAll: 'ivCounter=n/a'.
> aStream
> nextPutAll: ', CIVCounter=';
> print: CIVCounter.
> aStream
> nextPutAll: ', CVCounter=';
> print: CVCounter.
> aStream nextPutAll: ' )'. TestCounter class >> reset
> CVCounter := nil.
> CIVCounter := nil.
>
> "---------"
> Then from Workspace evaluate these...
>
> tc1 := TestCounter new.
> tc2 := TestCounter new.
>
> tsc1 := TestSubCounter new.
> tsc2 := TestSubCounter new.
>
> TestCounter reset.
> TestSubCounter reset.
>
> tc1 next. tc2 next.
> tc1 next.
> tc2 next.
>
> tsc1 next.
> tsc2 next.
> tsc1 next.
> tsc2 next.
>
> TestCounter next2.
> TestCounter next2.
> TestCounter next2.
>
>
> TestSubCounter next2.
> TestSubCounter next2.
> TestSubCounter next2.
>
> cheers -ben
>
>
>
>
>

... [show rest of quote]

pharo4Stef@free.fr

Mar 20, 2014; 7:42am

Re: Class vars and inst vars of a class

In reply to this post by Uko2

read the pharo by example book.
On 19 Mar 2014, at 23:26, Yuriy Tymchuk <[hidden email]> wrote:

> Can anybody tell me what is the difference between the class variable on the "instance side” definition and instance variable on the “class side” definition?
>
> Thanks in advance.
> Uko

pharo4Stef@free.fr

Mar 20, 2014; 7:43am

Re: Class vars and inst vars of a class

In reply to this post by Uko2

On 20 Mar 2014, at 08:32, Yuriy Tymchuk <[hidden email]> wrote:

> Thank you all for your replies.
>
> I would say, that “Class Variable” name is a bit misleading, but now I know what is going on behind it. :)

Yes I would like to rename them to sharedVariable

>
> Cheers!
> Uko
>
> On 19 Mar 2014, at 23:53, Ben Coman <[hidden email]> wrote:
>
>> Yuriy Tymchuk wrote:
>>> Can anybody tell me what is the difference between the class variable on the "instance side” definition and instance variable on the “class side” definition?
>>>
>>> Thanks in advance.
>>> Uko
>>>
>>>
>> instance-variable: A variable that holds the private state of an object. In Smalltalk these can only be accessed by instance-methods.
>>
>> class-variable: An attribute of a class that is shared by that class, its subclasses, and all instances of those classes. Can be accessed by both instance-methods and class-methods.
>>
>> class-instance-variable: An attribute of a class that is not shared outside that specific class. Only the definition is inherited by subclasses, not the value . Each subclass has its own private value. Accessible only by class-methods.
>>
>>
>> You will understand it best by experimenting. Here is one I made for myself...
>>
>> ivCounter - instance variable counter
>> CVCounter - class variable counter
>> CIVCounter - class instance variable counter
>>
>> "----------"
>> Object subclass: #TestCounter
>> instanceVariableNames: 'ivCounter iName'
>> classVariableNames: 'CVCounter'
>> poolDictionaries: ''
>> category: 'MyExamples'
>>
>> TestCounter class "accessed by clicking the <Class> button"
>> instanceVariableNames: 'CIVCounter'
>>
>> TestCounter subclass: #TestSubCounter
>> instanceVariableNames: ''
>> classVariableNames: ''
>> poolDictionaries: ''
>> category: 'MyExamples'
>> "----------"
>> TestCounter name: myInstanceName
>> iName := myInstanceName.
>> TestCounter >> next
>> ivCounter ifNil: [ ivCounter := 0 ].
>> CVCounter ifNil: [ CVCounter := 0 ].
>> ivCounter := ivCounter + 1.
>> CVCounter := CVCounter + 1.
>> TestCounter >> printOn: aStream
>> super printOn: aStream.
>> aStream nextPutAll: ' ['. aStream
>> print: iName.
>> aStream nextPutAll: '] ( '. aStream
>> nextPutAll: ' ivCounter=';
>> print: ivCounter.
>> aStream
>> nextPutAll: ', CIVCounter=n/a'.
>> aStream
>> nextPutAll: ', CVCounter=';
>> print: CVCounter.
>> aStream nextPutAll: ' )'. "----------" TestCounter class >> next2
>> CIVCounter ifNil: [ CIVCounter := 0 ].
>> CVCounter ifNil: [ CVCounter := 0 ].
>>
>> CIVCounter := CIVCounter + 1.
>> CVCounter := CVCounter + 1.
>> TestCounter class >> printOn: aStream
>> super printOn: aStream.
>> aStream nextPutAll: ' ( '. aStream
>> nextPutAll: 'ivCounter=n/a'.
>> aStream
>> nextPutAll: ', CIVCounter=';
>> print: CIVCounter.
>> aStream
>> nextPutAll: ', CVCounter=';
>> print: CVCounter.
>> aStream nextPutAll: ' )'. TestCounter class >> reset
>> CVCounter := nil.
>> CIVCounter := nil.
>>
>> "---------"
>> Then from Workspace evaluate these...
>>
>> tc1 := TestCounter new.
>> tc2 := TestCounter new.
>>
>> tsc1 := TestSubCounter new.
>> tsc2 := TestSubCounter new.
>>
>> TestCounter reset.
>> TestSubCounter reset.
>>
>> tc1 next. tc2 next.
>> tc1 next.
>> tc2 next.
>>
>> tsc1 next.
>> tsc2 next.
>> tsc1 next.
>> tsc2 next.
>>
>> TestCounter next2.
>> TestCounter next2.
>> TestCounter next2.
>>
>>
>> TestSubCounter next2.
>> TestSubCounter next2.
>> TestSubCounter next2.
>>
>> cheers -ben
>>
>>
>>
>>
>>
>
>

... [show rest of quote]

Nicolas Cellier

Mar 20, 2014; 8:07am

Re: Class vars and inst vars of a class

2014-03-20 8:43 GMT+01:00 Pharo4Stef <[hidden email]>:

On 20 Mar 2014, at 08:32, Yuriy Tymchuk <[hidden email]> wrote:

> Thank you all for your replies.
>
> I would say, that “Class Variable” name is a bit misleading, but now I know what is going on behind it. :)

Yes I would like to rename them to sharedVariable

It's exactly a shared variable, but that does not describe the scope.

It's a shared variable accessible in all methods of a class, its metaclass and their subclasses...

>
> Cheers!
> Uko
>
> On 19 Mar 2014, at 23:53, Ben Coman <[hidden email]> wrote:
>
>> Yuriy Tymchuk wrote:
>>> Can anybody tell me what is the difference between the class variable on the "instance side” definition and instance variable on the “class side” definition?
>>>
>>> Thanks in advance.
>>> Uko
>>>
>>>
>> instance-variable: A variable that holds the private state of an object. In Smalltalk these can only be accessed by instance-methods.
>>
>> class-variable: An attribute of a class that is shared by that class, its subclasses, and all instances of those classes. Can be accessed by both instance-methods and class-methods.
>>
>> class-instance-variable: An attribute of a class that is not shared outside that specific class. Only the definition is inherited by subclasses, not the value . Each subclass has its own private value. Accessible only by class-methods.
>>
>>
>> You will understand it best by experimenting. Here is one I made for myself...
>>
>> ivCounter - instance variable counter
>> CVCounter - class variable counter
>> CIVCounter - class instance variable counter
>>
>> "----------"
>> Object subclass: #TestCounter
>> instanceVariableNames: 'ivCounter iName'
>> classVariableNames: 'CVCounter'
>> poolDictionaries: ''
>> category: 'MyExamples'
>>
>> TestCounter class "accessed by clicking the <Class> button"
>> instanceVariableNames: 'CIVCounter'
>>
>> TestCounter subclass: #TestSubCounter
>> instanceVariableNames: ''
>> classVariableNames: ''
>> poolDictionaries: ''
>> category: 'MyExamples'
>> "----------"
>> TestCounter name: myInstanceName
>> iName := myInstanceName.
>> TestCounter >> next
>> ivCounter ifNil: [ ivCounter := 0 ].
>> CVCounter ifNil: [ CVCounter := 0 ].
>> ivCounter := ivCounter + 1.
>> CVCounter := CVCounter + 1.
>> TestCounter >> printOn: aStream
>> super printOn: aStream.
>> aStream nextPutAll: ' ['. aStream
>> print: iName.
>> aStream nextPutAll: '] ( '. aStream
>> nextPutAll: ' ivCounter=';
>> print: ivCounter.
>> aStream
>> nextPutAll: ', CIVCounter=n/a'.
>> aStream
>> nextPutAll: ', CVCounter=';
>> print: CVCounter.
>> aStream nextPutAll: ' )'. "----------" TestCounter class >> next2
>> CIVCounter ifNil: [ CIVCounter := 0 ].
>> CVCounter ifNil: [ CVCounter := 0 ].
>>
>> CIVCounter := CIVCounter + 1.
>> CVCounter := CVCounter + 1.
>> TestCounter class >> printOn: aStream
>> super printOn: aStream.
>> aStream nextPutAll: ' ( '. aStream
>> nextPutAll: 'ivCounter=n/a'.
>> aStream
>> nextPutAll: ', CIVCounter=';
>> print: CIVCounter.
>> aStream
>> nextPutAll: ', CVCounter=';
>> print: CVCounter.
>> aStream nextPutAll: ' )'. TestCounter class >> reset
>> CVCounter := nil.
>> CIVCounter := nil.
>>
>> "---------"
>> Then from Workspace evaluate these...
>>
>> tc1 := TestCounter new.
>> tc2 := TestCounter new.
>>
>> tsc1 := TestSubCounter new.
>> tsc2 := TestSubCounter new.
>>
>> TestCounter reset.
>> TestSubCounter reset.
>>
>> tc1 next. tc2 next.
>> tc1 next.
>> tc2 next.
>>
>> tsc1 next.
>> tsc2 next.
>> tsc1 next.
>> tsc2 next.
>>
>> TestCounter next2.
>> TestCounter next2.
>> TestCounter next2.
>>
>>
>> TestSubCounter next2.
>> TestSubCounter next2.
>> TestSubCounter next2.
>>
>> cheers -ben
>>
>>
>>
>>
>>
>
>

... [show rest of quote]

Uko2

Mar 20, 2014; 8:15am

Re: Class vars and inst vars of a class

It’s an evil variable :)

On 20 Mar 2014, at 09:07, Nicolas Cellier <[hidden email]> wrote:

2014-03-20 8:43 GMT+01:00 Pharo4Stef <[hidden email]>:

On 20 Mar 2014, at 08:32, Yuriy Tymchuk <[hidden email]> wrote:

> Thank you all for your replies.
>
> I would say, that “Class Variable” name is a bit misleading, but now I know what is going on behind it. :)

Yes I would like to rename them to sharedVariable

It's exactly a shared variable, but that does not describe the scope.
It's a shared variable accessible in all methods of a class, its metaclass and their subclasses...

>
> Cheers!
> Uko
>
> On 19 Mar 2014, at 23:53, Ben Coman <[hidden email]> wrote:
>
>> Yuriy Tymchuk wrote:
>>> Can anybody tell me what is the difference between the class variable on the "instance side” definition and instance variable on the “class side” definition?
>>>
>>> Thanks in advance.
>>> Uko
>>>
>>>
>> instance-variable: A variable that holds the private state of an object. In Smalltalk these can only be accessed by instance-methods.
>>
>> class-variable: An attribute of a class that is shared by that class, its subclasses, and all instances of those classes. Can be accessed by both instance-methods and class-methods.
>>
>> class-instance-variable: An attribute of a class that is not shared outside that specific class. Only the definition is inherited by subclasses, not the value . Each subclass has its own private value. Accessible only by class-methods.
>>
>>
>> You will understand it best by experimenting. Here is one I made for myself...
>>
>> ivCounter - instance variable counter
>> CVCounter - class variable counter
>> CIVCounter - class instance variable counter
>>
>> "----------"
>> Object subclass: #TestCounter
>> instanceVariableNames: 'ivCounter iName'
>> classVariableNames: 'CVCounter'
>> poolDictionaries: ''
>> category: 'MyExamples'
>>
>> TestCounter class "accessed by clicking the <Class> button"
>> instanceVariableNames: 'CIVCounter'
>>
>> TestCounter subclass: #TestSubCounter
>> instanceVariableNames: ''
>> classVariableNames: ''
>> poolDictionaries: ''
>> category: 'MyExamples'
>> "----------"
>> TestCounter name: myInstanceName
>> iName := myInstanceName.
>> TestCounter >> next
>> ivCounter ifNil: [ ivCounter := 0 ].
>> CVCounter ifNil: [ CVCounter := 0 ].
>> ivCounter := ivCounter + 1.
>> CVCounter := CVCounter + 1.
>> TestCounter >> printOn: aStream
>> super printOn: aStream.
>> aStream nextPutAll: ' ['. aStream
>> print: iName.
>> aStream nextPutAll: '] ( '. aStream
>> nextPutAll: ' ivCounter=';
>> print: ivCounter.
>> aStream
>> nextPutAll: ', CIVCounter=n/a'.
>> aStream
>> nextPutAll: ', CVCounter=';
>> print: CVCounter.
>> aStream nextPutAll: ' )'. "----------" TestCounter class >> next2
>> CIVCounter ifNil: [ CIVCounter := 0 ].
>> CVCounter ifNil: [ CVCounter := 0 ].
>>
>> CIVCounter := CIVCounter + 1.
>> CVCounter := CVCounter + 1.
>> TestCounter class >> printOn: aStream
>> super printOn: aStream.
>> aStream nextPutAll: ' ( '. aStream
>> nextPutAll: 'ivCounter=n/a'.
>> aStream
>> nextPutAll: ', CIVCounter=';
>> print: CIVCounter.
>> aStream
>> nextPutAll: ', CVCounter=';
>> print: CVCounter.
>> aStream nextPutAll: ' )'. TestCounter class >> reset
>> CVCounter := nil.
>> CIVCounter := nil.
>>
>> "---------"
>> Then from Workspace evaluate these...
>>
>> tc1 := TestCounter new.
>> tc2 := TestCounter new.
>>
>> tsc1 := TestSubCounter new.
>> tsc2 := TestSubCounter new.
>>
>> TestCounter reset.
>> TestSubCounter reset.
>>
>> tc1 next. tc2 next.
>> tc1 next.
>> tc2 next.
>>
>> tsc1 next.
>> tsc2 next.
>> tsc1 next.
>> tsc2 next.
>>
>> TestCounter next2.
>> TestCounter next2.
>> TestCounter next2.
>>
>>
>> TestSubCounter next2.
>> TestSubCounter next2.
>> TestSubCounter next2.
>>
>> cheers -ben
>>
>>
>>
>>
>>
>
>

... [show rest of quote]

... [show rest of quote]

Sean P. DeNigris

Mar 20, 2014; 2:39pm

Re: Class vars and inst vars of a class

Administrator

In reply to this post by Uko2

Uko2 wrote

what is the difference between the class variable...

pharo-users@lists.pharo.org is a better place to ask questions like this, so that they do not get missed amongst the dev threads, and to give the devs' inboxes a break.

Cheers,
Sean