How to migrate an object to a subclass of its class but with new inst vars ?

Hi,

i'm trying to migrate an object from a given class A to a subclass of A with new inst vars. Can't get it to work.

I have a class and an instance of it, say:

Object subclass: #A
    instanceVariableNames: ''

|a|

a := A new.

A has no instance variables. Now i create a subclass B of A with a new inst var:

A subclass: #B
    instanceVariableNames: 'x'

I want to do the following:

B adoptInstance: a

It does'nt work, for what i understand it is because the format of the classes are different (B has an inst var and A has not).

How can i make the a object become an instance of B (subclass of a's class), making a acquiring the new inst var x (and in more complex cases keeping its own previous inst vars) ?

Steven.

Hi Vincent, thanks for your quick answer.

This is the default behavior of adoptInstance: which calls primitiveChangeClassTo:

But i specifically want to gain new inst vars, which does not work that way.

I can't add an instvar to the superclass since i don't want to change my system.

Steven.

Le 2017-06-07 15:09, Blondeau Vincent a écrit :

Hi,

 

You should be able to do it using the method primitiveChangeClassTo: but it not recommended to do it. Are you sure that it is the only way to resolve your problem?

 

If you really want to, an example:

 

Object subclass: #Toto

                slots: { #tata }

                classVariables: {  }

                category: 'Temp'.

 

Toto subclass: #Titi

                slots: {  }

                classVariables: {  }

                category: 'Temp'.

obj := Toto new.

obj primitiveChangeClassTo: Titi new.

 

obj has a new class.

 

However, you cannot add a new instance variable this way but you keep the values of the instance.

But nothing avoid you to add an empty instance variable in a superclass that only the subclass will use.

 

I hope that it will help you

 

Vincent

 

 

De : Pharo-users [mailto:[hidden email]] De la part de Steven Costiou
Envoyé : mercredi 7 juin 2017 14:54
À : Pharo users users
Objet : [Pharo-users] How to migrate an object to a subclass of its class but with new inst vars ?

 

Hi,

i'm trying to migrate an object from a given class A to a subclass of A with new inst vars. Can't get it to work.

I have a class and an instance of it, say:

Object subclass: #A
    instanceVariableNames: ''

|a|

a := A new.

A has no instance variables. Now i create a subclass B of A with a new inst var:

A subclass: #B
    instanceVariableNames: 'x'

I want to do the following:

B adoptInstance: a

It does'nt work, for what i understand it is because the format of the classes are different (B has an inst var and A has not).

 

How can i make the a object become an instance of B (subclass of a's class), making a acquiring the new inst var x (and in more complex cases keeping its own previous inst vars) ?

 

Steven.

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I managed to do it :)

What i did not say is that the subclasses (with new inst var) i want to use are anonymous subclasses.

So i subclassed the SlotClassBuilder and i did:

- generate an anonymous subclass from the source class

- migrate my object

- modify the anonymous subclass with new instance variables

- rebuild the class with my own SlotClassBuilder

It seems to work. I did not try how to migrate back my object to its original class but i think i might have a problem there. Also it seems very slow.

Indeed it is a very certain, specific application :)

Steven.

Le 2017-06-07 15:32, Blondeau Vincent a écrit :

So, I do not think that is possible...

 

Any solution I see is either to change the superclass or to add an external object with a mapping between your new objects and the values of your new IVs.

Or you do the migration and after you add the IV once all instances are migrated.

 

But, I repeat that, as written in comments of primitiveChangeClassTo,  "The facility is really provided for certain, very specific applications (mostly related to classes changing shape) and not for casual use."

 

Vincent

 

De : Pharo-users [mailto:[hidden email]] De la part de Steven Costiou
Envoyé : mercredi 7 juin 2017 15:18
À : Any question about pharo is welcome
Objet : Re: [Pharo-users] How to migrate an object to a subclass of its class but with new inst vars ?

 

Hi Vincent, thanks for your quick answer.

This is the default behavior of adoptInstance: which calls primitiveChangeClassTo:

But i specifically want to gain new inst vars, which does not work that way.

I can't add an instvar to the superclass since i don't want to change my system.

 

Steven.

Le 2017-06-07 15:09, Blondeau Vincent a écrit :

Hi,

 

You should be able to do it using the method primitiveChangeClassTo: but it not recommended to do it. Are you sure that it is the only way to resolve your problem?

 

If you really want to, an example:

 

Object subclass: #Toto

                slots: { #tata }

                classVariables: {  }

                category: 'Temp'.

 

Toto subclass: #Titi

                slots: {  }

                classVariables: {  }

                category: 'Temp'.

obj := Toto new.

obj primitiveChangeClassTo: Titi new.

 

obj has a new class.

 

However, you cannot add a new instance variable this way but you keep the values of the instance.

But nothing avoid you to add an empty instance variable in a superclass that only the subclass will use.

 

I hope that it will help you

 

Vincent

 

 

De : Pharo-users [[hidden email]] De la part de Steven Costiou
Envoyé : mercredi 7 juin 2017 14:54
À : Pharo users users
Objet : [Pharo-users] How to migrate an object to a subclass of its class but with new inst vars ?

 

Hi,

i'm trying to migrate an object from a given class A to a subclass of A with new inst vars. Can't get it to work.

I have a class and an instance of it, say:

Object subclass: #A
    instanceVariableNames: ''

|a|

a := A new.

A has no instance variables. Now i create a subclass B of A with a new inst var:

A subclass: #B
    instanceVariableNames: 'x'

I want to do the following:

B adoptInstance: a

It does'nt work, for what i understand it is because the format of the classes are different (B has an inst var and A has not).

 

How can i make the a object become an instance of B (subclass of a's class), making a acquiring the new inst var x (and in more complex cases keeping its own previous inst vars) ?

 

Steven.

Yes but when you do that you loose all the states from "a" (at least the values). You have to do state migration management...

Le 2017-06-07 16:18, Denis Kudriashov a écrit :

2017-06-07 14:53 GMT+02:00 Steven Costiou <[hidden email]>:

I want to do the following:

B adoptInstance: a

It does'nt work, for what i understand it is because the format of the classes are different (B has an inst var and A has not).

Yes, #adopt checks class format. 

Use need to use #become to achieve what you want:

a becomeForward: B new 

 

Hmm ok, but can i easily make b become a again later ? I guess i would just need to instanciate a new a...

But would it not be faster to just change the class (which finally works with a little tuning) ?

I can't test right now but i will compare the two things...

Le 2017-06-07 21:59, Denis Kudriashov a écrit :

2017-06-07 16:43 GMT+02:00 Steven Costiou <[hidden email]>:

Yes but when you do that you loose all the states from "a" (at least the values). You have to do state migration management...

You just need extra step to copy all state from old object:

b := B new.

b copyFrom: a.

a becomeForward: b.