Cast instance of superclass to instance of subclass

[MartinCz]

Hi,

I need to cast an instance of a superclass to an instance of a subclass. Is there any way?

I can't change the code that generates the instance of the superclass, so there's no way it could generate an instance of the subclass.

Thank you for any tips.

[TriSebastian]

Hi!

Well there are different approches available. It depends on the kind of Object you have.

You can either have a look at the class "String" and its implementations like asSymbol and so on.

Another way, that you can find in the RBChangeObjectsApp>>RenameClassChange

convertInstances
| oldClass newClass |
oldClass := self oldClass.
newClass := self newClass.
oldClass allInstances do:
[:each |
| newInst |
newInst := each basicSize > 0
ifTrue: [newClass basicNew: each basicSize]
ifFalse: [newClass basicNew].
1 to: newClass instSize
do: [:i | newInst instVarAt: i put: (each instVarAt: i)].
1 to: each basicSize do: [:i | newInst basicAt: i put: (each basicAt: i)].
each become: newInst]

Hope this helps!

Sebastian

[MartinCz]

Hi,

thank you for the prompt reply - in the mean time, I've come to the same conclusion as you - all instance variables need to be copied one by one as in Object>>deepCopy or in the method that you found.

Rather than doing this, I decided to choose composition instead of inheritance, so I don't have to deal with this problem

Anyway, thank you for your ideas and help.

Have a nice day!

[marten]

That was a nice question. Here a primitive, that - I would think - could do the work, if both classes have the same structure (same instance variables etc)...

Code: Select all

EsUserPrimitive(mskClassConvert)
{
   EsObject object = NULL;
   EsObject newClass = NULL;

   object = EsPrimArgument(1);
   newClass = EsPrimArgument(2);
   object->class = newClass;
   EsPrimSucceed(object);
}


you would call it via:

Code: Select all

MySuperclass new convertTo: MySubclass

with:

Code: Select all

MySuperclass >>convertTo: aClass

   ^self primConvert: self to: aClass


and:

Code: Select all

MySuperclass >>primConvert: anObject to: aClass

   <primitive: 'mskClassConvert.dll':mskClassConvert>
   ^self primitiveFailed


and you would get an instance of MySubclass ...

[MartinCz]

Wow

Thank you for this comprehensive answer!!

[waynej]

When I read the question, I did not understand it as needing to convert an instance of one class to that of another class. But it sounds like that is indeed what is desired.