Check if an object is an instance of a given class or of a subclass of it

Question

Is there an easy way to do this in Smalltalk? I'm 80% sure that there is some method but can't find it anywhere.

I know that I can use

(instance class = SomeClass) ifTrue:

And I know that I can use superclass etc... but I hope that there is something built in :)

aka.nice · Accepted Answer · 2012-12-08 17:11:33Z

22

To test whether anObject is instance of aClass:

(anObject isMemberOf: aClass)

To test whether it is an instance of aClass or one of it subclasses:

(anObject isKindOf: aClass)

answered Dec 8, 2012 at 17:11

aka.nice

9,2671 gold badge29 silver badges41 bronze badges

Add a comment |

Janko Mivšek · Accepted Answer · 2012-12-08 17:13:17Z

6

You are right, to check for exact class you use (using identity instead):

instance class == SomeClass ifTrue: []

Usefull is also isKindOf: which tests if instance is a class or subclass of given class:

(instance isKindOf: SomeClass) ifTrue: []

Nicest and most elegant is to write a testing method in superclass and peer classes, then use it like:

instance isSomeClass ifTrue: []

answered Dec 8, 2012 at 17:13

Janko Mivšek

3,9643 gold badges24 silver badges29 bronze badges

1

Nicest and most elegant is to not ask, tell, e.g.: instead of: object isSomething ifTrue: [ do something ] use: object doSomething
– Igor Stasenko
Commented Dec 9, 2012 at 5:33
2

I agree with Igor. Moreover, "nicest and most elegant" is in the eye of the beholder. What isInteger and friends do is definitely faster as they are a single message send that immediately returns true/false versus isKindOf: which has to loop up the class hierarchy. The downside for some people is that you have to add a isSomeClass method to Object which returns false.
– Dave Mason
Commented Dec 9, 2012 at 14:31

Add a comment |

2 Answers 2