2011/6/1 Juan Vuletich <juan at jvuletich.org>:
> Kamil Tomsik wrote:
>>
>>
>>> So, #new: creates a point without values for x and y... What if I want to
>>> only build objects that make sense, meaning that their instance variables
>>> must conform to some invariant defined by the class?
>>>
>>
>> you can't do this even in smalltalk.
>
> How come? In your Point example it is very easy to get a 'broken' point...
> How would you get an equally broken OrderedCollection in Smalltalk? In my
> opinion doing 'OrderedCollection basicNew' or "anOrderedCollection
> instVarNamed: 'array' put: nil" don't count, because it's not happening by
> accident: You're clearly stating the intention to break it!

Well, it's _technically_ true. Consider Point>>x:y:. First you
evaluate "self basicNew", at which point you have an uninitialised
object, and then you call the private #setX:setY:, after which you
have a well-initialised object. From the outside, you only ever see a
properly initialised object, of course.

But I say "technically". Practically speaking, having constructor
messages on the class side that call private initialisers on the
instance side, and having the discipline not to send those private
messages from other objects, means No Problem.

frank

>
> Cheers,
> Juan Vuletich
>
>>
>>
>>
>> Best,
>> Kamil
>
>
>