[squeak-dev] Re: Question about class and instance

Yanni Chiu yanni at rogers.com
Fri Oct 12 19:50:46 UTC 2012

On 12/10/12 2:23 PM, Joseph J Alotta wrote:
> Is there a general rule for class and instance?
> For example, if there should be only one instance of an object, should it be a class object?

That is the Singleton pattern. Look for examples in the image - try 
looking for senders/implementers of #default or #instance.

There's arguments on both sides, whether to use the class object itself 
as the singleton, vs. using a single default instance. I think it comes 
down to how likely it is that what you thought was a singleton, turns 
out not to be, and then you have to move all the methods over from the 
class-side to the instance-side.

> Or if I expect it to stay in memory and be available all the time, should it be a class object?

Your words "stay in memory" is a big concern. It indicates that you are 
bringing some assumptions about programming from another programming 
language/environment, to your learning of Smalltalk/Squeak. IMHO, that's 
going to impair your ability to learn how to effectively learn and use 
Smalltalk. Try to find articles/tutorials that explain the Smalltalk 
image concept, and the concept of live-ness.

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