[Newbies] System Browser: instance vs. class

[Jerome Peace]

Hi lanas

Your instincts are correct.

The 'factory' variable should belong to the instance.

The important thing about messages is trying to keep strait who they are going to.

In this case the #factory message is sent to an instance of CarAssembler.
The message new is sent to the class usually denoted as CarAssembler class.
The message #new makes an instance which the message #factory then supplys with a value.

The class is there to manufacture assemblers and to keep track of the methods they know (e.g. #factory:). The factory ivar is there to keep track of a second object which in your example was an instance of a class that assembles a particular type of car.

The message #using: is a convenience. It is sent fo CarAssembler class inorder to create an instance of a CarAssembler that will assemble a particular type of car according to its argument.

To touch on the question you asked, which was both methods work so which one is preferred. The answer is it's designer choice. In the context of an application that made only a few different instances of car assembler it might be simpler to get along with out the convenience method. If you are turning out car assemblers by the dozens then the #using: method would keep your code shorter and clearer. 

Often times the answer to questions like your are "It depends." 

hth,

Yours in friendship and service, --Jerome Peace





--- On Sun, 1/31/10, lanas <lanas at securenet.net> wrote:

> From: lanas <lanas at securenet.net>
> Subject: Re: [Newbies] System Browser: instance vs. class
> To: beginners at lists.squeakfoundation.org
> Date: Sunday, January 31, 2010, 7:22 PM
> Le Dimanche, 31 Janvier 2010 15:13:50
> -0800,
> merlyn at stonehenge.com
> (Randal L. Schwartz) a écrit :
> 
> > A class is always a singleton.  There is only one
> class named Car
> > in the system.  However, there can be many car
> instances, usually
> > created by sending "new" to "Car".
> > 
> > In the Car class, you'd hold information collectively
> about all cars,
> > such as the default specifications or number of cars
> produced.
> > 
> > In each car instance, you'd hold information about a
> specific car,
> > like its color or owner.
> 
> Thanks for pointing that out.  In the book's example
> though, the
> variable is declared as instance variable.  To go
> along with class
> variables holding information for all instances, it should
> instead
> be declared as class variable isn't it ? (Although it does
> work like
> this).
> 
> Object subclass: #CarAssembler
>   instanceVariableNames: 'factory'
>   classVariableNames: ''
> 
> CarAssembler class>>using: aCarFactory
>   ^self new factory: aCarFactory
> 
> That is, if one thinks that a single car factory should be
> shared
> amongst all instances of CarAssembler.  I think not
> and I would make it
> local to each object instance.
> 
> Al
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