System Browser: instance vs. class

Hello all,

  I would like to know what is the practical difference between
instance and class in the System Browser.  Based on the Abstract
Factory example from Alpert's 'Desgin Patterns' book:

1) Not using class:

  Object subclass: #CarAssembler
          instanceVariableNames: 'factory'

  factory: aCarFactory
          factory := aCarFactory.

And in the workspace:

  assembler := CarAssembler new factory: RenaultFactory new.

  car := assembler assembleCar.

Works fine.


2) Using class:

The book example actually uses:

  Object subclass: #CarAssembler
          instanceVariableNames: 'factory'

  CarAssembler class>> using aCarFactory

And then in the workspace:

  assembler := CarAssembler using: RenaultFactory new.

  car := assembler assembleCar.

That also works fine.

I see it as a semantic difference only.  There must be more than
that.  The class approach would nevertheless have an instance
variable of the name 'factory' in which the wished factory object
will be found. That variable is here initialized to a
RenaultFactory in both cases.  What are the pros and cons of
using or not using the class approach ?

Thanks,

Al
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>>>>> "lanas" == lanas  <[hidden email]> writes:

lanas>   I would like to know what is the practical difference between
lanas> instance and class in the System Browser.  Based on the Abstract
lanas> Factory example from Alpert's 'Desgin Patterns' book:

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.

Does that help?

--
Randal L. Schwartz - Stonehenge Consulting Services, Inc. - +1 503 777 0095
<[hidden email]> <URL:http://www.stonehenge.com/merlyn/>
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Le Dimanche, 31 Janvier 2010 15:13:50 -0800,
[hidden email] (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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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 <[hidden email]> wrote:



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