From: "David A. Black" <dblack@...>
Date: 2009-01-29T11:48:19+09:00
Subject: Re: proper use of classes

Hi --

On Thu, 29 Jan 2009, Tom Cloyd wrote:

> 1. HOW do you use a class?
>
> I was assuming that since I couldn't pass data to an instance, after 
> creation, I have no option but to write data into its instance vars as 
> needed. Sometimes, it seems there simply is no other option.

You can pass data to an instance:

   instance.some_method(data)

> But, is it approved practice to do something like
>
> junk = MyClass.new( var_1...var_n ).mymethod
>
> which creates an instance and calls a method which leaves its results in some 
> instance vars., then simple read the results with...but how? there's no 
> instance object!

What you've written here is similar to:

   upstring = String.new("David").upcase

or something like that, and it's perfectly legit, though you might
also want to grab the object separately:

   string = String.new("David")
   upstring = string.upcase

> (Not that I can find, anyway.) That seems to leave me only 
> with this possibility:
>
> myclass = MyClass.newMyClass.new( var_1...var_n )

I don't quite get that line. What's newMyClass? (Or maybe it's
garbling again between our machines?)

> myclass.mymethod
> varA = myclass.var_whatever
> etc....until I have all my results back out of the instance.
>
> Compared to a simple method call, this seems designed to make me crazy 
> quickly. Is there a better way?

I'm not sure I'm following. I guess the short answer is that there's a
ton of different patterns you can follow, depending on what you need
to do.

> Then, to call the instance again, I have to write new data into its instance 
> vars. This simply looks like nonsense, unless one really needs to have the 
> encapsulation that an instance offers. Am I missing something? Is this just 
> the facts of life when using classes?
>
> 2. Is it accepted practice to simply create a new instance every time the 
> class is needed, thus setting the instance's state once, using it with one or 
> more method calls, then moving on to the next new instance? It occurs to me 
> that maybe Ruby's garbage collection would sweep the old instance right up, 
> knowing it won't be used again, but I don't know.
>
> My nightmare case is a class which operates on an input record, but 
> differently each time, depending upon a number of factors in the environment 
> outside the class. I just can't see a graceful way to do this. I'm struggling 
> to see why I do OO programming at all in this case.

Normally you'd write a class in cases where you want more than one of
something. I'm not sure that's the case here. What exactly do you mean
by a class operating on an input record? Or, to go at it a different
way, what exactly is the flow of events that you want to handle? It
may be that you could use a class called InputHandler (or whatever),
and you'd do something like:

   ih = InputHandler.new(filename)
   fields = ih.parse_into_fields

A class is a generalization. So if what you're doing isn't general,
you may not need or want to model it in classes. If you're writing a
script to parse one particular file, there's quite likely no point
writing a generalized handler class.

I feel like I'm failing to get beyond the rather unhelpful "it
depends" level, but maybe prompt me with some further specifics....


David

-- 
David A. Black / Ruby Power and Light, LLC
Ruby/Rails consulting & training: http://www.rubypal.com
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