From: Mohammad Khan <mkhan@...>
Date: 2005-01-20T02:42:42+09:00
Subject: Re: value by reference

On Wed, 2005-01-19 at 12:31, Florian Gross wrote:
> Mohammad Khan wrote:
> 
> > But what about scenario 2? [methods changes state of an Object that
> > it gets as an argument] Scenario 2 is not acting as scenario 1, is it
> > because scenario 2 gets object rather than variable? Or, Scenario 2
> > gets clonable object while scenario 1 not?
> 
> First, let's agree on using the term "mutable" (or "changeable") instead
> of "cloneable". (Immutable objects can not be cloned most of the time,
> but the both terms are not strictly the same things.)

agreed on using term 'mutable'.
> 
> Scenario 2 changes attributes of an Object. This is something that is
> entirely possible. It makes sense for a CarFactory object to be stopped
> or running -- it also makes sense that you are able to stop or (re)start
> such an object at run time. What can not be done is changing the value
> of a variable that appeared in the method invocation because the method
> does not know anything about the variable -- it gets the Object. Objects
> can have state that can be changed. This can happen via accessors that
> are invoked like  obj.foo = x  or other methods like  car_factory.stop.
> But while  obj.foo  looks similar to variable assignment it is something
> entirely else.
> 
> Why do different things look so similar in Ruby? It's uniform access at
> work.
> 
> In other languages there usually is the distinction between Object's
> fields and methods. Having public fields is usually a bad idea (it
> breaks encapsulation) and looks like this:
> 
>    obj.field += 5
> 
> Having methods that modify state looks like this:
> 
>    obj.setField(obj.getField() + 5)
> 
> In Ruby both look like this:
> 
>    obj.field += 5
> 
> Even if obj.field=() or obj.field() do not directly map to an instance
> variable at all like in this case:
> 
>    def obj.radius()
>      @diameter / 2
>    end
> 
>    def obj.radius=(new_radius)
>      @diameter = new_radius * 2
>    end
> 
> I hope this explains how Ruby works. If there is still anything left
> that you don't understand, feel free to ask.

Thanks for your GREAT explanation.

In my two scenarios, we were dealing with two things:  variable and object.
If everything is object ... variable is coming from where?
In other words, what is the relation between variable and object.
Are non-mutable objects considered as variables?


Thanks again,
MOhammad