From: Stefan Lang <langstefan@...>
Date: 2005-01-20T03:11:40+09:00
Subject: Re: value by reference

Mohammad Khan wrote:

> 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?
> 

You have to tell ruby what object you want to pass
in a method or to what object a message should be sent.
A variable is just a name for an object.

def do_something(a)
  # inside this method you can access
  # the object passed to the method with the name a
  ...
end

x = Object.new
# You can do something with the new created
# object by specifying its name, the variable x

# Tell ruby to pass the object named by x
# to the method do_something
do_something(x)
# Inside do_something the method can access
# our object with the name a (look above)