From: Austin Ziegler <halostatue@...>
Date: 2005-01-20T02:05:51+09:00
Subject: Re: value by reference

On Thu, 20 Jan 2005 01:10:22 +0900, Mohammad Khan <mkhan@lextranet.com> wrote:
> def do_something(a, b, c)
>   a += 1
>   b += 2
>   c += 3
> end
> 
> def another_do_something(foo = Foo.new)
>   foo.do_something
> end
> 
> class Foo
>   def initialize
>     @value = 0
>   end
> 
>   def value
>     return @value
>   end
>   
>   def value=(val)
>     @value = val
>   end
>   
>   def do_something
>     @value += 1
>     @value += 2
>     @value += 3
>   end
> end

> a, b, c = 5, 6, 7
> f = Foo.new
[...]
> If everything is object, Why can't I do
>   do_something(a.clone, b.clone, c.clone) # I don't want them to
>                                           # be changed
>   do_something(a, b, c)                   # change a, b, c like
>                                           # other objects
> Hope, I could explain properly what I am trying to understand.

I don't know why you're not getting this, as it is very simple.

a, b, and c are not objects. They're variables.

When you call Foo#do_something, you're changing the internal state
of an object. When you call Object#do_something, you're passing
three variables that can refer to just about any object.

It's no different than if I defined do_something as:

  def do_something(a, b, c)
    a += Foo.new
    b += Foo.new
    c += Foo.new
  end

As long as Foo can be added to the values of a, b, and c, I'm going
to get three completely new objects.

Try it with this:

  def do_something(a, b, c)
    a += "-half"
    b += "-thirds"
    c += "-fourths"
    puts a.id, b.id, c.id
    [ a, b, c ]
  end

  def do_something2(a, b, c)
    a << "-half"
    b << "-thirds"
    c << "-fourths"
    puts a.id, b.id, c.id
    [ a, b, c ]
  end

  a = "one"
  b = "two"
  c = "three"

  puts "#{a}<#{a.id}>, #{b}<#{b.id}>, #{c}<#{c.id}>"

  x, y, z = do_something(a, b, c)
  puts "#{x}<#{x.id}>, #{y}<#{y.id}>, #{z}<#{z.id}>"
  puts "#{a}<#{a.id}>, #{b}<#{b.id}>, #{c}<#{c.id}>"

  x, y, z = do_something2(a, b, c)
  puts "#{a}<#{a.id}>, #{b}<#{b.id}>, #{c}<#{c.id}>"
  puts "#{x}<#{x.id}>, #{y}<#{y.id}>, #{z}<#{z.id}>"

This will help you understand. #<< modifies a String in-place. It's
concatenation. += concatenates and creates a new string.

-austin
-- 
Austin Ziegler * halostatue@gmail.com
               * Alternate: austin@halostatue.ca