From: Brian Candler <B.Candler@...>
Date: 2003-06-01T06:04:54+09:00
Subject: Re: newbie questions

On Sun, Jun 01, 2003 at 05:45:43AM +0900, Steven Ketcham wrote:
> I am probably confusing concepts but in C++ you can pass a parameter by reference, pointer or value. Copy constructors, 
> variable scope and  things like that are very important to track. If I pass a complex object by value to a function, the copy 
> of the object will automatically run it's destructor  when the function
> returns. This could be devasting when working with file or database handles.
> 
> How are objects handled when passed to a function with Ruby?

- the thing passed is always a reference to an object (even if it's just
  a number! Numbers are [immutable] objects in Ruby)

- the object is never destroyed until the mark-and-sweep garbage collector
  finds that it is not being referenced from anywhere. The nearest
  equivalent to a destructor is the "finalizer", but I have never had a
  use for it myself.

- although almost everything is an object in Ruby, local variables are
  not: they contain a reference to an object, but you cannot get a
  reference to the variable itself.

     a = Foo.new
     bar(a)       # 'bar' can modify the object which 'a' refers to, but
                  # it cannot change 'a' to point to a different object [*]

Regards,

Brian.

[*] This is not strictly true, as you can pass a block or proc object
(closure) which binds to a local variable:

  def bar
    yield "hello"
  end

  a = 123
  puts a.id
  bar { |x| a=x }
  p a            # >> a now points to a String object
  puts a.id 

But this is cheating - you are not passing a reference to the variable 'a'
itself, but rather a reference to a piece of code which modifies the
variable 'a'