From: Stefan Lang <langstefan@...>
Date: 2005-01-20T00:46:01+09:00
Subject: Re: value by reference

Michel Martens wrote:

> It's easy to accomplish this task with strings:
> 
> def do_something(a, b, c)
>    a.replace((a.to_i + 1).to_s)
>    b.replace((b.to_i + 2).to_s)
>    c.replace((c.to_i + 3).to_s)
> end
> 
> a, b, c = '5', '6', '7'
> do_something(a, b, c)
> 
> It would be nice to have a replace method for every object, then we could
> write:
> 
> def do_something(a, b, c)
>    a.replace(a + 1)
>    b.replace(b + 2)
>    c.replace(c + 3)
> end
> 
> a, b, c = 5, 6, 7
> do_something(a, b, c)

But replace doesn't replace the object the variable
points to, it just replaces instance variables of the
string object:

irb(main):001:0> s = "abc"
=> "abc"
irb(main):002:0> s.object_id
=> 538462462
irb(main):003:0> s.replace "xyz"
=> "xyz"
irb(main):004:0> s
=> "xyz"
irb(main):005:0> s.object_id
=> 538462462

You can write your replace method if you like:

class A
  attr_reader :ivar
  def replace(a_obj)
    @ivar = a_obj.ivar
    # or generally copy all
    # instance vars
  end
end

def do_something(obj)
  ...
  obj.replace(other)
end

a = A.new
do_something(a)
p a

-- 
Stefan