From: Joel VanderWerf <vjoel@...>
Date: 2005-05-05T07:09:59+09:00
Subject: Re: object reference handle (like perl's reference to scalar)

Eric Mahurin wrote:
> In ruby, is there a way to get a handle of an object reference?
>  In perl, this is the \ operator:
> 
> $x = 1; # \$x is a handle to change $x
> $a = [1,2,3]; # \$a->[1] is a handle to change an element in $a
> 
> As far as I can tell, the closest that Ruby has to this is a
> symbol.  But, this only works for object references that have
> an associated variable name.  For example, there is no symbol
> associated with an element of an array (or hash).

You can do something like this with closures:

irb(main):001:0> a = [1,2,3]
=> [1, 2, 3]
irb(main):002:0> set_a_1, get_a_1 = proc {|v| a[1]=v}, proc {a[1]}
=> [#<Proc:0x401e8a6c@(irb):2>, #<Proc:0x401e89b8@(irb):2>]
irb(main):003:0> set_a_1[5]
=> 5
irb(main):004:0> a
=> [1, 5, 3]
irb(main):005:0> get_a_1[]
=> 5

Note that if the binding of a changes, then set_a_1 and get_a_1 refer to 
the new value. If you want the two procs always to refer to the same 
array, you need to introduce a new variable (probably better be in a new 
scope, as well):

irb(main):009:0> def make_elt_1_refs(x)
irb(main):010:1>   [proc {|v| x[1]=v}, proc {x[1]}]
irb(main):011:1> end
=> nil
irb(main):012:0> set_1, get_1 = make_elt_1_refs(a)
=> [#<Proc:0x401f6f18@(irb):10>, #<Proc:0x401f6e00@(irb):10>]
irb(main):013:0> a = []
=> []
irb(main):014:0> get_1[]
=> 5