From: Robert Klemme <bob.news@...>
Date: 2005-06-02T00:35:23+09:00
Subject: Re: Implementing a Read-Only array

Gavin Kistner wrote:
> On Jun 1, 2005, at 7:49 AM, Brian Schr�der wrote:
>> On 01/06/05, Gavin Kistner <gavin@refinery.com> wrote:
>>> class ReadOnlyArray < Array
>>>      alias_method :'__ro_<<', :'<<' #:nodoc:
>>>      alias_method :__ro_insert, :insert #:nodoc:
>>>      alias_method :__ro_delete_at, :delete_at #:nodoc:
>>>      affectors = %w| << []= clear concat delete delete_at delete_if
>>> fill flatten! insert map! pack pop push reject! replace reverse!
>>> shift slice! sort! uniq! unshift |
>>>      affectors.each{ |name| undef_method name }
>>> end
>> Why not create a proxy class that contains an array and only forwards
>> the reading methods. It seems to me, that this would achieve exactly
>> what you want.
>
> That's an interesting idea. What advantage do you think that would
> offer compared to my implementation above? How would you modify the
> internal representation when necessary? (Perhaps the constructor for
> the proxy class receives the array that it is supposed to wrap?)
>
> One advantage of mine versus the proxy approach would be that by
> actually inheriting from Array, my class supports additional methods
> defined by the user for Array or Enumerable. (If the user has added a
> special Array#my_collect method, my array is extended as well.)
>
> (I think that it's correct that it would not be possible for a custom
> user method to modify the array, if I'm undef'ing all core methods
> for modifying it, yes?)

require 'delegate'

class ImmutableArray < DelegateClass(Array)
  def []=(*a) raise "Immutable!" end
  def <<(*a) raise "Immutable!" end
  def push(*a) raise "Immutable!" end
  def unshift(*a) raise "Immutable!" end
end

?> a = [1,2,3]
=> [1, 2, 3]
>> ia = ImmutableArray.new a
=> [1, 2, 3]
>> ia << "bust"
RuntimeError: Immutable!
        from (irb):5:in `<<'
        from (irb):13

:-))

Kind regards

    robert