From: Gavin Kistner <gavin@...>
Date: 2005-06-02T11:11:24+09:00
Subject: Re: Implementing a Read-Only array

On Jun 1, 2005, at 4:11 PM, Brian Schr�der wrote:
>> Gavin Kistner wrote:
>>>>> affectors = %w| << []= clear concat delete delete_at delete_if
>>>>> fill flatten! insert map! pack pop push reject! replace reverse!
>>>>> shift slice! sort! uniq! unshift |
>
> Though this means that you have to override all methods that can
> change the array. I'd shurely forget something in this case. You
> forgot e.g. #clear. Therefore I'd propose to explicitly delegate the
> needed methods for the application because this is easier to control.

But again, that way prevents users' convenience methods for Arrays/ 
Enumerables from being available.

The 'affectors' list above is (I believe) precisely the list of  
methods that can change the array. (Except I accidentally included  
pack, which is non-mutating.)

The final solution that I'm currently using is:

require 'delegate'

class ReadOnlyArray < DelegateClass(Array)
     mutators = %w| << []= clear concat delete delete_at delete_if  
fill |
     mutators.concat %w| flatten! insert map! pop push reject! replace |
     mutators.concat %w| reverse! shift slice! sort! uniq! unshift |
     mutators.each do |name|
         define_method( name ){ raise "#{self} is read-only!" }
     end
end

a = [1,2,3]
ro = ReadOnlyArray.new( a )

a << 4
p ro
#=> [1, 2, 3, 4]

p ro.reverse
#=> [4, 3, 2, 1]

p ro.reverse!

/Users/gkistner/Desktop/tmp.rb:8:in `reverse!': 1234 is read-only!  
(RuntimeError)
     from /Users/gkistner/Desktop/tmp.rb:8:in `reverse!'
     from /Users/gkistner/Desktop/tmp.rb:18