From: Robert Klemme <bob.news@...>
Date: 2005-06-02T18:55:23+09:00
Subject: Re: Implementing a Read-Only array

Gavin Kistner wrote:
> 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.

Yeah, I prefer the method to explicitely exclude mutators, too.  The
reason being that IMHO there are more often non modifying convenience
methods defined on Array than modifying methods.

> 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

You don't even need the variable and the ugly concat

require 'delegate'

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

Kind regards

    robert