From: James French <James.French@...>
Date: 2009-11-07T01:01:36+09:00
Subject: Re: encapsulation issue



> -----Original Message-----
> From: Tom Stuart [mailto:tom@therye.org]
> Sent: 06 November 2009 14:06
> To: ruby-talk ML
> Subject: Re: encapsulation issue
> 
> 2009/11/6 David A. Black <dblack@rubypal.com>:
> > Hi --
> >
> > On Fri, 6 Nov 2009, James French wrote:
> >
> >> Hi,
> >>
> >> Is there any way of providing read only access to an array? (problem
> shown
> >> by code below).
> >>
> >> class A
> >>
> >>  def initialize
> >>   @dependencies = []
> >>  end
> >>
> >>  # intended to be read only access
> >>  def dependencies
> >>   @dependencies
> >>  end
> >>
> >>  def addDependency(d)
> >>   @dependencies << d
> >>   puts "adding #{d}"
> >>  end
> >> end
> >>
> >>
> >> a = A.new
> >> a.addDependency("foo")
> >> a.dependencies << "bar" # encapsulation subverted
> >>
> >> puts a.dependencies   # foo and bar both in array
> >
> > You can freeze the array:
> >
> >  a = [].freeze
> >  a << 1          # TypeError: can't modify frozen array
> >
> >
> 
> You could also dup it:
> 
> class A
>   def dependencies
>     @dependencies.dup
>   end
> end
> 
> a = A.new
> a.addDependency "foo"
> a.dependencies << "bar"
> puts a.dependencies # => ["foo"]

I don't think it solves it that nicely because as well as duplicating data which doesn't feel good, you've then got a line that looks as though it does something but doesn't. I guess it could raise an error somehow.