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On Fri, Nov 6, 2009 at 7:50 AM, James French <James.French / naturalmotion.com
> 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  .new
> a.addDependency("foo")
> a.dependencies << "bar" # encapsulation subverted
>
> puts a.dependencies   # foo and bar both in array
>
>
> Any suggestions appreciated,
> James
>
>
Ruby is a dynamic language, as David pointed out, you can always get around
whatever someone does. I think the point isn't so much to make it impossible
for them to do something, but rather to make it clear how it was anticipated
that it would be used. If they want to go so far as to override the methods,
or perform instance_eval to get at the variable, then I'd take that as a
very deliberate effort, so due to their determination to get around your
design, I would just assume that they had a legitimate reason to do so, or
at least if it blows up, they'll have no cause to be upset with you for it.

I don't know how you are trying to use the class, that you feel the need to
return an array that cannot be altered, but you could define methods which
give this functionality without ever exposing the array itself, something
like this:


# consider defining methods to give the functionality you might want
# without exposing the guts of your class to the world
class Configuration
  def initialize()       @dependencies  ]  end
  def add_dependency(d)  @dependencies << d  end

  def each_dependency
    @dependencies.each { |dep| yield dep }
  end
end

config  onfiguration.new
config.add_dependency :dependency1
config.add_dependency :dependency2
config.add_dependency :dependency3

config.each_dependency do |dependency|
  puts "I am accessing #{dependency}, " \
       "without ever seeing the array."
end



# Of course, they can always get around whatever you have done,
# in the above, there is no way to get at @dependencies from the
# outside world, but in their code they could just add the code below
class Configuration
  attr_accessor :dependencies
end

p config.dependencies

__END__
Ruby is not about strict rigid code, or forcing people to use it
a certain way, it is dynamic. You have made your intentions clear,
at this point, if the user is not satisfied with your intentions,
then you must trust them to know what is best for their needs.
Something like this will not occur by accident, they have quite
deliberately decided that they need to break the encapsulation.

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