[ruby-talk:9520] Enhancing the functionality of an object

I have a OOP programming problem.

Imagine I have to keep the sum of a list of numbers. I'd think "list of
numbers, use an array". Now how could I make the Array class keep a sum
of all its elements? In my imaginary application, the sum is going to be
looked up a lot more than the list is going to be changed.

My first thought would be "have an instance veriable to keep the sum".

So I want to write something like:

  class SumArray < Array
    attr_read :sum
    def initialize *args
      @sum = 0
      super args
    end
    def []= index, value
      @sum += value - self[index]
      super index, value
    end
  end

(no idea if the above is even near correct, I haven't used classes in
Ruby much)

This is nice and clean, doesn't require any knowledge of how Array is
implemented, and provides the additional call "sum" which returns the
cached value, as the calculation is performed only when changing the
list.

The problem is that just defining the []= operator wouldn't be enough.
There isn't a single point in which a value gets set in an Array, so I
have no place I can put a hook into when a value gets changed.

Would I have to write a mirror class that intercepts all the places the
values could be changed, like the initial value of an array, out of range
array extensions, iterators, etc?

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