Ronnie Aa wrote in post #995580:
> For extra clearness:
>
> My class Do_this and my class Do_that both have to use that method..
> They both use the same variable and both have to apply the same
> operations on that variable (amongst other things of course)..

One way to share common code is using a module, and including it into 
those classes where you need it.

module MyUsefulMethods
  def product
    @foo.inject { |x,y| x*y }
  end
end

class DoThis
  include MyUsefulMethods
  def initialize
    @foo = [1,2,3]
  end
end

class DoThat
  include MyUsefulMethods
  def initialize
    @foo = [4,5,6]
  end
end

p DoThis.new.product
p DoThat.new.product

Note that I'm using an instance variable (@foo) not a global variable 
($foo). Each instance of those classes has its own @foo.

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