delegation_example.rb

class MyString < SimpleDelegator
  def cut!(start_pos, end_pos)
    new_string = begin
      # your magic
    end
    
    __getobj__.replace(new_string)
  end
end

my_string = MyString.new('whatever whatever')
my_string.cut!('what', 'what')

# or if you only needed a few methods

class MyString
  extend Forwardable
  
  attr_reader :string
  
  def_delegators :string, :to_s, :sub, :gsub # Whatever methods you need
  
  def initialize(string)
    @string = string
  end
  
  def cut!(start_pos, end_pos)
    new_string = begin
      # your magic
    end
    
    string.replace(new_string)
  end
end