person.rb

class Person
  attr_reader :first_name, :last_name

  def initialize(first_name, last_name)
    @first_name, @last_name = first_name, last_name
  end
end

class PersonWithFullName < SimpleDelegator
  def full_name
    first_name + " " + last_name
  end
end

person = Person.new("Alireza", "Bashiri")
PersonWithFullName.new(Person).full_name # => Alireza Bashiri
person.full_name # => undefined method `full_name' for main:Person (NoMethodError)