by_ref.rb

class A
  attr_accessor :b
end

class B
end

def change_A x
  x.b = 1
end

def change_locally x # => locally named variable now pointing to passed reference
  x = B.new # => local variable now pointing to something new
end

a = A.new # => assign instance of object A to a
puts change_A(a)
puts a.b
puts change_locally(a)
puts a.b