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>> puts File.read('foo.rb')
class Foo
  def hello
    "Hello, world!"
  end
end
=> nil
>> require 'foo'
=> true
>> f = Foo.new
=> #
>> f.hello
=> "Hello, world!"
>> Foo.object_id
=> 9180910

# Now I update foo.rb...

>> puts File.read('foo.rb')
class Foo
  def hello
    "BRAND NEW Hello World!"
  end
end
=> nil

# Reload the file using remove_const and Kernel.load

>> Object.send(:remove_const, :Foo)
=> Foo
>> Kernel.load('foo.rb')
=> true

# Existing instance of Foo DOES NOT use the updated method.

>> f.hello
=> "Hello, world!"
>> Foo.object_id
=> 9078500

# New object DOES use the updated method.

>> f2 = Foo.new
=> #
>> f2.hello
=> "BRAND NEW Hello World!"