Ruby: why does an undeclared instance variable return nil

问题

Given the following class:

class Something
  def initialize
    @test = "test"
  end
end

Why does ruby return nil when invoking an undeclared instance variable?

thingy = Something.new
thingy.instance_variable_get(:@var)  # nil
thingy.instance_variable_get(:@test) # "test"

As opposed to some error message indicating the variable is missing from the instance. The answer I am hoping for is an explanation as to the reasoning behind ruby's implementation of instance variables in this way.

回答1:

It's just the way instance variables implemented in Ruby. Same for global variables. Also you can see warnings for such things if you pass -w parameter to ruby.

✗ irb -w
2.3.1 :001 > @a
(irb):1: warning: instance variable @a not initialized
=> nil 

回答2:

Because, instead of throwing an error, objects are allowed to have empty values. Such as a user, must have a name and address but email address is optional, a check can be done for nil before continuing.

if customer1.name == nil #personalized error code here
if customer1.email == nil customer1.email = "not given"

来源：https://stackoverflow.com/questions/37753581/ruby-why-does-an-undeclared-instance-variable-return-nil