Does Ruby's attr_accessor automatically create instance variables for attributes?

Question

For example, if we write

class MyClass
  attr_accessor :something
end

but did not explicitly create an initialize method with instance variable @something, does Ruby automatically create it?

Answer 1

No. Instance variables are not defined until you assign to them, and attr_accessor doesn't do so automatically.

Attempting to access an undefined instance variable returns nil, but doesn't define that variable. They don't actually get defined until you write to them. attr_accessor relies on this behaviour and doesn't do anything except define a getter/setter.

You can verify this by checking out .instance_variables:

class Test
  attr_accessor :something
end

A new instance of x has no instance variables:

x = Test.new # => #<Test:0xb775314c>
x.instance_variables # => []

Invoking the getter does not cause @something to become defined:

x.something # => nil
x.instance_variables # => []

Invoking the setter does cause @something to become defined:

x.something = 3 # => 3
x.instance_variables # => ["@something"]

Setting something back to nil doesn't cause instance_variables to revert, so we can be sure that the first empty array returned isn't simply a case of instance_variables omitting nil values:

x.something = nil # => nil
x.instance_variables # => ["@something"]

You can also verify that this isn't simply behaviour specific to attr_accessor:

class Test
  def my_method

    @x # nil

    instance_variables # []

    @x = 3 

    instance_variables # ["@x"]
  end 
end

Test.new.my_method

Answer 2

Sort of. In Ruby, instance variables are created when they are first assigned. This is completely transparent to the programmer. They default to nil until assigned.

Ex:

class Foo
  attr_accessor :bar
  def baz
    @nonexistant
  end
end
f.bar #=> nil
f.baz #=> nil
f.bar = 4
f.bar #=> 4

Until you assign a value to an instance variable, it floats in a undefined nil state.