Ruby metaprogramming: Initialize singleton_class variable

Question

Why does Foo.val return nil instead of "foo" before calling Foo.set?
Is there any mechanism for initializing @val on class evaluation?

In which scope is @val = "foo" stored into?

class Foo
  class << self
    @val  = "foo"
    attr_reader :val

    def set(val)
      @val = val
    end
  end
end

p Foo.val # nil
Foo.set("bar")
p Foo.val # "bar"

Andrew Grimm · Accepted Answer · 2011-08-21 22:46:21Z

9

You can initialize @val in Foo like this:

class Foo
  @val  = "foo"
  class << self
    attr_reader :val

    def set(val)
      @val = val
    end
  end
end

p Foo.val         #=> "foo"
Foo.set("bar")
p Foo.val         #=> "bar"

Your code initializes @val not on Foo, but on Foo's metaclass

edited Aug 21, 2011 at 22:46

Andrew Grimm

82.3k58 gold badges210 silver badges355 bronze badges

answered Aug 6, 2011 at 17:13

Victor Deryagin

12.3k1 gold badge32 silver badges38 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Community · Accepted Answer · 2017-05-23 12:14:57Z

3

Ruby generally executes expressions upon parsing them. The reason why your code did not perform as expected is because you are setting a class instance variable for the singleton class of Foo, but on the other hand you are accessing the class instance variable of Foo itself, that's why it doesn't work:

class << self
  @val  = "foo" # scope is class scope of singleton class of Foo
  attr_reader :val

  def set(val)
    # scope is instance scope of singleton class of Foo (equal to Foo itself)
    @val = val 
  end
end

That's why Foo.val yields nil in your case - it hasn't been set yet.

Setting val on class evaluation can be achieved in the way that Victor already demonstrated.

See also this post for a discussion about scope.

edited May 23, 2017 at 12:14

CommunityBot

11 silver badge

answered Aug 6, 2011 at 17:33

emboss

39.7k7 gold badges103 silver badges109 bronze badges

Comments

user904990 · Accepted Answer · 2011-08-21 21:43:33Z

0

No need to use #set. Simply define #val method and use nil guard:

class Foo
    class << self
        def val; @val ||= "bar" end
    end
end
p Foo.val  #=> "bar"

answered Aug 21, 2011 at 21:43

user904990

Collectives™ on Stack Overflow

Ruby metaprogramming: Initialize singleton_class variable

3 Answers 3

Comments

Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related