comp.lang.ruby

Brian Candler

9/19/2008 10:15:00 AM

Ben Johnson wrote:
> How do you grab the value of a constant if it is inside a class << self?
>
> example
>
> class A
> class << self
> B = "HI"
> end
> end
>
> A::B does not work. How would you go about grabbing the value for B?

You've explicitly defined the constant inside the metaclass (singleton
class) of A.

I'd say it's better not to define the constant there in the first place,
but define it under A:

irb(main):001:0> class A
irb(main):002:1> B = "HI"
irb(main):003:1> class << self
irb(main):004:2> def test
irb(main):005:3> puts "I say #{B}"
irb(main):006:3> end
irb(main):007:2> end
irb(main):008:1> end
=> nil
irb(main):009:0> A.test
I say HI
=> nil
irb(main):010:0> A::B
=> "HI"

But if you *really* want to store a constant in a metaclass, it *is*
possible to get at it:

irb(main):001:0> class A
irb(main):002:1> class << self
irb(main):003:2> B = "HI"
irb(main):004:2> end
irb(main):005:1> end
=> "HI"
irb(main):006:0> A::B
NameError: uninitialized constant A::B
from (irb):6
from :0
irb(main):007:0> class Class
irb(main):008:1> def meta
irb(main):009:2> class <<self; self; end
irb(main):010:2> end
irb(main):011:1> end
=> nil
irb(main):012:0> A.meta::B
=> "HI"

I wrote a SingletonTutorial on the rubygarden.org wiki, but
unfortunately that site appears to have closed.

Regards,

Brian.
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