Checking for 'nil' Class (not instance) variables?

johnpetterson · September 1, 2008, 4:27pm

See below, what’s the proper way of checking for initialized
class-variables ?

Cheers

John

irb(main):031:0> @nosuchvar.nil?
=> true
irb(main):032:0> @@nosuchvar.nil?
NameError: uninitialized class variable @@nosuchvar in Object
from (irb):32
irb(main):033:0>

johnpetterson · September 1, 2008, 4:30pm

Hi –

On Mon, 1 Sep 2008, John Pritchard-williams wrote:

NameError: uninitialized class variable @@nosuchvar in Object
from (irb):32
irb(main):033:0>

defined?(@@nosuchvar)

David

johnpetterson · September 1, 2008, 4:33pm

Thanks !
David A. Black wrote:

//defined?(@@nosuchvar)

johnpetterson · September 1, 2008, 4:53pm

John Pritchard-williams wrote:

See below, what’s the proper way of checking for initialized
class-variables ?
[…]
irb(main):031:0> @nosuchvar.nil?
=> true

That doesn’t check whether @nosuchvar was initialized. Look:

defined?(@var)
=> nil

@var.nil?
=> true

@var = nil # @var is now initialized
=> nil

@var.nil?
=> true

defined?(@var)
=> “instance-variable”

HTH,
Sebastian

johnpetterson · September 1, 2008, 4:54pm

//
You forgot the disclaimer that class variables are fragile and should
be avoided.
//

Nah, I’ll check each one with:

fragile?(@@var)

(that’s just a bad joke)…but seriously : ‘fragile’ in implementation ?
or ‘fragile’ from a high-level/idealistic point of view ?

johnpetterson · September 1, 2008, 5:04pm

irb(main):031:0> @nosuchvar.nil?
=> true
irb(main):032:0> @@nosuchvar.nil?
NameError: uninitialized class variable @@nosuchvar in Object
from (irb):32
irb(main):033:0>

Well, to query about class variables/methods, I would prefer to use
class methods.

use self.class to get the class object of current instance, then query
the class variables of this class using class_variable_defined? method.

irb(main):026:0> self.class.class_variable_defined? :@@nosuchvar
=> false

johnpetterson · September 1, 2008, 4:36pm

2008/9/1 David A. Black [email protected]:

On Mon, 1 Sep 2008, John Pritchard-williams wrote:

See below, what’s the proper way of checking for initialized
class-variables ?

–
irb(main):031:0> @nosuchvar.nil?
=> true
irb(main):032:0> @@nosuchvar.nil?
NameError: uninitialized class variable @@nosuchvar in Object
from (irb):32
irb(main):033:0>

defined?(@@nosuchvar)

You forgot the disclaimer that class variables are fragile and should
be avoided.

Kind regards

robert