Check if a constant is already defined

Question

This is a simple one, I hope. How do I check, in the following example, if a constant is already defined?

#this works var = var||1 puts var var = var||2 puts var #this doesn't CONST = CONST||1 puts CONST CONST = CONST||2 puts CONST => 1 1 uninitialized constant CONST (NameError)

jibiel · Accepted Answer · 2014-09-17 20:40:23Z

145

CONST = 2 unless defined? CONST

See here for more about awesome defined? operator.

^{P.S. And in the future I guess you'll want var ||= 1 instead of var = var||1.}

edited Sep 17, 2014 at 20:40

answered Apr 16, 2012 at 10:01

jibiel

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5 Comments

Jared Beck Over a year ago

There's also const_defined? but it doesn't work for me; not sure why.

jibiel Over a year ago

const_defined? is a method of Module class and it will tell you whether the constant is defined in that module and its ancestors (optionally). Check the docs for some examples — ruby-doc.org/core-1.9.3/Module.html#method-i-const_defined-3F

Donato Over a year ago

And what about this? ruby-doc.org/core-1.9.3/Module.html#method-i-const_defined-3F

Chuck van der Linden Over a year ago

Thanks for this answer.. great workaround for not being able to use ||= reliably with a boolean

Joshua Pinter Over a year ago

This works great for methods as well, not just constants.

Xavi · Accepted Answer · 2019-08-27 20:58:26Z

31

const_defined? API

pry> User.const_defined?("PER_PAGE") => true pry> User.const_defined?("PER_PAGE123") => false

edited Aug 27, 2019 at 20:58

Xavi

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answered Aug 3, 2015 at 8:39

rusllonrails

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1 Comment

DickieBoy Over a year ago

This is especially helpful when you are checking if this is defined in a dynamic way.

akostadinov · Accepted Answer · 2015-05-01 13:13:36Z

4

CONST ||= :default_value

the above works for me on ruby 1.9.3 but fails on 1.8... well 1.8 is ancient now.

edited May 1, 2015 at 13:13

answered May 1, 2015 at 13:08

akostadinov

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2 Comments

kitschmaster Over a year ago

:) 1.8 may be ancient, but still in use in 2017, example: Dreamhost shared hosting.

akostadinov Over a year ago

I see most projects have already dropped support for ruby 1.8. I know that there are still "supported" ruby 1.8 versions, e.g. shipped by Red Hat Enterprise Linux. They are getting security patches but one exposes to many unknown security flaws in old gem versions used in whatever application is installed. So yes, there is, but is irrelevant for most people. Like there are and will be for a long time computers running XP (just recently spotted again such an ATM).

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