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 · 2014-09-17T20:40:23.110

138

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 '14 at 20:40

answered Apr 16 '12 at 10:01

jibiel

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There's also `const_defined?` but it doesn't work for me; not sure why. – Jared Beck Jun 05 '13 at 22:46
8

`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 — http://www.ruby-doc.org/core-1.9.3/Module.html#method-i-const_defined-3F – jibiel Jun 08 '13 at 21:01
And what about this? http://ruby-doc.org/core-1.9.3/Module.html#method-i-const_defined-3F – Donato Mar 03 '15 at 00:09
Thanks for this answer.. great workaround for not being able to use ||= reliably with a boolean – Chuck van der Linden Apr 16 '15 at 22:35
This works great for methods as well, not just constants. – Joshua Pinter Mar 11 '18 at 17:08

score 26 · Answer 2 · edited Aug 27 '19 at 20:58

26

const_defined? API

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

edited Aug 27 '19 at 20:58

Xavi

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answered Aug 03 '15 at 08:39

rusllonrails

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This is especially helpful when you are checking if this is defined in a dynamic way. – DickieBoy Feb 20 '17 at 13:43

akostadinov · Answer 3 · 2015-05-01T13:13:36.200

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 01 '15 at 13:13

answered May 01 '15 at 13:08

akostadinov

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:) 1.8 may be ancient, but still in use in 2017, example: Dreamhost shared hosting. – manitu Jan 30 '17 at 16:00
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). – akostadinov Jan 31 '17 at 09:31

Check if a constant is already defined

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