What does uninitialized constant mean?

Question

I'm getting the error uninitialized constant SessionsController I've searched, and can only find explanations of this error in reference to a NameError Does anyone know what the error means?

It means exactly what it says, that the variable by that name is uninitialized. It's referring to it as a constant because it's capitalized. — Peter Alfvin, Sep 27 '13 at 18:31
Can you elaborate a little bit? SessionsController is supposed to be capitalized. — Arel, Sep 27 '13 at 18:39
Right. But it hasn't been defined yet. This is a standard error message for any attempt to access a capitalized variable that is undefined. — Peter Alfvin, Sep 27 '13 at 18:49
If you thought it _should_ have been defined, you'll have to share some code and assert why you believe that to be the case. — Peter Alfvin, Sep 27 '13 at 18:49
That's fine, I have another questions that is specifically about solving the error. I just wanted to know what the error message specifically meant so I could better understand what was going on. http://stackoverflow.com/questions/19057217/uninitialized-constant-sessionscontroller-in-api/19058081#19058081 — Arel, Sep 27 '13 at 18:51

score 3 · Answer 1 · answered Sep 27 '13 at 19:08

It means that you're trying to use a Class or a Module that wasn't defined yet. Probably because you forgot to require them.

Make sure that this class SessionsController has been declared in your sessions_controller.rb.

Read more: http://ruby.about.com/od/faqs/qt/Nameerror-Uninitialized-Constant-Object-Something.htm

score 3 · Accepted Answer · answered Sep 27 '13 at 19:09

That is one of two "subtypes of" Name Error dealing with uninitialized variables. The language of "uninitialized constant" is due to the fact that SessionsController is capitalized. Both types are illustrated below:

new-host-3:bot palfvin$ irb
2.0.0p247 :001 > foobar
NameError: undefined local variable or method `foobar' for main:Object
    from (irb):1
    from /Users/palfvin/.rvm/rubies/ruby-2.0.0-p247/bin/irb:16:in `<main>'
2.0.0p247 :002 > Foobar
NameError: uninitialized constant Foobar
    from (irb):2
    from /Users/palfvin/.rvm/rubies/ruby-2.0.0-p247/bin/irb:16:in `<main>'
2.0.0p247 :003 >

What does uninitialized constant mean?

2 Answers2