# Is there a way to make class and object constants in Ruby? **URL:** \ **Category:** Ruby\ **Created:** [April 15, 2010, 3:01am UTC](https://www.ruby-forum.com/t/is-there-a-way-to-make-class-and-object-constants-in-ruby/186969 "2010-04-15T03:01:16Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![Xeno\_C](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/x/958977/32.png) [@Xeno\_C](https://www.ruby-forum.com/u/Xeno_C)\ **Post date:** [April 15, 2010, 3:01am UTC](https://www.ruby-forum.com/t/is-there-a-way-to-make-class-and-object-constants-in-ruby/186969/1 "2010-04-15T03:01:16Z")
I am reading up in other areas, and it occurs to me much of the items I have as class and object variables really should be constants, but ideally still in the class or object context. Is there a way to do this? xc ---
**Author:** ![Xeno\_C](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/x/958977/32.png) [@Xeno\_C](https://www.ruby-forum.com/u/Xeno_C)\ **Post date:** [April 15, 2010, 3:36am UTC](https://www.ruby-forum.com/t/is-there-a-way-to-make-class-and-object-constants-in-ruby/186969/2 "2010-04-15T03:36:24Z")
On Wed, Apr 14, 2010 at 8:00 PM, Xeno C. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid)wrote: > I am reading up in other areas, and it occurs to me much of the items I > have as class and object variables really should be constants, but ideally > still in the class or object context. Is there a way to do this? > > ## xc > > “It’s the preponderance, stupid!” - Professor Stephen Schneider, IPCC > member Variables that begin with capital letters are constants initial\_constants = Module.constants class MyClass MY\_FAVOURITE\_NUMBER = 12 MyFavouriteNumber = 12 end Module.constants - initial\_constants # =\> [:MyClass] MyClass.constants # =\> [:MY\_FAVOURITE\_NUMBER, :MyFavouriteNumber] So you can see that classes are already constants, and an example of how to create constants within a class. In Ruby, this really means that the reference may not be changed (or at least you will be warned if you try to do so), not that the object being referenced may not be mutated. class MyClass MyFavouriteNumber = 12 MyFavouriteNumber = 13 # !\> already initialized constant MyFavouriteNumber MyFavouriteNumber # =\> 13 end If you change your constant, you will know, it will warn you (ie, it won’t happen by accident). If your code breaks because of this… well… you’ll have a pretty good idea where to look. While Ruby is dynamic enough to make such a change, it is not something you should do. If you need to change your constants, then they shouldn’t be constants. If you really need your data to not change, to the point where you don’t even want to allow yourself the opportunity to do so, you can freeze it, and it will throw an exception class MyClass MyFavouriteWord = “ambivalent” MyFavouriteWord \<\< ‘l’ MyFavouriteWord # =\> “ambivalentl” MyFavouriteWord.freeze MyFavouriteWord \<\< ‘y’ MyFavouriteWord # =\> end # ~\> -:6:in `class:MyClass’: can’t modify frozen string (RuntimeError) # ~\> from -:1:in `’ But as far as I am aware, there is no way to recursively free any references it has (probably a good thing, you could recursively freeze your entire program) class MyClass SomeArray = [‘abc’ , ‘def’].freeze SomeArray # =\> [“abc”, “def”] begin SomeArray \<\< ‘ghi’ rescue =\> e e # =\> #\ end SomeArray.last # =\> “def” SomeArray.last \<\< ‘ghi’ SomeArray.last # =\> “defghi” end Generally, constants are used for data that should be hard coded in, you use a constant so that you can easily change it later, like using a macro in C. You can then drop it into your code, and it is replaced with the actual value (you can redefine macros too). And they are also used for keeping important references, like the constant STDIN. We can change the global we use as stdin, but we don’t want to lose track the actual stdin, so we keep it in the constant. STDIN # =\> #\ $stdin # =\> #\ $stdin = DATA # =\> #\ gets # =\> “this is the data\n” $stdin = STDIN # =\> #\ **END** this is the data ---
**Author:** ![Xeno\_C](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/x/958977/32.png) [@Xeno\_C](https://www.ruby-forum.com/u/Xeno_C)\ **Post date:** [April 15, 2010, 3:42am UTC](https://www.ruby-forum.com/t/is-there-a-way-to-make-class-and-object-constants-in-ruby/186969/3 "2010-04-15T03:42:51Z")
Am Wed, 14 Apr 2010 20:00:54 -0500 schrieb Xeno C.: > I am reading up in other areas, and it occurs to me much of the items I > have as class and object variables really should be constants, but > ideally still in the class or object context. Is there a way to do > this? > > xc In class context you could do: class MyClass MYCONSTANT=6 def initialize puts MYCONSTANT end end But I wonder, whether it makes sense at all to have constants at object level. This sounds weird to me. Thomas ---
**Author:** ![Xeno\_C](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/x/958977/32.png) [@Xeno\_C](https://www.ruby-forum.com/u/Xeno_C)\ **Post date:** [April 20, 2010, 1:38am UTC](https://www.ruby-forum.com/t/is-there-a-way-to-make-class-and-object-constants-in-ruby/186969/4 "2010-04-20T01:38:47Z")
Thank you Josh and Thomas. The source of this question is my study of Erlang, where all ‘Variables’ do not vary after the initial assignment. I realized when reading up on the language that there are many areas in my classes where the thing I use, especially with Object Variables, are effectively constants, so I would do best to have them as actual constants. It turns out there is a lot of value in knowing that something cannot change, even if it can change but will have at least an obvious error. I don’t expect to Erlangize my programs, but I have already identified things I can do to make my stuff safer and clearer just by making class and object variables into constants. Too bad there is not a special prefix for these as there is for the variables. That would be nice for referencing things. Still, no big deal. xc ---
**Author:** ![Xeno\_C](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/x/958977/32.png) [@Xeno\_C](https://www.ruby-forum.com/u/Xeno_C)\ **Post date:** [April 20, 2010, 4:23pm UTC](https://www.ruby-forum.com/t/is-there-a-way-to-make-class-and-object-constants-in-ruby/186969/5 "2010-04-20T16:23:15Z")
To make objects immutable you can ‘freeze’ them. After being frozen any attempt to change will result in an error: > > x = ‘test’ > > =\> > > “test” > > > x.freeze > > =\> > > “test” > > > x[0] = ‘w’ > > RuntimeError: can’t modify frozen string ---
**Author:** ![Xeno\_C](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/x/958977/32.png) [@Xeno\_C](https://www.ruby-forum.com/u/Xeno_C)\ **Post date:** [April 15, 2010, 5:27am UTC](https://www.ruby-forum.com/t/is-there-a-way-to-make-class-and-object-constants-in-ruby/186969/6 "2010-04-15T05:27:36Z")
On Wed, Apr 14, 2010 at 8:40 PM, Thomas Volkmar W. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > But I wonder, whether it makes sense at all to have constants at object > level. This sounds weird to me. > > Thomas I do for things that don’t (or shouldn’t) change. For example, I’m working on a Rails project where the admin can assign a level of trust to a user such that they can post images directly to the main page from Twitter. The database doesn’t inherently support enumerated types, so the attribute is an integer, then I have class level constants that map those values to their meanings, and a set of methods that allow you to get booleans back when asking if that user is able to do that authorized thing, and they are embedded into named scopes so that I can pull all users who have this authorization level, or that authorization level, and so forth. So I just store a single integer in the database, but my application interprets it the same across every instance.