How to I make private class constants in Ruby

Question

In Ruby how does one create a private class constant? (i.e one that is visible inside the class but not outside)

class Person
  SECRET='xxx' # How to make class private??

  def show_secret
    puts "Secret: #{SECRET}"
  end
end

Person.new.show_secret
puts Person::SECRET # I'd like this to fail

Of course, "private" in Ruby doesn't mean what it means in most languages. Ruby is not that big on hiding, so if you want to access something, you always can (but maybe not with the most obvious syntax). Ruby's `private` only says it can't be preceded by a dot (`.`) (which I guess is also a hint why it won't work on `::`) — Amadan, May 20 '10 at 13:50

score 174 · Answer 1 · edited Jul 06 '17 at 10:53

174

Starting on ruby 1.9.3, you have the Module#private_constant method, which seems to be exactly what you wanted:

class Person
  SECRET='xxx'.freeze
  private_constant :SECRET

  def show_secret
    puts "Secret: #{SECRET}"
  end
end

Person.new.show_secret
# => "Secret: xxx"

puts Person::SECRET
# NameError: private constant Person::SECRET referenced

edited Jul 06 '17 at 10:53

leviathan

11,080
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answered Aug 09 '12 at 14:09

Renato Zannon

28,805
6
38
42

1

This is so freaking awesome. I had no idea this was even a thing. – Brad Rice Jul 16 '13 at 16:18
1

I don't see this method in the `Module` API until http://www.ruby-doc.org/core-2.0.0/Module.html (http://www.ruby-doc.org/core-1.9.3/Module.html) – Kris Robison Jun 03 '14 at 21:34
@KrisRobison it appears to be some RDoc bug. Take a look at the comments section, on the end of the 1.9.3 page – Renato Zannon Jun 04 '14 at 07:02
@KrisRobison just to be sure: This answer is from 08/2012, and Ruby 2.0 went out on 02/2013. – Renato Zannon Jun 04 '14 at 15:33
Thanks @RenatoZannon. I had not scrolled down in the docs to read the comments. – Kris Robison Jun 06 '14 at 21:53
This prevents accessing the constant from the outside but does not prevent from overwriting its value!! – silverdr Jan 13 '17 at 14:09
@silverdr His sample code has .freeze already how can we overwritting its value? – truongnm Nov 04 '20 at 10:08

score 14 · Accepted Answer · answered May 20 '10 at 13:22

14

You can also change your constant into a class method:

def self.secret
  'xxx'
end

private_class_method :secret

This makes it accessible within all instances of the class, but not outside.

answered May 20 '10 at 13:22

harald

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Perhaps a little meta-programming would make this more palatable. – DMisener May 25 '10 at 15:20
I thought about it a little longer and then realized the above approach doesn't make the constant available to instance methods so I think I'll stick with @@var approach. – DMisener May 25 '10 at 15:51
@DMisener: Ruby has a method for missing constants, if anyone wants meta-programming magic. – Andrew Grimm Apr 11 '11 at 23:08
you can access this class method from your instances using `self.class.secret` in an instance method – brad May 25 '11 at 20:01
1

@brad Instances can't call private class methods, even if the instance belongs to the class. The only way is to use `send` which defeats the whole purpose. – Kelvin Sep 09 '14 at 19:41
You can use private constant as well. Just as private as you with your methods/calls. – Rubyrider Sep 19 '16 at 05:57
If constant is based on some values from database or http request, it's very bad idea to use a method which may be called in a loop. You can store it in the instance variable only once, but I'm not sure, constants are available for a reason. – ciemborowicz Dec 26 '22 at 21:11

score 12 · Answer 3 · answered May 20 '10 at 13:19

Instead of a constant you can use a @@class_variable, which is always private.

class Person
  @@secret='xxx' # How to make class private??

  def show_secret
    puts "Secret: #{@@secret}"
  end
end
Person.new.show_secret
puts Person::@@secret
# doesn't work
puts Person.class_variable_get(:@@secret)
# This does work, but there's always a way to circumvent privateness in ruby

Of course then ruby will do nothing to enforce the constantness of @@secret, but ruby does very little to enforce constantness to begin with, so...

Thanks...Seems obvious in hindsight. – DMisener May 25 '10 at 15:22 — DMisener, May 25 '10 at 15:22

score 0 · Answer 4 · answered May 20 '10 at 14:41

0

Well...

@@secret = 'xxx'.freeze

kind of works.

answered May 20 '10 at 14:41

Anonymous

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1

score 0 · Answer 5 · answered Jan 30 '23 at 21:10

You can just literally make it a private method in the first place ‍♂️

class Person
  def SECRET
    'xxx'
  end

  def show_secret
    puts SECRET
  end
end

Person::SECRET    # Error: No such constant
Person.SECRET     # Error: No such method
Person.new.SECRET # Error: Call private method
person.new.show_secret # prints "xxx"

How to I make private class constants in Ruby

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