Create module variables in Ruby

Question

Is there any way to create a variable in a module in Ruby that would behave similar to a class variable? What I mean by this is that it would be able to be accessed without initializing an instance of the module, but it can be changed (unlike constants in modules).

coreyward · Accepted Answer · 2019-07-26T17:08:41.400

176

Ruby natively supports class variables in modules, so you can use class variables directly, and not some proxy or pseudo-class-variables:

module Site
  @@name = "StackOverflow"

  def self.setName(value)
    @@name = value
  end

  def self.name
    @@name
  end
end

Site.name            # => "StackOverflow"
Site.setName("Test")
Site.name            # => "Test"

edited Jul 26 '19 at 17:08

answered Apr 16 '11 at 23:52

coreyward

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+1 Actually, I have been thinking that the term 'class variable' is misleading. Classes are special cases of modules, and class variables are definable on modules. They should be called module variables. – sawa Apr 17 '11 at 02:54
4

@sawa: It's somewhat misleading, but it's what Ruby itself uses: `defined?(@@foo) => "class variable"`. – Andrew Grimm Jan 09 '12 at 05:57
1

Or they could be called static fields. Seems that's what they are. – Peter Ajtai May 11 '12 at 01:11
1

@coreyward Hey my mistake. Why the need for two '@@' class variables? Isn't it considered a code smell, especially if the class is extended to use class variables? I was testing this and I realized I could get the same result from a single `@` instance variable. Is there a specific reason for using class variables? Thanks for the reply. – MrPizzaFace Jan 25 '14 at 02:14
@feed_me_code It's just a part of the language. A class variable is on the actual class, not an instance of the class. In most cases these will behave similarly, but not always. See http://stackoverflow.com/questions/5890118/what-does-variable-mean-in-ruby for a more in-depth explanation. – coreyward Jan 25 '14 at 20:08
2

why the different calls at the end: `T.get` and `T::get` ? – intrixius Aug 05 '14 at 10:05
@intrixius This being so long ago I'm not sure what prompted me to use the `::` notation, perhaps just an effort to show that there are multiple ways of accessing the methods. These two are essentially identical in behavior, but the dot notation is preferred. – coreyward Aug 05 '14 at 17:06

sawa · Answer 2 · 2011-04-17T03:34:52.163

32

If you do not need to call it from within an instance, you can simply use an instance variable within the module body.

module SomeModule
  module_function
  def param; @param end
  def param= v; @param = v end
end

SomeModule.param
# => nil
SomeModule.param = 1
SomeModule.param
# => 1

The instance variable @param will then belong to the module SomeModule, which is an instance of the Module class.

edited Apr 17 '11 at 03:34

answered Apr 17 '11 at 03:02

sawa

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score 25 · Answer 3 · answered Jan 09 '12 at 05:07

25

you can set a class instance variable in the module.

module MyModule
   class << self; attr_accessor :var; end
end

MyModule.var = 'this is saved at @var'

MyModule.var    
=> "this is saved at @var"

answered Jan 09 '12 at 05:07

Orlando

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+1, but I'll just emphasize that class instance variables are different to class variables. – Andrew Grimm Jan 09 '12 at 05:58

score 8 · Answer 4 · answered Mar 04 '16 at 12:40

8

You can also initialize value within module definition:

module MyModule
  class << self
    attr_accessor :my_variable
  end
  self.my_variable = 2 + 2
end

p MyModule.my_variable

answered Mar 04 '16 at 12:40

Nakilon

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Create module variables in Ruby

4 Answers4

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