In Ruby, a class variable is a variable declared inside a class that's shared between the class and all its subclasses (i.e. each class instance has the same copy of a class variable). It is:
- Static — i.e. only a single copy of the variable exists, regardless of however many instances of the class you create;
- Created by adding a double at sign (
@@) before a variable name (e.g.
- Always private to outside access;
- Potentially problematic when it comes to inheritance.
For example, consider the following class where a static
counter is incremented each time a new object is instantiated:
class Entity @@counter = 0 def initialize @@counter += 1 end def counter @@counter end end a = Entity.new b = Entity.new c = Entity.new puts a.counter #=> 3 puts b.counter #=> 3 puts c.counter #=> 3
As you can see from the example above, all instances of the class return the same value from
counter() method, because
@@counter is a class variable that's shared across all instances of the
Consider another example, where a delimiter is declared as a class variable:
class Collection @@delimiter = ", " def self.from_str(str) self.new(str.split(@@delimiter)) end end
This can be inherited by other classes, where the same
delimiter would be used across all subclasses:
class Fruits < Collection def initialize(fruits) @fruits = fruits end attr_reader :fruits end a = Fruits.from_str('orange, apple, grapes') b = Fruits.from_str('banana, mango') print a.fruits #=> ["orange", "apple", "grapes"] print b.fruits #=> ["banana", "mango"]
You should use class variables with caution as all subclasses inherit the same class variables from its superclass, which can lead to unexpected/confusing results.
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