How to Set a Ruby Class Variable From Outside the Class?

In Ruby, you cannot directly set class variables (i.e. @@variable) from outside the class:

class Foo
    @@bar = "foobar"
end

# undefined method `bar=' for Foo:Class (NoMethodError)
Foo.bar = "foo"

To set a class variable, you must create a class method that allows you to set the value of the class variable:

class Foo
    @@bar = "foobar"

    # setter
    def self.bar=(val)
        @@bar = val
    end

    # getter
    def self.bar
        @@bar
    end
end

puts Foo.bar #=> "foobar"

Foo.bar = "foo"

puts Foo.bar #=> "foo"

It is also possible to set a class variable via a class instance by creating a setter instance method.