What Is a Class Method in Ruby?

In Ruby, a class method is a method declared in a class, that's:

• Created by adding "self" (or the class name) before the method name;
• Callable without creating an instance of the class;
• Able to access/modify class variables;
• Not able to access class instance variables or instance methods.

Created by Adding "self" (or the Class Name) Before the Method Name

To create a class method, you can add "self." before the method name, for example, like so:

class Foo
    def self.bar
        # ...
    end
end

Alternatively, you may use the class name to instead of self, to create a class method:

class Foo
    def Foo.bar
        # ...
    end
end

Callable Without Creating an Instance of the Class

Class methods are static (i.e. methods are attached to the class itself). Therefore, they can be called on the class itself, without having to create an instance of the class:

class Foo
    def self.bar
        "bar"
    end
end

puts Foo.bar #=> "bar"

If you try calling this method on an instance of the class, it will raise an error:

foo = Foo.new

# undefined method `bar` for #<Foo:0x000055e734934d08> (NoMethodError)
puts foo.bar

Able to Access/Modify Class Variables

You may access class variables inside a class method:

class Foo
    @@baz = "baz"

    def self.bar
        @@baz
    end
end

puts Foo.bar #=> "baz"

Similarly, a class method may modify values of class variables:

class Foo
    @@separator = ","

    def self.separator=(val)
        @@separator = val
    end

    def self.separator
      @@separator
    end
end

puts Foo.separator #=> ","
Foo.separator = "|"
puts Foo.separator #=> "|"

Not Able to Access Class Instance Variables or Instance Methods

A class method does not have access to class instance variables (as they're only relevant to individual class instances). Therefore, if you try to access them in a class method, it will always be nil (because instance variables are always nil before initialization):

class Foo
    def initialize()
        @name = "foo"
    end

    def self.bar
        @name
    end
end

puts Foo.bar #=> nil

When you try to access a class instance method from a class method, it will raise an error:

class Foo
    def baz
        @name
    end

    def self.bar
        baz
    end
end

# undefined local variable or method `baz` for Foo:Class (NameError)
puts Foo.bar