Class and Instance Methods in Ruby

Class methods are methods that are called on a class and instance methods are methods that are called on an instance of a class. Here is a quick example and then we’ll go into a bit more detail.

class Foo
  def self.bar
    puts 'class method'
  end
  
  def baz
    puts 'instance method'
  end
end

Foo.bar # => "class method"
Foo.baz # => NoMethodError: undefined method ‘baz’ for Foo:Class

Foo.new.baz # => instance method
Foo.new.bar # => NoMethodError: undefined method ‘bar’ for # 

Class Methods

Ruby is very flexible and as such it allows several ways to define a class method. The following is a sample of the most commonly used ways.

# Way 1
class Foo
  def self.bar
    puts 'class method'
  end
end

Foo.bar # "class method"

# Way 2
class Foo
  class << self
    def bar
      puts 'class method'
    end
  end
end

Foo.bar # "class method"

# Way 3
class Foo; end
def Foo.bar
  puts 'class method'
end

Foo.bar # "class method"

Instance Methods
Instance methods are a bit more simple. Here are a few common ways that instance methods are defined.

# Way 1
class Foo
  def baz
    puts 'instance method'
  end
end

Foo.new.baz # "instance method"

# Way 2
class Foo
  attr_accessor :baz
end

foo = Foo.new
foo.baz = 'instance method'
puts foo.baz

# Way 3
class Foo; end

foo = Foo.new
def foo.bar
  puts 'instance method'
end

Foo.new.baz # "instance method"