When to use self for Ruby Class

user_2861567 0
Viewed 151

Can somebody help me distinguish When we create methods inside class << self block and when we define normal methods.

I saw somewhere code like this, but I don't know concisely the use cases of them

class Foo
  def initialize
  end

  def bar
  end

  class << self
    def foobar
    end
  end
end
ruby
2 Answers

The methods defined right inside a class block are instance methods:

class Foo
  def bar
  end
end

Methods defined within class << self inside a class block are class methods:

class Foo
  class << self
    def baz
    end
  end
end

Instance methods become available to any instance of a given class:

foo = Foo.new
foo.bar

Whereas class methods can be called directly on the class:

Foo.baz

Attempting to call instance methods on the class or vice versa results in an error:

Foo.bar #=> NoMethodError: undefined method `bar' for Foo:Class
foo.baz #=> NoMethodError: undefined method `baz' for #<Foo:0x00007ffe20055a20>

Another way to define class methods is by prefixing the method name with self.:

class Foo
  def self.baz
  end
end

You could also define them outside the class block, although this is rarely seen:

def Foo.baz
end

Or likewise:

class << Foo
  def baz
  end
end

Note that defining methods this way is not limited to classes. You can add methods to arbitrary objects, e.g.:

o = Object.new

def o.hello
  "hello from o"
end

o.hello
#=> "hello from o"

Or via:

class << o
  def hello
    "hello from o"
  end
end

Internally, these methods are added to the object's singleton class. It's a special purpose class to hold methods for just that instance:

o.singleton_class.instance_methods(false)
#=> [:hello]

For the Foo class above:

Foo.instance_methods(false)                 #=> [:bar]
Foo.singleton_class.instance_methods(false) #=> [:baz]

So technically, a class method is just an instance method defined on the class' singleton class.

user_4770370

You may need to read up on Ruby's instance and class methods. But personally, I'd do

class Foo
  class << self
    def foobar
    end
  end
end

instead of 

class Foo
  def self.foobar
  end
end

whenever I want to add some class level attributes, or make a method private etc as 

class Foo
  private
  def self.foobar
  end
end

wouldn't work the same as 

class Foo
  class << self
    private
    def foobar
    end
  end
end
user_38937100
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