class self idiom in Ruby

Question

What does class  lt  lt  self do in Ruby

User · Answer

In fact if you write any C extensions for your Ruby projects there is really only one way to define a Module method.

rb_define_singleton_method

I know this self business just opens up all kinds of other questions so you could do better by searching each part.

Objects first.

foo = Object.new

Can I make a method for foo?

Sure

def foo.hello
 'hello'
end

What do I do with it?

foo.hello
 ==>"hello"

Just another object.

foo.methods

You get all the Object methods plus your new one.

def foo.self
 self
end

foo.self

Just the foo Object.

Try to see what happens if you make foo from other Objects like Class and Module. The examples from all the answers are nice to play with but you have to work with different ideas or concepts to really understand what is going on with the way the code is written. So now you have lots of terms to go look at.

Singleton, Class, Module, self, Object, and Eigenclass was brought up but Ruby doesn't name Object Models that way. It's more like Metaclass. Richard or __why shows you the idea here. http://viewsourcecode.org/why/hacking/seeingMetaclassesClearly.html And if the blows you away then try looking up Ruby Object Model in search. Two videos that I know of on YouTube are Dave Thomas and Peter Cooper. They try to explain that concept too. It took Dave a long time to get it so don't worry. I'm still working on it too. Why else would I be here? Thanks for your question. Also take a look at the standard library. It has a Singleton Module just as an FYI.

This is pretty good. https://www.youtube.com/watch?v=i4uiyWA8eFk

User · Answer

I found a super simple explanation about class  lt  lt  self   Eigenclass and different type of methods   In Ruby  there are three types of methods that can be applied to a class    Instance methods Singleton methods Class methods   Instance methods and class methods are almost similar to their homonymous in other programming languages   class Foo     def an instance method       puts  I am an instance method      end     def self a class method       puts  I am a class method      end   end  foo   Foo new  def foo a singleton method   puts  I am a singletone method  end   Another way of accessing an Eigenclass which includes singleton methods  is with the following syntax  class  lt  lt     foo   Foo new  class  lt  lt  foo   def a singleton method     puts  I am a singleton method    end end   now you can define a singleton method for self which is the class Foo itself in this context   class Foo   class  lt  lt  self     def a singleton and class method       puts  I am a singleton method for self and a class method for Foo      end   end end

User · Answer

singleton method is a method that is defined only for a single object    Example   class SomeClass   class  lt  lt  self     def test     end   end end  test obj   SomeClass new  def test obj test 2 end  class  lt  lt  test obj   def test 3   end end  puts  Singleton s methods of SomeClass  puts SomeClass singleton methods puts  ------------------------------------------  puts  Singleton s methods of test obj  puts test obj singleton methods   Singleton s methods of SomeClass  test    Singleton s methods of test obj  test 2  test 3

User · Answer

Usually  instance methods are global methods  That means they are available in all instances of the class on which they were defined  In contrast  a singleton method is implemented on a single object   Ruby stores methods in classes and all methods must be associated with a class  The object on which a singleton method is defined is not a class  it is an instance of a class   If only classes can store methods  how can an object store a singleton method  When a singleton method is created  Ruby automatically creates an anonymous class to store that method  These anonymous classes are called metaclasses  also known as singleton classes or eigenclasses  The singleton method is associated with the metaclass which  in turn  is associated with the object on which the singleton method was defined   If multiple singleton methods are defined within a single object  they are all stored in the same metaclass   class Zen end  z1   Zen new z2   Zen new  class  lt  lt  z1   def say hello     puts  Hello     end end  z1 say hello      Output  Hello  z2 say hello      Output  NoMethodError  undefined method  say hello       In the above example  class  lt  lt  z1 changes the current self to point to the metaclass of the z1 object  then  it defines the say hello method within the metaclass   Classes are also objects  instances of the built-in class called Class   Class methods are nothing more than singleton methods associated with a class object    class Zabuton   class  lt  lt  self     def stuff       puts  Stuffing zabuton         end   end end   All objects may have metaclasses  That means classes can also have metaclasses  In the above example  class  lt  lt  self modifies self so it points to the metaclass of the Zabuton class  When a method is defined without an explicit receiver  the class object on which the method will be defined   it is implicitly defined within the current scope  that is  the current value of self  Hence  the stuff method is defined within the metaclass of the Zabuton class  The above example is just another way to define a class method  IMHO  it s better to use the def self my new clas method syntax to define class methods  as it makes the code easier to understand  The above example was included so we understand what s happening when we come across the class  lt  lt  self syntax   Additional info can be found at this post about Ruby Classes

User · Answer

First  the class  lt  lt  foo syntax opens up foo s singleton class  eigenclass   This allows you to specialise the behaviour of methods called on that specific object   a    foo  class  lt  lt  a   def inspect       bar     end end a inspect       gt   bar   a    foo      new object  new singleton class a inspect       gt   foo      Now  to answer the question  class  lt  lt  self opens up self s singleton class  so that methods can be redefined for the current self object  which inside a class or module body is the class or module itself   Usually  this is used to define class module   static   methods   class String   class  lt  lt  self     def value of obj       obj to s     end   end end  String value of 42       gt   42    This can also be written as a shorthand   class String   def self value of obj     obj to s   end end   Or even shorter   def String value of obj   obj to s end     When inside a function definition  self refers to the object the function is being called with  In this case  class  lt  lt  self opens the singleton class for that object  one use of that is to implement a poor man s state machine   class StateMachineExample   def process obj     process hook obj   end  private   def process state 1 obj               class  lt  lt  self       alias process hook process state 2     end   end    def process state 2 obj               class  lt  lt  self       alias process hook process state 1     end   end      Set up initial state   alias process hook process state 1 end   So  in the example above  each instance of StateMachineExample has process hook aliased to process state 1  but note how in the latter  it can redefine process hook  for self only  not affecting other StateMachineExample instances  to process state 2  So  each time a caller calls the process method  which calls the redefinable process hook   the behaviour changes depending on what state it s in

User · Answer

What class  lt  lt  thing does  class Hi   self    gt  Hi   class  lt  lt  self  same as  class  lt  lt  Hi      self    gt    lt Class Hi gt      self    Hi singleton class    gt  true   end end   it makes self    thing singleton class in the context of its block    What is thing singleton class  hi   String new def hi a end  hi class instance methods include   a    gt  false hi singleton class instance methods include   a    gt  true  hi object inherits its  methods from its  singleton class instance methods and then from its  class instance methods  Here we gave hi s singleton class instance method  a  It could have been done with class  lt  lt  hi instead  hi s  singleton class has all instance methods hi s  class has  and possibly some more   a here    instance methods of thing s  class and  singleton class can be applied directly to thing  when ruby sees thing a  it first looks for  a method definition in thing singleton class instance methods and then in thing class instance methods   By the way - they call object s singleton class    metaclass    eigenclass

[ruby] class << self idiom in Ruby

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