Ruby: metaprogramming

Definitions
Singleton method: a method defined on a particular object
Singleton class: the anonymous class created in which a singleton method is defined

Object Model

• Instance variables live in objects, and methods live in classes.
• You can get a list of object's methods by calling obj.methods.
• For a class, you can call klass.methods to see what class methods are available, and klass.instance_methods to know the instance methods. klass.instance_methods(false) returns methods defined by the class and not inherited.
• Classes themselves are nothing but objects, and have their own class called Class.
• All classes ultimately inherit from Object.

  "hello".class # => String
  String.class # => Class
  Class.instance_methods(false) # => [:allocate, :new, :superclass]
  # :superclass is only a method a class, not an object. 
  String.superclass # => Object
  

• The methods of an a class are the instance methods of Class.

Self

• Every line of Ruby code is executed inside an object - the so-called current object. The current object is also known as self.
•  In a class or module definition, the role of self is taken by the class or module.

Singleton Methods

• Class methods are Singleton Methods of a class.
• Introspection methods:
• singleton_methods returns all the singleton methods for the object (also the ones in included modules).
• singleton_methods(false) returns all the singleton methods for the object, but not those declared in included modules.
• methods(false) supposedly returns the singleton methods by calling singleton_methods, but it also passes the parameter false to it.

  String.methods(false) == String.singleton_methods(false) # => true