[Rails] Re: Class, Module, Object
Yes, Object, Class, and Module are more than just constants. Object, Class, and Module are instances of Class class: 1.9.3p0 :031 Class.class = Class 1.9.3p0 :032 Module.class = Class 1.9.3p0 :033 Object.class = Class Since Object, Class, and Module are instances of Class class, they don't have access to Class's singleton methods, but rather they have access to Class's instance methods: 1.9.3p0 :052 class Class 1.9.3p0 :053? def Class.abc 1.9.3p0 :054? puts 'abc' 1.9.3p0 :055? end 1.9.3p0 :056? end = nil 1.9.3p0 :076 Module.abc NoMethodError: undefined method `abc' for Module:Class 1.9.3p0 :077 Object.abc NoMethodError: undefined method `abc' for Object:Class However, since we defined the singleton method on Class, it will work on a Class invocation: 1.9.3p0 :078 Class.abc abc = nil But instance methods work: 1.9.3p0 :012 Class.instance_methods(false) = [:allocate, :new, :superclass, :cattr_reader, :cattr_writer, :cattr_accessor, …] 1.9.3p0 :015 Object.cattr_writer = [] 1.9.3p0 :017 Module.cattr_reader = [] 1.9.3p0 :018 Class.cattr_writer = [] This here is an indication that Object and Module are instances of Class but DO NOT inherit from Class. Let's check that out with the superclass call: 1.9.3p0 :079 Class.superclass = Module 1.9.3p0 :080 Module.superclass = Object 1.9.3p0 :081 Object.superclass = BasicObject 1.9.3p0 :082 BasicObect.superclass = nil Thus, Class inherits from Module, and Module in turn inherits from Object, which in turn inherits from BasicObject in ruby 1.9. We can test that out by seeing if we define a singleton method on Module, then Class should be able to invoke it, since there is inheritance between them. 1.9.3p0 :067 class Module 1.9.3p0 :068? def Module.qqqw 1.9.3p0 :069? puts 'qqqw' 1.9.3p0 :070? end 1.9.3p0 :071? end = nil 1.9.3p0 :072 Module.singleton_methods(false) = [:nesting, :constants, :attr_internal_naming_format, :attr_internal_naming_format=, :qqqw] 1.9.3p0 :073 Class.qqqw qqqw = nil 1.9.3p0 :074 Object.qqqw NoMethodError: undefined method `qqqw' for Object:Class 1.9.3p0 :084 class Object 1.9.3p0 :085? def Object.ttyy 1.9.3p0 :086? puts 'ttyy' 1.9.3p0 :087? end 1.9.3p0 :088? end = nil 1.9.3p0 :089 Module.ttyy ttyy = nil 1.9.3p0 :090 Class.ttyy ttyy = nil This shows the role of inheritance. We defined a singleton method on Object, and thus since Module inherits from Object, it had access to it, and so did Class, since Class inherits from Module, which in turn inherits from Object. Now that's the metaclass (eigenclass) inheritance relationship (the fact that the eighenclass of Class inherits the singleton methods of the eighenclass of Module which in turn inherits the singleton methods of eigenclass of Object). If the inheritance relationship did not exist as it does, then we wouldnt be able to call singleton methods of Module on Class, for example. Furthermore, we have a circular relationship. Since Class is an instance of Class, Module is an instance of Class, and Object is an instance of Class, they all have access to instance methods of Class, as already stated. What's more interesting is the circular relationship that has been created via the combination of instantiation and inheritance. If Object is an instance of Class, and Class inherits from Module, then that means Class is an instance of Module, and so since Object is an instance of Class, it itself is an instance of Module. Hence, we can access the INSTANCE METHODS of any of these classes via the other Class. 1.9.3p0 :094 Class.is_a?(Module) = true 1.9.3p0 :095 Module.is_a?(Class) = true 1.9.3p0 :096 Object.is_a?(Module) = true 1.9.3p0 :097 Object.is_a?(Class) = true Now lets access an instance method of Module on Object: 1.9.3p0 :098 Module.instance_methods(false) = [:freeze, :===, :==, :=, :, :=, :, :=, :to_s, :included_modules, :include?, :name, :ancestors, :instance_methods, :public_instance_methods, :protected_instance_methods, :private_instance_methods, :constants, :const_get, :const_set, :const_defined?, :class_variables, :remove_class_variable, :class_variable_get, :class_variable_set, :class_variable_defined?, :public_constant, :private_constant, :module_exec, :class_exec, :module_eval, :class_eval, :method_defined?, :public_method_defined?, :private_method_defined?, :protected_method_defined?, :public_class_method, :private_class_method, :autoload, :autoload?, :instance_method, :public_instance_method, :psych_yaml_as, :yaml_as, :deprecate, :alias_method_chain, :alias_attribute, :mattr_reader, :mattr_writer, :mattr_accessor, :remove_possible_method, :redefine_method, :delegate, :instance_method_names, :method_names, :attr_internal_reader, :attr_internal_writer, :attr_internal_accessor, :attr_internal, :anonymous?, :duplicable?, :reachable?, :parent_name, :parent, :parents, :local_constants, :local_constant_names, :attr_accessor_with_default, :synchronize,
[Rails] Re: Class, Module, Object
I want to fix a typo. I said: Since Object, Class, and Module are instances of Class class, they don't have access to Class's singleton methods, but rather they have access to Class's instance methods: I meant to say: Object and Module do not inherit from Class and so they don't have access to Class's singleton methods, but since Object, Class, and Module are instances of Class class, they all have access to Class's instance methods: On Oct 3, 11:08 pm, John Merlino stoici...@aol.com wrote: Yes, Object, Class, and Module are more than just constants. Object, Class, and Module are instances of Class class: 1.9.3p0 :031 Class.class = Class 1.9.3p0 :032 Module.class = Class 1.9.3p0 :033 Object.class = Class Since Object, Class, and Module are instances of Class class, they don't have access to Class's singleton methods, but rather they have access to Class's instance methods: 1.9.3p0 :052 class Class 1.9.3p0 :053? def Class.abc 1.9.3p0 :054? puts 'abc' 1.9.3p0 :055? end 1.9.3p0 :056? end = nil 1.9.3p0 :076 Module.abc NoMethodError: undefined method `abc' for Module:Class 1.9.3p0 :077 Object.abc NoMethodError: undefined method `abc' for Object:Class However, since we defined the singleton method on Class, it will work on a Class invocation: 1.9.3p0 :078 Class.abc abc = nil But instance methods work: 1.9.3p0 :012 Class.instance_methods(false) = [:allocate, :new, :superclass, :cattr_reader, :cattr_writer, :cattr_accessor, …] 1.9.3p0 :015 Object.cattr_writer = [] 1.9.3p0 :017 Module.cattr_reader = [] 1.9.3p0 :018 Class.cattr_writer = [] This here is an indication that Object and Module are instances of Class but DO NOT inherit from Class. Let's check that out with the superclass call: 1.9.3p0 :079 Class.superclass = Module 1.9.3p0 :080 Module.superclass = Object 1.9.3p0 :081 Object.superclass = BasicObject 1.9.3p0 :082 BasicObect.superclass = nil Thus, Class inherits from Module, and Module in turn inherits from Object, which in turn inherits from BasicObject in ruby 1.9. We can test that out by seeing if we define a singleton method on Module, then Class should be able to invoke it, since there is inheritance between them. 1.9.3p0 :067 class Module 1.9.3p0 :068? def Module.qqqw 1.9.3p0 :069? puts 'qqqw' 1.9.3p0 :070? end 1.9.3p0 :071? end = nil 1.9.3p0 :072 Module.singleton_methods(false) = [:nesting, :constants, :attr_internal_naming_format, :attr_internal_naming_format=, :qqqw] 1.9.3p0 :073 Class.qqqw qqqw = nil 1.9.3p0 :074 Object.qqqw NoMethodError: undefined method `qqqw' for Object:Class 1.9.3p0 :084 class Object 1.9.3p0 :085? def Object.ttyy 1.9.3p0 :086? puts 'ttyy' 1.9.3p0 :087? end 1.9.3p0 :088? end = nil 1.9.3p0 :089 Module.ttyy ttyy = nil 1.9.3p0 :090 Class.ttyy ttyy = nil This shows the role of inheritance. We defined a singleton method on Object, and thus since Module inherits from Object, it had access to it, and so did Class, since Class inherits from Module, which in turn inherits from Object. Now that's the metaclass (eigenclass) inheritance relationship (the fact that the eighenclass of Class inherits the singleton methods of the eighenclass of Module which in turn inherits the singleton methods of eigenclass of Object). If the inheritance relationship did not exist as it does, then we wouldnt be able to call singleton methods of Module on Class, for example. Furthermore, we have a circular relationship. Since Class is an instance of Class, Module is an instance of Class, and Object is an instance of Class, they all have access to instance methods of Class, as already stated. What's more interesting is the circular relationship that has been created via the combination of instantiation and inheritance. If Object is an instance of Class, and Class inherits from Module, then that means Class is an instance of Module, and so since Object is an instance of Class, it itself is an instance of Module. Hence, we can access the INSTANCE METHODS of any of these classes via the other Class. 1.9.3p0 :094 Class.is_a?(Module) = true 1.9.3p0 :095 Module.is_a?(Class) = true 1.9.3p0 :096 Object.is_a?(Module) = true 1.9.3p0 :097 Object.is_a?(Class) = true Now lets access an instance method of Module on Object: 1.9.3p0 :098 Module.instance_methods(false) = [:freeze, :===, :==, :=, :, :=, :, :=, :to_s, :included_modules, :include?, :name, :ancestors, :instance_methods, :public_instance_methods, :protected_instance_methods, :private_instance_methods, :constants, :const_get, :const_set, :const_defined?, :class_variables, :remove_class_variable, :class_variable_get, :class_variable_set, :class_variable_defined?, :public_constant, :private_constant, :module_exec, :class_exec, :module_eval, :class_eval, :method_defined?, :public_method_defined?, :private_method_defined?,
[Rails] Re: Class, Module, Object
Module.is_a?(Class) Module inherits from Object, Object is an instance of Class. Object.is_a?(Module) Object is an instance of Class, Class inherits from Module. -- Posted via http://www.ruby-forum.com/. -- You received this message because you are subscribed to the Google Groups Ruby on Rails: Talk group. To post to this group, send email to rubyonrails-talk@googlegroups.com. To unsubscribe from this group, send email to rubyonrails-talk+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/groups/opt_out.
