Deciphering the Ruby Object Model

Who am I?KarthikSirasanagandlaA Pragmatic Programmer @ ThoughtWorksVB freelance programmer for about a yearJava/JEE apps. developer for about 5 yearsRuby Enthusiast for over 8 months nowHaskell newbieOccasionally blog at kartzontech.blogspot.comNot so frequently tweet using @kartzontech

Why Learn Ruby Object Model?Good if you know “syntax and semantics”Better if you know “Ruby’s Object Model”“A language the does not affect the way you think about programming is not worth learning at all” – Alan Perlis

The AxiomBasicObjectKernelSuperclassObjectClassSuperclassSuperclassModuleAnyOtherClassSuperclassYou don’t question an axiom,rather start with it…ClassClass

ClassesClasses should be closed for modification and open to extensionclass MyClassendclass MyClassdef im# instance method puts “Instance method” end def self.cm #class method puts “Class method”endend

Classes“With more power comes more responsibilities” opines Matz, like uncle Ben in the spiderman movie Open Classes ???Monkey Patching ???

ClassesClass methods can be defined in number of ways:class MyClassdef MyClass.cm1 puts “cm1() invoked…” end def self.cm2 puts “cm2() invoked…”end class << self def cm3 puts “cm3() invoked…” endendend

ClassesClasses are objectsclass MyClass def im puts “Instance method” end endis the same as:MyClass = Class.newdo def im puts “Class method”endend

ClassesQuiz Time:class MyClass < String.new def self.to_s puts “Class method” end endMyClass.to_sWhat is the output? TypeError: Wrong argument type String (expected Class)Lesson:Classes inherit from classes only not from just any object

ClassesNo connection between a class and its objects’ instance variablesMyClass______________________my_instance_method()ClassClassClassmy_obj1______________________@var_1 = 1@var_2 = “Ruby Conf”my_obj2______________________@v = 123my_obj3______________________

ClassesQuiz:What is the Java/C# Interface equivalent in Ruby? [FYI: This has nothing to do with what I have been talking about all through!!]Answer:Interfaces are irrelevant in Ruby.But why? Duck Typing is the answer. Program to an interfaceNo need to inherit from a common interface.

The joy of “self” discoveryself is not the same as this in Javaself is synonymous to current/default objectWho gets to be self depends on Where self is

The joy of “self” discoveryCode is an unbeatable teaching aid. What say?p “At Topmost level, self is #{self}”class MyClass p “In class definition block, self is #{self}” def self.my_class_method p “In my_class_method, self is #{self}” end def my_instance_method p “In my_instance_method, self is #{self}” endend

Class, Instance and Class Instance VariablesExperience is the best teacher. class A y = 1@p = 2@q @@t = 3 def initialize @@r ||= 4 @s = 5 end endputs “Class instance variables of A are #{A.instance_variables}”[:@p]puts “Class variables of A are #{A.class_variables}”[:@@t]a = A.newputs “a, of type A, has instance variables #{a.instance_variables}”[:@s]puts “Class variables of A are #{A.class_variables}”[:@@t, :@@r]

Access modifiersRuby has 3 access modifiers for methods:Public (default)PrivateProtectedAccess modifiers apply until the end of scope, or until another access modifier pops-upclass MyClass #public is the default access modifier def m1; endprivate def m2; end; def m3; end;protected def m4; end;end

Access modifiersprivate, public, protectedare all methods, so you can pass method name as symbol to it and change its visibilityclass MyClass def m1; end def m2; end; def m3; end; def m4; end;public :m1private :m2, :m3protected :m4end

Access modifierspublic privateonly accessible within the scope of a single object in which it is defined (truly private)protectedaccessible from the scope of a method belonging to any object that’s an instance of the same classclass MyClassprotected def my_protected_method endpublic def my_public_method(other)self.my_protected_methodother.my_protected_method endendmc1 = MyClass.newmc2 = MyClass.newmc1.my_public_method(mc2) #=> Works mc1.my_protected_method #=> NoMethodError

Access modifiersQuizclass Speaker def talkself.confident? ? “lecture…” : “abscond!” endprivate def confident? true endendSpeaker.new.talkWhat is the output? NoMethodErrorLesson:Private methods can be called only with implicit receiver. No explicit receiver. Not even self.

Access modifiersQuizWe talked about access modifiers with methods as example. What about the access modifiers for instance variables? Answer:Access modifiers don’t apply to Instance variables (@inst_vari). Instance variables always remain private.Access to instance variables are only through getter and setter methods!

Singleton MethodsA method defined to be specific to a single objectstatement = “Karthik is speaking at Ruby Conf India 2011” def statement.really? trueendstatement.really? What is the output? #=> trueanother_statement = “Karthik is bull shitting at Ruby Conf 2011”another_statement.really? What is the output? #=> undefined method error

Singleton MethodsHow about a class method? Isn’t that tied to a single object?Eg:def MyClass.my_class_method#blah..endClass methods are indeed singleton methods!

Methods – where do they reside?It depends!!!MyClass______________________myclass_instance_method()#<Class: MyClass>______________________myclass_class_method()Class#<Class: #<Class: 0x1234567>>______________________myclass_class_method()my_obj1_specific_inst_meth()my_obj1_specific_class_meth()my_obj1______________________@var_1 = 1Class#<Class: #<Class: 0x7654321>>______________________myclass_class_method()my_obj2_specific_inst_meth()my_obj2______________________

Methods -> Function call or Message???No function call. It’s all messages!duck.quack()As Java programmer, I see it as looking up for “quack” as member function in a table and call it.As Ruby programmer, I see it as passing a message “quack” to the object “duck”.No Method Overloading.

Module Used for namespacingmodule MyModuleMyConstant = “MyModule::Myconstant” class MyClassMyConstant = “MyModule::MyClass::MyConstant” endendputs MyModule::MyConstant# => MyModule::MyConstantputs MyModule::MyClass::MyConstant#=> MyModule::MyClass::MyConstantAnother example:ActiveRecord::Base.connection.execute(sql_query)

ModuleBetter alternate to multiple inheritence.DP: “Favor composition over inheritence”Code Examples:module MyModule def my_meth puts “my_meth() of MyModule” endend class MyClassendCase 1: Include MyModule instance methods as instance methods of myClassclass MyClass include MyModule #   Just include it…end

Modulemodule MyModule def my_meth puts “my_meth() of MyModule” endend class MyClassendCase 2: Include instance methods of MyModule as class methods of MyClass

ModuleQuiz:module MyModule def self.my_freakin_meth puts “my_freakin_meth() of MyModule” endend class MyClass include MyModuleendMyClass.my_freakin_methWhat is the output?#NoMethodErrorLesson:When an object includes a moduleModule’s instance methods are includedModule’s class methods are excluded

Method LookupEigen/Singleton/Ghost ClassesRegular ObjectsClasses# <Class : Object>Object# <Class : S>SD# <Class : D># <D: 0x1234567># <Class : # <D:0x1234567>># <Class : # <D:0x7654321>># <D: 7654321>

ResourcesProgramming Ruby by Dave ThomasMetaprogramming Ruby by Paolo PerrottaThe Well-grounded Rubyist by David A BlackOneStepBack.org by Jim Weirichblog.jayfields.com

Thanks.Don’t forget to whisper your feedback in my ears!

Deciphering the Ruby Object Model

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Deciphering the Ruby Object Model