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Ruby Object Model: a first insight by Mind Map: Ruby Object Model:
a first insight
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Ruby Object Model: a first insight

Some useful methods I discovered


all constants in the current scope, module Piero class Paolo #... end class Pippo #... end end puts Piero.constants # => [Pippo, Paolo]


all the top level constants in the current program, including classnames!, Module.constants => ["IO", "Binding", "TypeError", "Module", "Config", "RUBY_PLATFORM", "SecurityError", "NilClass", "Mutex", "APPLE_GEM_HOME", "TrueClass", "ENV", "MAXHISTSIZE", "ARGF", "DTracer", "Precision", "LocalJumpError", "ZeroDivisionError", "ARGV", "RubyToken", "Symbol", "Time", "Continuation", "SyntaxError", "SystemStackError", "Integer", "Piero", "CROSS_COMPILING", "Math", "STDIN", "ArgumentError", "NoMemoryError", "Class", "RUBY_PATCHLEVEL", "Etc", "TRUE", "Struct", "ThreadError", "FloatDomainError", "RubyLex", "SignalException", "Enumerable", "UnboundMethod", "StringIO", "LoadError", "DateTime", "Queue", "RUBY_FRAMEWORK", "Proc", "Float", "Exception", "IndexError", "STDOUT", "Fixnum", "SystemCallError", "SLex", "VERSION", "Date", "YAML", "FalseClass", "MatchData", "SystemExit", "ObjectSpace", "Readline", "IOError", "NoMethodError", "Thread", "Numeric", "Exception2MessageMapper", "Interrupt", "FileTest", "NotImplementedError", "Object", "RUBY_VERSION", "Hash", "String", "GC", "ETC_IRBRC_LOADED", "STDERR", "RangeError", "Marshal", "Regexp", "Bignum", "Kernel", "RELEASE_DATE", "Method", "MatchingData", "FALSE", "Signal", "RegexpError", "Gem", "EOFError", "Kconv", "ConditionVariable", "TOPLEVEL_BINDING", "StandardError", "IRB", "File", "RuntimeError", "RUBY_RELEASE_DATE", "ThreadGroup", "RbConfig", "A", "Data", "Array", "Range", "NameError", "Process", "Rational", "HISTFILE", "Errno", "PLATFORM", "Dir", "NIL", "SizedQueue", "Comparable", "RUBY_FRAMEWORK_VERSION", "ScriptError"]


shows the current path

spell: Open Classes

you can always reopen existing classes and modify them on the fly

even Ruby standard library classes

example: the Money gem

class Numeric def to_money * 100) end end

Careful with That Axe, Eugene

spell: Monkeypatch, the dark side of Open Classes, you can end up with strange bugs...


Any reference that begins with an uppercase letter, including the names of classes and modules, is a constant

Constant scope

similar to a file system, directories => modules and classes, files => constants

Constants are uniquely identified by their paths, path separator is a double colon, MyModule::MyClass::MyConstant

you can change the value of a constant

but you'll get a warning from the interpreter

and as far as class names are constants, you can change the value of a class name, String = MyClass

spell: Namespace

using a module just as a container of constants

e.g. Rake, Rake old versions, had classes such as, Task, FileTask, Rake recent versions, define its classes inside a Rake module, module Rake class Task # ...

What's in an object?

instance variables




Object#methods, try yourself, for example "hi there!".methods

where are the methods?, in the classes, object methods, "hi!".size, vs, class' instance methods, String#size, String.instance_methods == "Hi man".methods, String.methods != "Hi man".methods

Classes themselves are nothing but objects!

this means that...

Class class has instance methods, that is, methods that every Class instance (e.g. the String class) has, Class.instance_methods(false), superclass, new

in Ruby a class is every instance of the Class class, to tell if an object obj is a class, make him this question, obj.is_a? Class

QUIZ TIME!, >> Class.is_a? Class => ? >> String.is_a? String => ? >> String.is_a? Class => ? >> "Hello there".is_a? String => ?

let's explore the Class hierarchy, Class.superclass # => Module, Module.superclass # => Object, therefore, a Class is a Module, plus the possibility to create instances (Class#new) and the capability to arrange its instances into hierarchies (Class#superclass), and a Module is just a bunch of methods

how do you create an instance?, obj =, obj is a reference to the new object, obj is a variable

how do you create a Class instance?, class MyClass; end, MyClass is a reference to the new class, MyClass is a constant, therefore, a class name is a reference to the relative Class instance, String is a reference to the String class, that is, an instance of the Class class

so, what's an object?, it's a bunch of instance variables, plus a link to a class, the object methods live in the object's class, where they're called "instance methods" of the class

so, what's a class?, it's an object, it's an instance of Class, plus a list of instance methods, e.g String#length, plus a link to a superclass, plus a "new" method