Object Introduction

Object is the base of the idea of Object Oriented Programming. Object is the mapping of real world objects or resources in to the digital paradigm.

Ruby is a true object oriented language because everything in Ruby is an object. Objects are the first-class citizens of the Ruby world. Even language constructs such as classes, instance variables, methods and modules are also objects. They live in a system known as Object Model. What do you think an object is? It’s just a bunch of instance variables, plus a link to a class.

# numbers are objects
num  = 1
num.object_id

chr = 'a'
chr.object_id

# classes are also objects
class.object_id

# Even methods are objects
method = num.method(:times)
method.object_id

# operators are also objects
# actually they are methods defined in an object and which are also objects
# see all the methods `num.methods`
plus = num.method(:+)
plus.object_id

Object Model

So basically, Object Model is a world or system where all these language constructs live together. Object Model is where you will find answers to the questions like “which class does this method come from?” , “what happens when I inherit this class?” and “what instance variables does this object have?”

class A; end
class B < A; end

B = B.new
B.class
B.ancestors
B.superclass
# modules do not fall in `superclass`

# which class does this method come from?
class A
  def new_method
     print “new method”
  end
end


class B < A; end

b= B.new
b.new_method
> b.method :new_method
=> #<Method: B(A)#new_method>

“what instance variables does this object have?”
b.instance_variables
# []

Introduction to Class

The object’s methods don’t live in the object—they live in the object’s class, where they’re called the instance methods of the class.

What’s a class?

It’s just an object (an instance of Class), plus a list of instance methods and a link to a superclass. Class is a subclass of Module, so a class is also a module.

Like any object, a class has its own methods, such as new( ) and other you define. These are instance methods of the Class class. They are also called class-methods. You define these methods using self. prefix.

# reference
class A
    # this method is local to this particular class
    # other descendants of the Class class don’t have it
    # this is singleton method
    def self.some_method
        print “some method”
    end
end

A.singleton_methods
# [:some_method]

A.some_method
a=A.new
a.some_method # is not defined

Also like any object, classes must be accessed through references. You already have a constant reference to each class i.e. the class’s name.