The document discusses Ruby classes and objects. It shows that everything in Ruby is an object, including classes. Classes are instances of the Class class. The hierarchy of built-in classes is: - BasicObject - Object - Module - Class It demonstrates that all objects inherit from BasicObject and classes inherit from Module. The document uses method calls like .class, .is_a?, and .superclass to explore and illustrate these relationships between Ruby objects and classes.

1. module Exlabs
class Presentation
attr_reader :author, :title, :date
def self.ish
new(author: 'Wojtek Widenka',
title: 'Selfish Presentation')
end
def initialize(params)
@author = params.fetch(:author, 'unknown')
@title = params.fetch(:title, 'unknown')
@date = params.fetch(:date, Date.today)
end
end
end

2. module Exlabs
class Presentation
attr_reader :author, :title, :date
puts "1: #{self}"
def self.ish
puts "2: #{self}"
new(author: 'Wojtek Widenka',
title: 'Selfish Presentation')
end
def initialize(params = {})
@author = params.fetch(:author, 'unknown')
@title = params.fetch(:title, 'unknown')
@date = params.fetch(:date, Date.today)
puts "3: #{self}"
end
def foo
puts "4: #{self}"
end
end
end

3. module Exlabs
class Presentation
attr_reader :author, :title, :date
puts "1: #{self}t#{self.class}"
def self.ish
puts "2: #{self}t#{self.class}"
new(author: 'Wojtek Widenka',
title: 'Selfish Presentation')
end
def initialize(params = {})
@author = params.fetch(:author, 'unknown')
@title = params.fetch(:title, 'unknown')
@date = params.fetch(:date, Date.today)
puts "3: #{self}t#{self.class}"
end
def foo
puts "4: #{self}t#{self.class}"
end
end
end
# enter the text from left side
1: Exlabs::Presentation Class
=> :foo
> object = Exlabs::Presentation.ish
2: Exlabs::Presentation Class
3: #<Exlabs::Presentation:0x007f8fa9eac028> Exlabs::Presentation
=> #<Exlabs::Presentation:0x007f8fa9eac028 @author="Wojtek
Widenka", @title="Selfish Presentation", @date=#<Date: 2016-10-26
((2457688j,0s,0n),+0s,2299161j)>>
> object.foo
4: #<Exlabs::Presentation:0x007f8fa9eac028> Exlabs::Presentation
=> nil

4. SystemStackError: stack level too deep
def self.h
@h = 'st'
puts h
end
module Exlabs
class Examples
->
end
end
def self.a
puts a
end
def self.b
puts self.b
end
st
=> nil
def self.c
c = 'st'
puts c
end
def self.d
d = 'st'
puts self.d
end
def self.e
self.e = 'st'
puts e
end
def self.f
self.f = 'st'
puts self.f
end
def self.g
@g = 'st'
puts @g
end
> Exlabs::Examples.a
NoMethodError: undefined method `e='
for Exlabs::Examples:Class
def self.i
@i = 'st'
puts self.i
end

5. SystemStackError: stack level too deep
def q
@q = 'st'
puts q
end
module Exlabs
class Examples
->
end
end
def j
puts j
end
def k
puts self.k
end
st
=> nil
def l
l = 'st'
puts l
end
def m
m = 'st'
puts self.m
end
def n
self.n = 'st'
puts n
end
def o
self.o = 'st'
puts self.o
end
def p
@p = 'st'
puts @p
end
> Exlabs::Examples.new.j
NoMethodError: undefined method `n=' for
#<Exlabs::Examples:0x007f8130af82d8>
def r
@r = 'st'
puts self.r
end

6. SystemStackError: stack level too deep
def self.h
@h = 'st'
puts h
end
module Exlabs
class Examples
->
end
end
def self.a
puts a
end
def self.b
puts self.b
end
st
=> nil
def self.c
c = 'st'
puts c
end
def self.d
d = 'st'
puts self.d
end
def self.e
self.e = 'st'
puts e
end
def self.f
self.f = 'st'
puts self.f
end
def self.g
@g = 'st'
puts @g
end
> Exlabs::Examples.a
NoMethodError: undefined method `e='
for Exlabs::Examples:Class
def self.i
@i = 'st'
puts self.i
end

7. SystemStackError: stack level too deep
def q
@q = 'st'
puts q
end
module Exlabs
class Examples
->
end
end
def j
puts j
end
def k
puts self.k
end
st
=> nil
def l
l = 'st'
puts l
end
def m
m = 'st'
puts self.m
end
def n
self.n = 'st'
puts n
end
def o
self.o = 'st'
puts self.o
end
def p
@p = 'st'
puts @p
end
> Exlabs::Examples.new.j
NoMethodError: undefined method `n=' for
#<Exlabs::Examples:0x007f8130af82d8>
def r
@r = 'st'
puts self.r
end

8. module Exlabs
class Presentation
attr_reader :author, :title, :date
puts "1: #{self}t#{self.class}"
def self.ish
puts "2: #{self}t#{self.class}"
new(author: 'Wojtek Widenka',
title: 'Selfish Presentation')
end
class << self
puts "5: #{self}t#{self.class}"
def bar
puts "6: #{self}t#{self.class}"
end
def self.foobar
puts "7: #{self}t#{self.class}"
end
end
def initialize(params = {})
@author = params.fetch(:author, 'unknown')
@title = params.fetch(:title, 'unknown')
@date = params.fetch(:date, Date.today)
puts "3: #{self}t#{self.class}"
end
def foo
puts "4: #{self}t#{self.class}"
end
end
end
# enter the text from left side
1: Exlabs::Presentation Class
5: #<Class:Exlabs::Presentation> Class
=> :foo
> object = Exlabs::Presentation.ish
2: Exlabs::PresentationClass
3: #<Exlabs::Presentation:0x007fce5514b218> Exlabs::Presentation
=> #<Exlabs::Presentation:0x007fce5514b218 @author=[…]>
> object.foo
4: #<Exlabs::Presentation:0x007fce5514b218> Exlabs::Presentation
=> nil
> object.bar
NoMethodError: undefined method `bar' for #<Exlabs::Presentation:0x007fce5514b218>
> Exlabs::Presentation.bar
6: Exlabs::Presentation Class
=> nil
> object.foobar
NoMethodError: undefined method `foobar' for #<Exlabs::Presentation:0x007fce5514b218>
> Exlabs::Presentation.foobar
NoMethodError: undefined method `foobar' for Exlabs::Presentation:Class
> Exlabs::Presentation.singleton_class.foobar
7: #<Class:Exlabs::Presentation> Class
=> nil

9. module Exlabs
class Presentation
attr_reader :author, :title, :date
puts "1: #{self}t#{self.class}"
def self.ish
puts "2: #{self}t#{self.class}"
new(author: 'Wojtek Widenka',
title: 'Selfish Presentation')
end
class << self
puts "5: #{self}t#{self.class}"
def bar
puts "6: #{self}t#{self.class}"
end
def self.foobar
puts "7: #{self}t#{self.class}"
end
end
def initialize(params = {})
@author = params.fetch(:author, 'unknown')
@title = params.fetch(:title, 'unknown')
@date = params.fetch(:date, Date.today)
puts "3: #{self}t#{self.class}"
end
def foo
puts "4: #{self}t#{self.class}"
end
end
end
module Exlabs
class Stranger
class << self
puts "8: #{self}t#{self.class}"
def bar
puts "9: #{self}t#{self.class}"
end
private
def private_method
puts 'Just a text'
end
end
class << Presentation
puts "10: #{self}t#{self.class}"
def bar
puts "11: #{self}t#{self.class}"
end
end
end
end
# enter the text from left side
8: #<Class:Exlabs::Stranger> Class
10: #<Class:Exlabs::Presentation> Class
=> :bar
> Exlabs::Stranger.bar
9: Exlabs::Stranger Class
=> nil
> Exlabs::Stranger.private_method
NoMethodError: private method `private_method'
called for Exlabs::Stranger:Class
> Exlabs::Presentation.bar
11: Exlabs::Presentation Class
=> nil

10. str1 = 'String 1'
str2 = 'String 2'
class << String
def foo
puts 'foo called'
puts self
end
end
class << str1
def bar
puts 'bar called'
puts self
end
end
> str1.foo
NoMethodError: undefined method `foo' for "String 1":String
> String.foo
foo called
String
=> nil
> str1.bar
bar called
String 1
=> nil
> str2.bar
NoMethodError: undefined method `bar' for "String 2":String
> String.singleton_class.method_defined? :bar
=> false
> str1.singleton_class.method_defined? :bar
=> true
> str2.singleton_class.method_defined? :bar
=> false

11. str1 = 'String 1'
str2 = 'String 2'
class << String
def foo
puts 'foo called'
puts self
end
end
class << str1
def bar
puts 'bar called'
puts self
end
end
> str1.foo
NoMethodError: undefined method `foo' for "String 1":String
> String.foo
foo called
String
=> nil
> str1.bar
bar called
String 1
=> nil
> str2.bar
NoMethodError: undefined method `bar' for "String 2":String
> String.singleton_class.method_defined? :bar
=> false
> str1.singleton_class.method_defined? :bar
=> true
> str2.singleton_class.method_defined? :bar
=> false

12. ▸ ruby is tricky
▸ in ruby everything is an object
▸ from ruby implementation point of view Class is slightly different than Object
struct RObject {
struct RBasic basic;
union {
struct {
uint32_t numiv;
VALUE *ivptr;
void *iv_index_tbl;
} heap;
VALUE ary[ROBJECT_EMBED_LEN_MAX];
} as;
};
struct RBasic {
VALUE flags;
const VALUE klass;
}
struct RClass {
struct RBasic basic;
VALUE super;
rb_classext_t *ptr;
struct rb_id_table *m_tbl;
};
▸ classes contains methods while objects variables (this is confusing, not sure if
can explain)

13. ▸ when we implementing new class
class NewClass
def new_method
end
end
▸ we are creating new instance of Class (we can use instead Class.new syntax)
▸ There is also constructed singleton class (eigenclass, metaclass) in which methods
are stored
▸ Since it is singleton it cannot be duplicated and is connected always with one object
▸ ruby is tricky, metaclass is transparent, so calling #class on object will ommit it

16. "Nathan".is_a?(BasicObject) # => true "Nathan" is an object.
"Nathan".class #=> String "Nathan" is an instance of the String class.
"Nathan".is_a?(Class) #=> false "Nathan" is not a class.
"Nathan".superclass # NoMethodError "Nathan" has no superclass, because "Nathan" is not a class.
String.is_a?(BasicObject) #=> true String is an object.
String.class #=> Class String is an instance of the Class class.
String.superclass #=> Object String's superclass is Object.
Object.is_a?(BasicObject) #=> true Object is an object.
Object.class #=> Class Object is an instance of the Class class.
Object.superclass #=> BasicObject Object's superclass is BasicObject.
BasicObject.is_a?(BasicObject) #=> true BasicObject is an object.
BasicObject.class #=> Class BasicObject is an instance of the Class class
BasicObject.superclass #=> nil BasicObject's superclass is nil.
nil.is_a?(BasicObject) #=> true nil is an object.
nil.class #=> NilClass nil is an instance of the NilClass class
nil.superclass # NoMethodError nil has no superclass, because it is not a class.

17. Class.is_a?(BasicObject) #=> true Class is an object.
Class.class #=> Class Class is an instance of the Class class.
# this is probably the most important part.
Class.superclass #=> Module Class's superclass is Module
# 2nd most important part
Module.is_a?(BasicObject) #=> true Module is an object
Module.class #=> Class Module is an instance of the Class class. # 3rd
Module.superclass #=> Object Module's superclass is Object # 4th
Object.is_a?(BasicObject) #=> true Object is an object.
Object.class #=> Class Object is an instance of the Class class.
Object.superclass #=> BasicObject Object's superclass is BasicObject.
BasicObject.is_a?(BasicObject) #=> true BasicObject is an object.
BasicObject.class #=> Class BasicObject is an instance of the Class class
BasicObject.superclass #=> nil BasicObject's superclass is nil.
nil.is_a?(BasicObject) #=> true nil is an object.
nil.class #=> NilClass nil is an instance of the NilClass class
nil.superclass # NoMethodError nil has no superclass, because it is not a class.