Ruby classes

ruby_classes_1.rb

# Class
# http://ruby-doc.org/core-2.3.0/Class.html

class A
end

A.new.class # A

class B < A
end

B.new.class # B
B.superclass # A
A.superclass # Object
Object.superclass # BasicObject
BasicObject.superclass # nil

(b = B.new) && b.is_a?(B) && b.kind_of?(B) # true
(b = B.new) && b.is_a?(A) && b.kind_of?(A) # true
B.new.instance_of? B # true
B.new.instance_of? A # false

a = 1
defined? a # "local-variable"

@a = 2
defined? @a # "instance-variable"

@@a = 3
defined? @@a # "class variable"

$a = 4
defined? $a # "global-variable"

ruby_classes_2.rb

class A
  def initialize(a)
    @a = a
  end
  
  def print_a
    p @a
  end
  
  def self.create_with_a
    A.new(22)
  end
end

A.new(1).print_a # 1
A.create_with_a.print_a #22

class C
  def a
    @a
  end
  
  def a=(val)
    @a = val
  end
end

c = C.new
c.a # nil
c.a = 1
c.a # 1

class D
  attr_accessor :a
  attr_writer :b
  attr_reader :c
 
  def initialize(a, b, c)
    @a = a
    @b = b
    @c = c
  end
end

d = D.new 1, 2, 3
d.a # 1
d.a = 10 
d.a # 10

d.b # NoMethodError: undefined method `b'
d.b = 4
d.instance_variable_get(:@b) # 4

d.c # 3
d.c = 11 # NoMethodError: undefined method `c='
d.instance_variable_set(:@c, 5)
d.c # 5

ruby_classes_3.rb

class Arc
  attr_accessor :radians
  
  def degrees
    @radians * 180 / Math::PI
  end

  def degrees=(degrees)
    @radians = degrees * Math::PI / 180
  end
end

   arc = Arc.new
   arc.degrees = 180
   arc.radians # => 3.14159265358979
   arc.radians = 3.141592653589793 / 2
   arc.degrees # => 90.0
	
class Summator
  attr_reader :sum
  
  def initialize(start_value = 0)
    @sum = start_value
  end
  
  def +(value)
    @sum += value if value.is_a? Numeric
  end
  
  def == (summator)
    (summator.is_a? self.class) && summator.sum == sum
  end
end

summator = Summator.new
summator.sum # 0
summator + 1 
summator.sum # 1

summator2 = Summator.new

summator == 1 #false
summator == summator # true
summator == summator2 # false
summator == (summator2 + 1) #false
summator == summator2 # true

class Summator
  attr_reader :sum
  
  def initialize(start_value = 0)
    @sum = start_value
  end
  
  def +(value)
    @sum += value if value.is_a? Numeric
    
    self
  end
  
  def == (summator)
    (summator.is_a? self.class) && summator.sum == sum
  end
end

summator = Summator.new
summator + 1 # summator

summator2 = Summator.new

summator == summator2 # false
summator == (summator2 + 1) #false

ruby_classes_4.rb

class Summator
  attr_reader :sum
  
  def initialize(start_value = 0)
    @sum = start_value
  end
  
  def +(value)
    @sum += value if value.is_a? Numeric
    
    self
  end
  
  def == (summator)
    (summator.is_a? self.class) && summator.sum == sum
  end
end

1 + Summator.new # TypeError: Summator can't be coerced into Fixnum

class Summator
  def coerce(other)
    [sum + other, other]
  end
end

class SummatorAgr < Summator
  def coerce(other)
    [self + other, other]
  end
end

1 + Summator.new # 2
(1 + SummatorAgr.new).sum # 2

ruby_classes_5.rb

class A
  attr_accessor :val1, :val2
  
  def print_self
    p self
  end
  
  def inspect
    "val1: #{@val1} val2: #{self.val2}"
  end
end

a = A.new
a.print_self # val1:  val2:
a.val1 = 1
a.val2 = 2 
a.print_self # val1: 1 val2: 2
a.inspect # "val1: 1 val2: 2"

ruby_classes_6.rb

class A
  attr_reader :val1
  
  @@has_at_least_one_instace = false
  
  def initialize(val1)
    @@has_at_least_one_instace = true
    @val1 = val1
  end
  
  def self.has_at_least_one_instace?
    @@has_at_least_one_instace
  end
  
  def self.create_with_parameters
    self.new 99
  end
end

A.has_at_least_one_instace? #false
A.create_with_parameters.val1 # 99
A.has_at_least_one_instace? #true

ruby_classes_7.rb

class A
  MY_CONSTANT = 3
  
  def self.constant
    MY_CONSTANT
  end
  
  def self.update_constant(val)
    # self::MY_CONSTANT = val # SyntaxError:  dynamic constant assignment
  end
end

A::MY_CONSTANT
A.constant