class Example

  # ---------------------------------------------------------------------------
  # Example 1

  # @ is an alias for self.
  # Self in the contest of the class declaration refers to the class itself.
  # Indeed, we use def "self.my_method" to define class methods.
  # This implies that @cls_var in the context of a class declaration (like follows)
  # is equivalent to @@cls_var inside an (instance) method definition.
  @cls_var = "I am class variable"

  # Since cls_var is a variable and not a constant, accessors method are always required.
  # And must be explicitly defined
  def self.cls_var
    # NOTE: self here always refer to the class Example and to an instance.
    @cls_var
  end


  # ---------------------------------------------------------------------------
  # Example 2

  # NOTE: the method name may be the same as of the variable.
  # implementation via explicit getter and setter.
  @cls_var_cname = 123
  def self.cls_var_cname; @cls_var_cname; end
  def self.cls_var_cname=(val); @cls_var_cname = val; end


  # ---------------------------------------------------------------------------
  # Example 3

  # This variable exists only while the class is built.
  cls_var2 = "I do exists only while the class is built"

  # Let's verify it, trying to access it.
  def self.cls_var2_1; defined?  cls_var2; end
  def self.cls_var2_2; defined?  @cls_var2; end
  def self.cls_var2_3; defined?  self.cls_var2; end
  def self.cls_var2_4; defined?  Example.cls_var2; end
  def cls_var2_5; defined?  cls_var2; end
  def cls_var2_6; defined?  @cls_var2; end
  def cls_var2_7; defined?  self.cls_var2; end
  def cls_var2_8; defined?  Example.cls_var2; end


  # ---------------------------------------------------------------------------
  # Example 4

  # Constants may be defined as class attributes and does not require
  # the use of "self" or getters.
  CLS_CONST_1 = 789


  # ---------------------------------------------------------------------------
  # Example 5

  # Instance attributes and Instance constants must be defined inside a method
  # like "initialize". Non constant attributed requires getters and setters,
  # where constant does not.
  def initialize
    # Constant
    @INST_CONST_1 = 12
    # Externally readable and writable var
    @var_1 = 13
    # Externally read only
    @read_only_var = 14
    # Externally write only
    @write_only_var = 15

    # Protected read only
    @prot_var = 42
  end

  attr_accessor :var_1
  attr_reader :read_only_var
  attr_writer :write_only_var

  # Visibility of getter and setter can be modified too.
  attr_reader :prot_var
  protected :prot_var
end


# `<main>': undefined method `cls_var' for Example:Class (NoMethodError)
# puts Example.cls_var

puts "Example 1: #{Example.cls_var}"

puts "Example 2: #{Example.cls_var_cname}"
Example.cls_var_cname = 345
puts "Example 2: #{Example.cls_var_cname}"

ex = Example.new
puts "Example 3: Is cls_var2 defined in any of the getter we tested? " + [
  Example.cls_var2_1,
  Example.cls_var2_2,
  Example.cls_var2_3,
  Example.cls_var2_4,
  ex.cls_var2_5,
  ex.cls_var2_6,
  ex.cls_var2_7,
  ex.cls_var2_8,
].any?.to_s