Eigenclass constants.rb

class Foo
  CONST_ON_FOO = "the constant on Foo"

  class << self
    puts "When we're defining CONST_ON_FOOS_CLASS, we're actually in: #{self}, a #{self.class}"
  	CONST_ON_FOOS_CLASS = "the constant on Foo's class"
  end
end

instance = Foo.new

puts "Foo's class: #{Foo}"
puts "The instance of Foo: #{instance}"

puts "Getting Foo's const directly from the Foo class: #{Foo::CONST_ON_FOO}"
begin
  puts "Getting Foo's const via the instance (does not work): #{instance::CONST_ON_FOO}"
rescue StandardError => e
  puts "Can't reference CONST_ON_FOOS_CLASS via the instance"
  puts e
end
puts "Getting Foo's const via the instance's class: #{instance.class::CONST_ON_FOO}"

begin
	puts "Getting Foo's eigenclass's const via Foo (doesn't work): #{Foo::CONST_ON_FOOS_CLASS}"
rescue StandardError => e
  puts "Can't reference Foo::CONST_ON_FOOS_CLASS"
  puts e
end

begin
	puts "Getting Foo's eigenclass's const via Foo.class (doesn't work): #{Foo.class::CONST_ON_FOOS_CLASS}"
rescue StandardError => e
  puts "Can't reference Foo.class::CONST_ON_FOOS_CLASS"
  puts e
end

puts "Getting Foo's eigenclass's const via Foo.singleton_class class: #{Foo.singleton_class::CONST_ON_FOOS_CLASS}"

# Output:
# When we're defining CONST_ON_FOOS_CLASS, we're actually in: #<Class:Foo>, a Class
# Foo's class: Foo
# The instance of Foo: #<Foo:0x00007fcfaf0ebcb8>
# Getting Foo's const directly from the Foo class: the constant on Foo
# Can't reference CONST_ON_FOOS_CLASS via the instance
# #<Foo:0x00007fcfaf0ebcb8> is not a class/module
# Getting Foo's const via the instance's class: the constant on Foo
# Can't reference Foo::CONST_ON_FOOS_CLASS
# uninitialized constant Foo::CONST_ON_FOOS_CLASS
# Can't reference Foo.class::CONST_ON_FOOS_CLASS
# uninitialized constant Class::CONST_ON_FOOS_CLASS
# Getting Foo's eigenclass's const via Foo.singleton_class class: the constant on Foo's class