How do all those variable-types work in different scopes?

foo.rb

class Foo
  def self.bar
    @bar ||= :self_bar
  end

  def self.bar=(b)
    @bar = b
  end

  def bar
    @bar ||= :instance_bar
  end

  def bar=(b)
    @bar = b
  end

  def self.baz
    @@baz ||= :self_baz
  end

  def self.baz=(b)
    @@baz = b
  end

  def baz
    @@baz ||= :instance_baz
  end

  def baz=(b)
    @@baz = b
  end
end

class Bing < Foo
end

zzzz.md

Class Instance Variables

Foo.bar
# => :self_bar

Bing.bar
# => :self_bar

Foo.bar = 'non-default value'
Foo.bar
# => 'non-default value'

Bing.bar
# => :self_bar

Instance Variables

f1 = Foo.new
f2 = Foo.new
b  = Bing.new

f1.bar
# => :instance_bar

f2.bar
# => :instance_bar

b.bar
# => :instance_bar

f1.bar = 'non-default value'
f1.bar
# => 'non-default value'

f2.bar
# => :instance_bar

b.bar
# => :instance_bar

Class Class Variables (?!?)

Foo.baz
# => :self_baz

Bing.baz
# => :self_baz

Foo.baz = 'non-default value'
Foo.baz
# => 'non-default value'

Bing.baz
# => 'non-default value'

Class Variables

f1 = Foo.new
f2 = Foo.new
b  = Bing.new

f1.baz
# => :instance_baz

f2.baz
# => :instance_baz

b.baz
# => :instance_baz

f1.baz = 'non-default value'
f1.baz
# => 'non-default value'

f2.baz
# => 'non-default value'

b.baz
# => 'non-default value'

I'd add a couple of variations (although it quickly becomes a combinatorial issue, and maybe to a limited benefit):

module Broda
def broda
# @Broda, @@Broda
end
end

and then:
class Foo
include Broda
extend Brode
end

class Bing
include Brodi
extend Brodo
end

f1 = Foo.new
f1.extend Brodu
etc.

There's also this variation

Foo.class_eval
def broda
end
end

(as well as instance_eval, which, iirc, has slightly different semantics on a class than class_eval)

And
f1.instance_eval do
def self.broda; end
end

Really, the details of inheritance and access in Ruby are worth exploring. I sometimes think a template or a gem or something would be helpful - just not sure how.