Share one variable amongst several objects - ruby

Question

class Collie
def speak
puts dog_generic
end
end
class Greyhound
def speak
puts dog_generic
end
end
class Labrador
def speak
puts dog_generic
end
end
dog_generic = "Woof"
chep = Collie.new
wrex = Collie.new
speedy = Greyhound.new
faithful = Labrador.new
chep.speak #=> Woof
wrex.speak #=> Woof
speedy.speak #=> Woof
faithful.speak #=> Woof
I'd like those last three methods to all return "Woof". However, this code will call an undefined variable dog_generic error. This seems to be because even global variables aren't available to objects. If I were to change all instances of dog_generic to ##dog_generic, it would work, but ## variables are rarely used, and based on that alone, I can't help thinking I'd be doing it wrong.
How can I share one variable amongst several objects?
And no, I don't want to pass in a string of "Woof" to every single object as a parameter.

Answer 1

Typically, one would use inheritance to provide this sort of behavior:
class Dog
def speak
puts "Woof"
end
end
class Collie < Dog
# whatever behavior that is specific to Collie here
end
chep = Collie.new
chep.speak #=> Woof

Answer 2

You seem to have some confusion about what a global variable in ruby is. You have to explicitly make them global, with the $ sigil, a la
$dog_generic = 'Woof'
and
def speak
puts $dog_generic
end
That said, using a global is probably your worst approach here. Defining either a generic dog class and having your specific types inherit, or else creating a dog mixin and including it would both be better solutions IMHO.

Answer 3

Make a constant. Any variable starting with a capital letter is a constant. Including classes, so constants can be scoped globally. class Foo; end is a constant that points to a class, you could also write this as Foo = Class.new.
class Collie
def speak
puts DOG_GENERIC
end
end
class Greyhound
def speak
puts DOG_GENERIC
end
end
class Labrador
def speak
puts DOG_GENERIC
end
end
DOG_GENERIC = "Woof"
chep = Collie.new
wrex = Collie.new
speedy = Greyhound.new
faithful = Labrador.new
chep.speak #=> Woof
wrex.speak #=> Woof
speedy.speak #=> Woof
faithful.speak #=> Woof
I agree with #perimosocordiae, you should probably use inheritance. I would disagree with him though on use of a class for inheritance. Modules in Ruby are included in the inheritance chain, however, they do not have constructors (you can think of them as abstract classes, but you can inherit multiple modules).
module Speach
WOOF = "Woof"
QUACK = "Quack"
module Dog
def speak
puts WOOF
end
end
module Duck
def speak
puts QUACK
end
end
end
class Collie
include Speach::Dog
end
class Greyhound
include Speach::Dog
# can overwrite ancestor
def speak
puts "Ruff"
end
end
class Mallard
include Speach::Duck
end
Collie.new.speak # => "Woof"
Greyhound.new.speak # => "Ruff"
Mallard.new.speak # => "Quack"

Answer 4

While others answered your question about subclassing Dog, I would like to introduce you to a better way of naming your instances: NameMagic. Type gem install y_support in your command line, and require NameMagic it by:
require 'y_support/name_magic'
And then:
class Dog
include NameMagic
def speak
puts "Woof"
end
end
Collie = Class.new Dog
Greyhound = Class.new Dog
Labrador = Class.new Dog
Chep = Collie.new
Wrex = Collie.new
Speedy = Greyhound.new
Faithful = Labrador.new
And then:
Dog.instances.each &:speak
#=> Woof
#=> Woof
#=> Woof
#=> Woof
NameMagic also allows you to query instances and their names:
Dog.instances
#=> [#<Collie:0xb90e3c38>, #<Collie:0xb90e1adc>, #<Greyhound:0xb90e7860>, #<Labrador:0xb90e5574>]
Collie.instances
#=> [#<Collie:0xb90e3c38>, #<Collie:0xb90e1adc>]
Labrador.instances
#=> [#<Labrador:0xb90e5574>]
Dog.instance_names
#=> [:Chep, :Wrex, :Speedy, :Faithful]
Collie.instance_names
#=> [:Chep, :Wrex]