Method declared out side the class is included automatically?

Can someone help me make sense of the following? I have the following code in test.rb:

class Dog
end
// bark is declared outside of Dog class
def bark
  puts 'W开发者_高级运维oof!'
end

then in irb:

>> source 'test.rb'
>> a = Dog.new
=> #<Dog:0x117f614>
>> a.bark
Woof!
=> nil

Why does method bark exist in Dog instance even though it is declared outside of the class? Because it's in the same file? Thanks!

When you create a method in the "global" scope (outside of any class), that method is made a private method of Object:

#!/usr/bin/ruby1.8

class Dog
end

p Object.respond_to?(:bark, true)     # => false

def bark
  puts "Woof!"
end

p Object.respond_to?(:bark, true)     # => true

Object is in the ancestry chain of all objects, including Dog:

dog = Dog.new
p dog.class.name               # => "Dog"
p dog.class.superclass.name    # => "Object"

Therefore dogs (and indeed all objects) now know how to bark. However, the method being private, you'll have to use instance_eval to call it with an explicit receiver:

dog.instance_eval { bark  }    # => "Woof!"

Or you can call it with an implicit receiver with no gymnastics needed:

bar    # => "Woof!"

Your exact example doesn't work in Ruby 1.9. (Apart from the bad comment syntax.)

However, declaring a method in the top level scope will make it a private method on Object, apparently:

>> Object.private_methods.include? :bark
=> true

Perhaps in your Ruby (1.8?), this is a public method?