What is Ruby's double-colon `::`?

Question

What is this double-colon ::? E.g. Foo::Bar.

I found a definition:

The :: is a unary operator that allows: constants, instance methods and class methods defined within a class or module, to be accessed from anywhere outside the class or module.

What good is scope (private, protected) if you can just use :: to expose anything?

Answer 1

Ruby on rails uses :: for namespace resolution.

class User < ActiveRecord::Base

  VIDES_COUNT = 10
  Languages = { "English" => "en", "Spanish" => "es", "Mandarin Chinese" => "cn"}

end

To use it :

User::VIDEOS_COUNT
User::Languages
User::Languages.values_at("Spanish") => "en"

Also, other usage is : When using nested routes

OmniauthCallbacksController is defined under users.

and route is like :

devise_for :users, controllers: {omniauth_callbacks: "users/omniauth_callbacks"}


class Users::OmniauthCallbacksController < Devise::OmniauthCallbacksController

end

Answer 2

No, it is not to access every method, it is a "resolution" operator, that is, you use it to resolve the scope (or location you can say) of a constant/static symbol.

For example in the first of your line, Rails use it to find the Base class inside the ActiveRecord.Module, in your second one it is used to locate the class method (static) of the Routes class, etc, etc.

It is not used to expose anything, its used to "locate" stuff around your scopes.

http://en.wikipedia.org/wiki/Scope_resolution_operator

Answer 3

:: is basically a namespace resolution operator. It allows you to access items in modules, or class-level items in classes. For example, say you had this setup:

module SomeModule
    module InnerModule
        class MyClass
            CONSTANT = 4
        end
    end
end

You could access CONSTANT from outside the module as SomeModule::InnerModule::MyClass::CONSTANT.

It doesn't affect instance methods defined on a class, since you access those with a different syntax (the dot .).

Relevant note: If you want to go back to the top-level namespace, do this: ::SomeModule – Benjamin Oakes

Answer 4

Adding to previous answers, it is valid Ruby to use :: to access instance methods. All the following are valid:

MyClass::new::instance_method
MyClass::new.instance_method
MyClass.new::instance_method
MyClass.new.instance_method

_{As per best practices I believe only the last one is recommended.}

Answer 5

This simple example illustrates it:

MR_COUNT = 0        # constant defined on main Object class
module Foo
  MR_COUNT = 0
  ::MR_COUNT = 1    # set global count to 1
  MR_COUNT = 2      # set local count to 2
end

puts MR_COUNT       # this is the global constant
puts Foo::MR_COUNT  # this is the local "Foo" constant

Taken from http://www.tutorialspoint.com/ruby/ruby_operators.htm

Answer 6

module Amimal
      module Herbivorous
            EATER="plants" 
      end
end

Amimal::Herbivorous::EATER => "plants"

:: Is used to create a scope . In order to access Constant EATER from 2 modules we need to scope the modules to reach up to the constant