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

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Ruby on rails uses    for namespace resolution   class User  lt  ActiveRecord  Base    VIDEOS COUNT   10   Languages      English    gt   en    Spanish    gt   es    Mandarin Chinese    gt   cn    end   To use it       User  VIDEOS COUNT User  Languages User  Languages values at  Spanish     gt   en    Also  other usage is   When using nested routes  OmniauthCallbacksController is defined under users   And routed as    devise for  users  controllers   omniauth callbacks   users omniauth callbacks     class Users  OmniauthCallbacksController  lt  Devise  OmniauthCallbacksController  end

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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

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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

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Surprisingly  all 10 answers here say the same thing  The      is a namespace resolution operator  and yes it is true  But there is one gotcha that you have to realize about the namespace resolution operator when it comes to the constant lookup algorithm  As Matz delineates in his book   The Ruby Programming Language   constant lookup has multiple steps  First  it searches a constant in the lexical scope where the constant is referenced  If it does not find the constant within the lexical scope  it then searches the inheritance hierarchy  Because of this constant lookup algorithm  below we get the expected results   module A   module B       PI   3 14       module C         class E           PI   3 15         end         class F  lt  E           def get pi             puts PI           end         end       end   end end f   A  B  C  F new f get pi  gt  3 14   While F inherits from E  the B module is within the lexical scope of F  Consequently  F instances will refer to the constant PI defined in the module B  Now if module B did not define PI  then F instances will refer to the PI constant defined in the superclass E   But what if we were to use      rather than nesting modules  Would we get the same result  No   By using the namespace resolution operator when defining nested modules  the nested modules and classes are no longer within the lexical scope of their outer modules  As you can see below  PI defined in A  B is not in the lexical scope of A  B  C  D and thus we get uninitialized constant when trying to refer to PI in the get pi instance method   module A end  module A  B   PI   3 14 end  module A  B  C   class D     def get pi       puts PI     end   end end d   A  B  C  D new d get pi NameError  uninitialized constant A  B  C  D  PI Did you mean   A  B  PI

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module Amimal       module Herbivorous             EATER  plants         end end  Amimal  Herbivorous  EATER   gt   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

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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

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Lets you access a constant  module  or class defined inside another class or module  It is used to provide namespaces so that method and class names don t conflict with other classes by different authors   When you see ActiveRecord  Base in Rails it means that Rails has something like  module ActiveRecord   class Base   end end   i e  a class called Base inside a module ActiveRecord which is then referenced as ActiveRecord  Base  you can find this in the Rails source in activerecord-n n n lib active record base rb   A common use of    is to access constants defined in modules e g   module Math   PI   3 141       end  puts Math  PI   The    operator does not allow you to bypass visibility of methods marked private or protected

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It is all about preventing definitions from clashing with other code linked in to your project   It means you can keep things separate    For example you can have one method called  run  in your code and you will still be able to call your method rather than the  run  method that has been defined in some other library that you have linked in

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What good is scope  private  protected  if you can just use    to expose anything    In Ruby  everything is exposed and everything can be modified from anywhere else   If you re worried about the fact that classes can be changed from outside the  class definition   then Ruby probably isn t for you   On the other hand  if you re frustrated by Java s classes being locked down  then Ruby is probably what you re looking for

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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  1 puts Foo  MR COUNT    this is the local constant  2   Taken from http   www tutorialspoint com ruby ruby operators htm

[ruby] What is Ruby's double-colon `::`?

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