Ruby : Ruby dot “.” and double Colon “::” Operators

Ruby dot “.” and double Colon “::” Operators:

You call a module method by preceding its name with the module’s name and a period, and you reference a constant using the module name and two colons.

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.

Remember: in Ruby, classes and methods may be considered constants too.

You need just to prefix the :: Const_name with an expression that returns the appropriate class or module object.

If no prefix expression is used, the main Object class is used by default.

Here are two examples:

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

Moreover,

module B
 def self.print
 puts 'its outer B print'
 end
end
 
module A
 def self.get_print_of_inner_B
 B.print
 end
 
 def self.get_print_of_outer_B
 ::B.print
 end
end
 
 
module A::B
 def self.print
 puts 'its inner B of A print'
 end
end

When you paste the code above in IRB, you get

2.2.1 :01 > A.get_print_of_inner_B
its inner B of A  print
 => nil 
2.2.1 :02 > A.get_print_of_outer_B
its outer B print
 => nil 

Source:

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

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