What does :: mean in Ruby syntax? [duplicate]

Question

What is ::?

@song ||= ::TwelveDaysSong.new

Answer 1

This is a method of lazily initializing the @song instance variable.

If @song is already set (to some truthy value, that is, not nil or false), then the expression just evaluates to that value.

If, however, @song is not already set to such a value, then it creates a new instance of the class TwelveDaysSong and assigns it to @song. Then, as before, the expression evaluates to the value of @song, but that value is now a reference to the newly-created TwelveDaysSong object.

The use of :: on the class name means that it is an absolute, top-level class; it will use the top-level class even if there is also a TwelveDaysSong class defined in whatever the current module is.

Answer 2

Ruby :: (double semi colons)

Top level constants are referenced by double colons

class TwelveDaysSong
end

a = TwelveDaysSong.new
#I could wrote it like this too
a = ::TwelveDaysSong.new 

module Twelve
  class TwelveDaysSongs
  end
end

b = Twelve::TwelveDaysSong.new
#b is not equal to 
a = ::TwelveDaysSong.new
#neither
a = TwelveDaysSong.new

Classes are constant too so if you have a constant

HELLOWOLRD = 'hw'

you could call it like this ::HELLOWORLD