。 VS ::（點與雙冒號）調用方法

Question

可能重複：
What does :: mean in Ruby?

我從Poignant Guide to Ruby和一些代碼示例學習Ruby，

File::open('idea-' + idea_name + '.txt', 'w') do |f| 
    f << idea 
end 

在上面的代碼中，我對面，似乎是用於同一目的的雙冒號和點的用途來雙冒號正被用於訪問File類的open方法。不過，我後來整個代碼中的點爲同一目的傳來：

require 'wordlist' 
# Print each idea out with the words fixed 
Dir['idea-*.txt'].each do |file_name| 
    idea = File.read(file_name) 
    code_words.each do |real, code| 
    idea.gsub!(code, real) 
    end 
puts idea 
end 

這個時候，一個點被用來訪問File類的read方法。是什麼區別：

File.read() 

和

File::open() 

Answer 1

這是scope resolution operator。

維基百科的一個例子：

module Example 
    Version = 1.0 

    class << self # We are accessing the module's singleton class 
    def hello(who = "world") 
     "Hello #{who}" 
    end 
    end 
end #/Example 

Example::hello # => "Hello world" 
Example.hello "hacker" # => "Hello hacker" 

Example::Version # => 1.0 
Example.Version # NoMethodError 

# This illustrates the difference between the message (.) operator and the scope 
# operator in Ruby (::). 
# We can use both ::hello and .hello, because hello is a part of Example's scope 
# and because Example responds to the message hello. 
# 
# We can't do the same with ::Version and .Version, because Version is within the 
# scope of Example, but Example can't respond to the message Version, since there 
# is no method to respond with.