Ruby 的双冒号 (::) 运算符用法差异 [英] Ruby's double colon (::) operator usage differences

问题描述

有什么区别吗

module Foo
  class Engine < Rails::Engine
  end
end

和

module Foo
  class Engine < ::Rails::Engine
  end
end

推荐答案

Ruby 中的常量就像文件系统中的文件和目录一样嵌套.因此，常量由它们的路径唯一标识.

Constants in Ruby are nested like files and directories in filesystem. So, constants are uniquely identified by their paths.

以文件系统做类比:

::Rails::Engine #is an absolute path to the constant.
# like /Rails/Engine in FS.

Rails::Engine #is a path relative to the current tree level.
# like ./Rails/Engine in FS.

以下是可能错误的说明:

Here is the illustration of possible error:

module Foo

  # We may not know about this in real big apps
  module Rails
    class Engine 
    end
  end

  class Engine1 < Rails::Engine
  end

  class Engine2 < ::Rails::Engine
  end
end

Foo::Engine1.superclass
 => Foo::Rails::Engine # not what we want

Foo::Engine2.superclass
 => Rails::Engine # correct

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