then - ruby變量

Ruby的雙冒號(::)運算符使用差異 (2)

兩者之間有什麼區別嗎？

module Foo
  class Engine < Rails::Engine
  end
end

和

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

Ruby中的常量嵌套在文件系統中的文件和目錄中。因此，常量由它們的路徑唯一標識。

與文件系統類比：

::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.

以下是可能的錯誤說明：

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

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

這不是真的！

我們舉個例子：

module M
  Y = 1
  class M
    Y = 2
    class M
      Y = 3
    end
    class C
      Y = 4
      puts M::Y
    end
  end
end

# => 3

module M
  Y = 1
  class M
    Y = 2
    class C
      Y = 4
      puts M::Y
    end
  end
end

# => 2

module M
  Y = 1
  class M
    Y = 2
    class M
      Y = 4
      puts M::Y
    end
  end
end

# => 4

因此，當你說M :: Y ruby查找最接近的定義時，無論它是在當前範圍內還是在外部範圍內或外部外部範圍內等。