Ruby 的双冒号(::)操作符使用差异

Question

有什么区别吗

module Foo
  class Engine < Rails::Engine
  end
end

和

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

Answer 1

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。

用文件系统打个比方:

::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.

这里是可能的错误的说明:

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