在 `class << self` 中声明的变量

Question

我应该如何考虑在任何方法定义之外的class << self 块中声明的@variable？看看这个脚本的最后一部分：

class VarDemo
  @@class_var_1 = "This is @@class_var_1"
  @class_i_var = "This is a @class_i_var"

  attr_accessor :ivar_1
  attr_accessor :ivar_2

  def initialize(ivar_1: "base_ivar_1", ivar_2: "base_ivar_2")
    @@class_var_2 = "This is @@class_var_2, defined within an instance method"
    @ivar_1 = ivar_1
    @ivar_2 = ivar_2
    VarDemo.class_i_var_2 = "class_i_var_2 defined through accessor on the class object"
  end

  def print_vars
    puts "from within an instance method. here 'self' is: #{self}"

    # test instance variables
    puts "@ivar_1: #{@ivar_1}"
    puts "@ivar_2: #{@ivar_2}"
    puts "self.ivar_1: #{self.ivar_1}"
    puts "self.ivar_2: #{self.ivar_2}"
    puts "ivar_1: #{ivar_1}"
    puts "ivar_2: #{ivar_2}"

    # test class variables
    puts "@@class_var_1: #{@@class_var_1}"
    puts "@@class_var_2: #{@@class_var_2}"

    # test class instance variables (ivars on the class object)
    puts "VarDemo.class_i_var: #{VarDemo.class_i_var}"
    puts "VarDemo.class_i_var_2: #{VarDemo.class_i_var_2}"
    puts "@class_i_var: #{@class_i_var} (should be empty, as it doesn't exist in the instance"

    puts "self.class_i_var: #{self.class_i_var}"
  rescue Exception => ex
    puts "self.class_i_var is not defined in this scope (self is an instance)"
  end

  # open the class object
  class << self
    attr_accessor :class_i_var
    attr_accessor :class_i_var_2

    @what_about_this = "what would this be?"

    attr_accessor :what_about_this

    def print_what_about_this
      puts "@what_about_this: #{@what_about_this}" # is empty
      puts "VarDemo.what_about_this: #{VarDemo.what_about_this}" # is empty
    end
  end

end

@what_about_this 是什么？它在哪里定义？它只是解释器让我做的事情，但没有任何实际用途吗？我以为它是在Class 上定义的，但事实并非如此。

Answer 1

class <<self 实际上并没有打开 self 的类；它打开了 self 的 singleton 类。 （描述为here。）

class Foo
  class << self
    @bar = "baz"   # not really inside Class here.
  end
end

Foo.singleton_class.instance_eval { @bar }  #baz

Yehuda Katz 有一个很好的article，它描述了这里真正发生的事情。