Instance_eval：為什么子類是超類

Question

    def singleton_class
       class << self
         self
       end
    end

    class Human
      proc = lambda { puts 'proc says my class is ' + self.name.to_s }

      singleton_class.instance_eval do
        define_method(:lab)  do 
          proc.call
        end
      end
    end

    class Developer < Human
    end

    Human.lab # class is Human
    Developer.lab # class is Human ; oops

以下解決方案有效

def singleton_class
  class << self
    self
  end
end

class Human
  proc =  lambda { puts 'proc says my class is ' + self.name.to_s }
  singleton_class.instance_eval do
    define_method(:lab) do
      self.instance_eval &proc
    end
  end
end

class Developer < Human
end

Human.lab # class is Human
Developer.lab # class is Human ; oops

為什么Developer.lab報告它是Human？ 還有什么可以讓proc在調用Developer.lab時報告Developer。

Answer 1

閉包是在定義它的上下文中捕獲self - 就像閉包應該做的那樣。 因此，當它被調用時，它將使用對它捕獲的上下文的引用。 閉包不是定義預期功能的理想工具。 而不是proc.call，“define_method”調用的內容應該是“puts”，proc表示我的類是'+ name.to_s'

Answer 2

它是微妙的，但它歸結為簡單地調用塊（在這種情況下，它充當正常閉包， self對應於它定義的位置，即在Human ），或者使用它（直接）作為方法定義的塊或instance_eval ：

def singleton_class
   class << self
     self
   end
end


class Human

  PROC = proc { puts 'proc says my class is ' + self.name.to_s }

  singleton_class.instance_eval do
    define_method(:lab)  do 
      PROC.call
    end
    define_method(:lab2, &PROC.method(:call))

    define_method(:lab3)  do 
      instance_eval(&PROC)
    end
    define_method(:lab4, &PROC) 
  end
end

class Developer < Human
end

Human::PROC.call  # => "class is Human"  (original closure)
Developer.lab     # Same as previous line, so "class is Human"  (original closure)
Developer.lab2    # ditto

Developer.instance_eval(&Human::PROC)  # => "class is Developer"  (instance_eval changes sets a different scope)
Developer.lab3    # Same as previous line, so "class is Developer"
Developer.lab4    # ditto

Answer 3

我必須仔細考慮一下它為什么會起作用，但目前它確實有效：

class Human
  proc = -> { name }

  define_singleton_method(:lab, &proc)
end

class Developer < Human; end

require 'test/unit'
class TestClosures < Test::Unit::TestCase
  def test_that_the_human_class_is_named_human
    assert_equal 'Human', Human.lab
  end
  def test_that_the_developer_class_is_named_developer
    assert_equal 'Developer', Developer.lab
  end
end