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

以下解决方案有效。

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

为什么Developer.lab报告它是Human？还有什么可以让proc在调用Developer.lab时报告Developer。

Answer 1

闭包是在定义它的上下文中捕获 self - 就像闭包应该做的那样。因此，当它被调用时，它将使用对它捕获的上下文的引用。闭包不是定义预期功能的理想工具。而不是proc.call，“define_method”调用的内容应该是“puts”，proc表示我的类是'+ name.to_s“

Answer 2

它很微妙，但归结为简单地调用块（在这种情况下，它充当正常闭包，self对应于它的定义位置，即在Human），或使用它（直接）作为方法定义的块或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

我必须考虑一下为什么这是有效的，但目前它的工作：

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

Instance_eval：为什么子类是超类

3 个答案: