劑量self.class.class_eval等於instance_eval嗎？

Question

我想找出instance_eval和class_eval之間的區別，所以我做了一些工作。在互聯網上搜索后，我發現了一些問題，但是還有另一個我無法理解的問題！當我這樣做時：

class FindMe

  def method_missing method, *args, &block
    method = method.to_s
    self.class.class_eval <<-EOF, __FILE__, __LINE__
      def #{method}
        puts "I'm an instance method! I'm #{method}"
      end
    EOF
    send method, *args
  end

  def self.method_missing method, *args, &block
    method = method.to_s
    instance_eval <<-HERE, __FILE__, __LINE__
      def #{method}
        puts "I'm a class method! I'm #{method}"
      end
    HERE
    send method, *args
  end

end

FindMe.new.hello
FindMe.hello

我懂了

I'm an instance method! I'm hello
I'm a class method! I'm hello

當我將代碼更改為：

class FindMe

  def method_missing method, *args, &block
    method = method.to_s
    self.class.class_eval <<-EOF, __FILE__, __LINE__
      def #{method}
        puts "I'm an instance method! I'm #{method}"
      end
    EOF
    send method, *args
  end

  def self.method_missing method, *args, &block
    method = method.to_s
    self.class.class_eval <<-HERE, __FILE__, __LINE__
      def #{method}
        puts "I'm a class method! I'm #{method}"
      end
    HERE
    send method, *args
  end

end

FindMe.new.hello
FindMe.hello

我得到相同的輸出，任何人都可以告訴發生了什么嗎？

Answer 1

通過class_eval可以修改類，而instance_eval則只能修改當前實例。 看：

▶ class A
▷   def ceval
▷     self.class.class_eval "def on_class ; puts 'On Class' ; end"
▷   end
▷   def ieval
▷     self.instance_eval "def on_instance ; puts 'On Instance' ; end"
▷   end
▷ end
▶ a1 = A.new
#⇒ #<A:0xcf6a87c>
▶ a1.ceval
#⇒ :on_class
▶ a1.ieval
#⇒ :on_instance
▶ a1.on_class
#⇒ On Class
▶ a1.on_instance
#⇒ On Instance

▶ a2 = A.new
#⇒ #<A:0xd0e9f7c>
▶ a2.on_class
#⇒ On Class  !!! IT IS DEFINED ON NEWLY CREATED INSTANCE OF A
▶ a2.on_instance
#⇒ NoMethodError: undefined method `on_instance' for #<A:0xd0e9f7c>

后者失敗是因為我們在實例a1上聲明了on_instance方法，而a2對此一無所知。

Answer 2

請參見以下示例，並嘗試向自己解釋：

class Foo
  def a
    self.class.class_eval <<-EOS
      def x
        'This is an instance method.'
      end
    EOS
  end

  def b
    self.instance_eval <<-EOS
      def y
        'This is a singleton method.'
      end
    EOS
  end
end

foo1 = Foo.new
foo2 = Foo.new

foo1.a
foo1.b

foo1.x  #=> This is an instance method.
foo1.y  #=> This is a singleton method.

foo2.x  #=> This is an instance method.
foo2.y  #=> NoMethodError