Question

在 Programming Ruby 1.9＆2.0 一书的 24.2 Singletons 一章中，给出了以下代码：

animal = "cat"
def animal.speak
  puts "The #{self} says miaow"
end

并解释为：“当我们为“ cat ”对象定义单例方法时，Ruby创建了一个新的匿名类，并在该类中定义了 speak 方法。这个匿名类称为单类（有时称为本征类）。”

不幸的是，我无法验证Ruby（2.5.1）确实创建了自己的匿名类：

str = "a string"                 # => "a string"
[str, str.object_id]             # => ["a string", 47279316765840]
[str.class, str.class.object_id] # => [String, 47279301115420]

def str.greet
  "hello"
end                              # => :greet

str.greet                        # => "hello"

[str, str.object_id]             # => ["a string", 47279316765840]
[str.class, str.class.object_id] # => [String, 47279301115420]

从上面可以看出，定义单例方法 greet 后， str 的类不会更改：它仍显示为 String 具有相同的object_id 47279301115420。

那么，匿名类在哪里？

Answer 1

str = "a string"                 # => "a string"
[str, str.object_id]             # => ["a string", 47279316765840]
[str.class, str.class.object_id] # => [String, 47279301115420]

def str.greet
  "hello"
end                              # => :greet

str.greet                        # => "hello"

当您问str.class或查找祖先链（str.class.ancestors）时，

Ruby会隐藏本征类。但是，您可以使用<<

在检查其类之后，通过返回self来获取对eigenclass的引用

str_class = class << str
  self
end
# => #<Class:#<String:0x007fbba28b3f20>>

str_class.instance_methods(false) #=> [:greet] # the singleton method you defined becomes the instance method of this eigenclass. :)

str_class.ancestors
[#<Class:#<String:0x007fbba28b3f20>>, String, Comparable, Object, Kernel, BasicObject]

Ruby：单例类是否真的拥有匿名类？

1 个答案: