ruby - 为什么 fixnum 没有特征类？

Question

该方法返回任意对象的特征类:

class Object
  def eigenclass
    class << self; self; end
  end
end

字符串示例:

"abc".eigenclass # => #<Class:#<String:0x331df0>>

数组:

[1, 2].eigenclass # => #<Class:#<Array:0x0000000065d218>>

但是对于 Fixnum:

1.eigenclass # => TypeError: can't define singleton

为什么？

Answer 1

作为Ruby Docs说:

There is effectively only one Fixnum object instance for any given integer value, so, for example, you cannot add a singleton method to a Fixnum.

Bignum、Float 和 Symbol 也是如此