Ruby类方法与本征类中的方法 [英] Ruby Class Methods vs. Methods in Eigenclasses

问题描述

类的方法和方法在该类的本征类（或元类）只有两种方式来定义一个东西？

Are class methods and methods in the eigenclass (or metaclass) of that class just two ways to define one thing?

否则有什么区别？

class X
  # class method
  def self.a
    "a"
  end

  # eigenclass method
  class << self
    def b
      "b"
    end
  end
end

$ b b

Do Xa 和 Xb 以任何方式表现不同？

Do X.a and X.b behave differently in any way?

我知道我可以通过打开特征类来覆盖或别名类方法：

I recognize that I can overwrite or alias class methods by opening the eigenclass:

irb(main):031:0> class X; def self.a; "a"; end; end
=> nil
irb(main):032:0> class X; class << self; alias_method :b, :a; end; end
=> #<Class:X>
irb(main):033:0> X.a
=> "a"
irb(main):034:0> X.b
=> "a"
irb(main):035:0> class X; class << self; def a; "c"; end; end; end
=> nil
irb(main):036:0> X.a
=> "c"

推荐答案

'eigenclass'版本有助于使用attr_ *方法，例如：

The two methods are equivalent. The 'eigenclass' version is helpful for using the attr_* methods, for example:

class Foo
  @instances = []
  class << self;
    attr_reader :instances
  end
  def initialize
    self.class.instances << self
  end
end

2.times{ Foo.new }
p Foo.instances
#=> [#<Foo:0x2a3f020>, #<Foo:0x2a1a5c0>]

define_singleton_method 可在类上创建方法：

Foo.define_singleton_method :bim do "bam!" end

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