ruby - 在 Ruby 中使用类 << self

Question

我试图理解 Ruby 的“class << self”背后的底层机制。 我了解如何使用它并且我知道这是一个单例类定义，但我想真正了解它是如何工作的。

这是一个例子:

class Klass
  puts "#{self}" #=> Klass
  class << self
    puts "#{self}" #=> #<Class:Klass>
  end
end

puts Klass.class #=> Class
puts Klass.singleton_class #=> #<Class:Klass>

为什么这些语句会输出它们输出的内容？ Class和Class:Klass、class和singleton_class有什么区别？？

Answer 1

irb>
irb>
irb> class FightClub; def rule_1; 'You do not talk about FIGHT CLUB.'; end; end
=> nil
irb> club1=FightClub.new
=> #<FightClub:0x007fb4240c48d0>
irb> club2=FightClub.new
=> #<FightClub:0x007fb42394b770>
irb>
irb>
irb> def club2.rule_2; rule_1; end
=> nil
irb> club1.rule_1
=> "You do not talk about FIGHT CLUB."
irb> club1.rule_2
NoMethodError: undefined method `rule_2' for #<FightClub:0x007fb4240c48d0>
from (pry):6:in `__pry__'
irb>
irb> club2.rule_1
=> "You do not talk about FIGHT CLUB."
irb> club2.rule_2
=> "You do not talk about FIGHT CLUB."
irb>

到目前为止很简单..

irb>
irb> club1.class.__id__ # FightClub
=> 70205838357120
irb> club2.class.__id__ # FightClub
=> 70205838357120
irb>
irb>
irb> club2.singleton_class # eigenclass of this instance
=> #<Class:#<FightClub:0x007fb42394b770>>
irb> club1.singleton_class
=> #<Class:#<FightClub:0x007fb4240c48d0>>
irb> club1.class.__id__ == club2.class.__id__
=> true
irb> club1.singleton_class.__id__ == club2.singleton_class.__id__
=> false
irb> club1.singleton_class.superclass.__id__ == 
irb*          club2.singleton_class.superclass.__id__ # 70205838357120
=> true
irb>
irb>
irb> club1.singleton_class.instance_methods.select{|m| m.to_s.start_with?('rule')}
=> [:rule_1]
irb> club2.singleton_class.instance_methods.select{|m| m.to_s.start_with?('rule')}
=> [:rule_2, :rule_1]
irb>
irb> club1.singleton_methods
=> []
irb> club2.singleton_methods
=> [:rule_2]
irb>

如果还不清楚:

• singleton_class 是 eigenclass的一个实例。这里有个好explanation of eigenclass .
• 任何实例方法(本例中的 rule_2)都是 singleton_methods 并在 eighenclass 上创建。
• superclass 所有特征类都是相同的(并且是 = instance.class)。
• 意见:所以实际上 Class 代表 any_ruby_object 类的实例实际上是 any_ruby_object。 singleton_class 而不是 any_ruby_object.class

不相关但只是为了让它变得困惑:

irb> club2.methods.select{ |m| m.to_s.start_with?('rule') }
=> [:rule_2, :rule_1]
irb> club1.methods.select{ |m| m.to_s.start_with?('rule') }
=> [:rule_1]
irb>