如何计算ruby中类的现有实例？

这是 this question的一个想法：在创建对象时,增加一个类变量.当对象被收集时,减少它.如您所见,调用终结器,并且@@ no_foo减少.但是当我稍后查询它时,减量就消失了.似乎价值只会上升,永不下降(如果我创建两个对象,它将显示2).我错过了一些明显的东西吗

class Foo
  @@no_foo = 0

  def initialize
    puts 'creating object'
    @@no_foo += 1
    ObjectSpace.define_finalizer(self,proc { self.delete })
  end

  def delete
    puts 'deleting object'
    @@no_foo # => 1
    @@no_foo -= 1
    @@no_foo # => 0
  end

  def self.no_foo
    @@no_foo # => 0,1
  end
end

Foo.no_foo # => 0
f = Foo.new
f = nil

GC.start
Foo.no_foo # => 1

# >> creating object
# >> deleting object

class Foo
  @@no_foo = 0

  def initialize
    @@no_foo += 1
    ObjectSpace.define_finalizer(self,Foo.method(:delete))
  end

  def self.delete id # also this argument seems to be necessary
    @@no_foo -= 1
  end

  def self.no_foo
    @@no_foo
  end
end

Foo.no_foo # => 0
1000.times{Foo.new}
Foo.no_foo # => 1000

GC.start
Foo.no_foo # => 0