如何计算 ruby​​ 中一个类的现有实例? [英] How to count existing instances of a class in ruby?

问题描述

这是这个问题的一个想法: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，finalizer 被调用，并且 @@no_foo 被递减.但是当我稍后查询它时，减量消失了.似乎价值只会上升，永远不会下降(如果我创建两个对象，它将显示 2).我错过了什么明显的东西吗?

Here's an idea from this question: Upon object creation, increment a class variable. When object gets collected, decrement it. As you can observe, finalizer is called, and @@no_foo gets decremented. But when I query it a moment later, decrement is gone. Seems that value is going only up, never down (if I create two objects, it will show 2). Am I missing something obvious?

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

推荐答案

可以，但是finalization里面有循环引用.您的终结器取决于应收集的对象的绑定.请参阅此解决方案.

It can work, but there's circular reference in finalization. Your finalizer depends on the binding of an object that should be collected. See this solution.

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

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