How to count existing instances of a class in ruby?

Question

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

Answer 1

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