Method: ObjectSpace#define_finalizer

Defined in:

gc.c

#define_finalizer(obj, aProc = proc()) ⇒ Object (private)

Adds aProc as a finalizer, to be called after obj was destroyed. The object ID of the obj will be passed as an argument to aProc. If aProc is a lambda or method, make sure it can be called with a single argument.

The return value is an array [0, aProc].

The two recommended patterns are to either create the finaliser proc in a non-instance method where it can safely capture the needed state, or to use a custom callable object that stores the needed state explicitly as instance variables.

class Foo
  def initialize(data_needed_for_finalization)
    ObjectSpace.define_finalizer(self, self.class.create_finalizer(data_needed_for_finalization))
  end

  def self.create_finalizer(data_needed_for_finalization)
    proc {
      puts "finalizing #{data_needed_for_finalization}"
    }
  end
end

class Bar
 class Remover
    def initialize(data_needed_for_finalization)
      @data_needed_for_finalization = data_needed_for_finalization
    end

    def call(id)
      puts "finalizing #{@data_needed_for_finalization}"
    end
  end

  def initialize(data_needed_for_finalization)
    ObjectSpace.define_finalizer(self, Remover.new(data_needed_for_finalization))
  end
end

Note that if your finalizer references the object to be finalized it will never be run on GC, although it will still be run at exit. You will get a warning if you capture the object to be finalized as the receiver of the finalizer.

class CapturesSelf
  def initialize(name)
    ObjectSpace.define_finalizer(self, proc {
      # this finalizer will only be run on exit
      puts "finalizing #{name}"
    })
  end
end

Also note that finalization can be unpredictable and is never guaranteed to be run except on exit.

# File 'gc.c', line 3540

static VALUE
define_final(int argc, VALUE *argv, VALUE os)
{
    VALUE obj, block;

    rb_scan_args(argc, argv, "11", &obj, &block);
    should_be_finalizable(obj);
    if (argc == 1) {
  block = rb_block_proc();
    }
    else {
  should_be_callable(block);
    }

    if (rb_callable_receiver(block) == obj) {
        rb_warn("finalizer references object to be finalized");
    }

    return define_final0(obj, block);
}