// it's decided to add functionality to close it in the version later than v2.
	if res.ttl != noEvictionTTL {
		go func(done <-chan struct{}) {
			ticker := time.NewTicker(res.ttl / numBuckets)
			defer ticker.Stop()
			for {
				select {
				case <-done:
					return
				case <-ticker.C:
					res.deleteExpired()
				}
			}
		}(res.done)
	}
	return &res
}