Method size_approx

source/util/concurrentqueue.h:1737–1742

		inline ProducerBase* next_prod() const { return static_cast<ProducerBase*>(next); }
		
		inline size_t size_approx() const
		{
			auto tail = tailIndex.load(std::memory_order_relaxed);
			auto head = headIndex.load(std::memory_order_relaxed);
			return details::circular_less_than(head, tail) ? static_cast<size_t>(tail - head) : 0;
		}
		
		inline index_t getTail() const { return tailIndex.load(std::memory_order_relaxed); }
	protected: