Method try_dequeue

source/util/concurrentqueue.h:1125–1156

	// Never allocates. Thread-safe.
	template<typename U>
	bool try_dequeue(U& item)
	{
		// Instead of simply trying each producer in turn (which could cause needless contention on the first
		// producer), we score them heuristically.
		size_t nonEmptyCount = 0;
		ProducerBase* best = nullptr;
		size_t bestSize = 0;
		for (auto ptr = producerListTail.load(std::memory_order_acquire); nonEmptyCount < 3 && ptr != nullptr; ptr = ptr->next_prod()) {
			auto size = ptr->size_approx();
			if (size > 0) {
				if (size > bestSize) {
					bestSize = size;
					best = ptr;
				}
				++nonEmptyCount;
			}
		}
		
		// If there was at least one non-empty queue but it appears empty at the time
		// we try to dequeue from it, we need to make sure every queue's been tried
		if (nonEmptyCount > 0) {
			if ((details::likely)(best->dequeue(item))) {
				return true;
			}
			for (auto ptr = producerListTail.load(std::memory_order_acquire); ptr != nullptr; ptr = ptr->next_prod()) {
				if (ptr != best && ptr->dequeue(item)) {
					return true;
				}
			}
		}
		return false;
	}
	
	// Attempts to dequeue from the queue.
	// Returns false if all producer streams appeared empty at the time they