Method try_get

deps/concurrentqueue/concurrentqueue.h:1452–1489

		}
		
		inline N* try_get()
		{
#ifdef MCDBGQ_NOLOCKFREE_FREELIST
			debug::DebugLock lock(mutex);
#endif		
			auto head = freeListHead.load(std::memory_order_acquire);
			while (head != nullptr) {
				auto prevHead = head;
				auto refs = head->freeListRefs.load(std::memory_order_relaxed);
				if ((refs & REFS_MASK) == 0 || !head->freeListRefs.compare_exchange_strong(refs, refs + 1, std::memory_order_acquire, std::memory_order_relaxed)) {
					head = freeListHead.load(std::memory_order_acquire);
					continue;
				}
				
				// Good, reference count has been incremented (it wasn't at zero), which means we can read the
				// next and not worry about it changing between now and the time we do the CAS
				auto next = head->freeListNext.load(std::memory_order_relaxed);
				if (freeListHead.compare_exchange_strong(head, next, std::memory_order_acquire, std::memory_order_relaxed)) {
					// Yay, got the node. This means it was on the list, which means shouldBeOnFreeList must be false no
					// matter the refcount (because nobody else knows it's been taken off yet, it can't have been put back on).
					assert((head->freeListRefs.load(std::memory_order_relaxed) & SHOULD_BE_ON_FREELIST) == 0);
					
					// Decrease refcount twice, once for our ref, and once for the list's ref
					head->freeListRefs.fetch_sub(2, std::memory_order_release);
					return head;
				}
				
				// OK, the head must have changed on us, but we still need to decrease the refcount we increased.
				// Note that we don't need to release any memory effects, but we do need to ensure that the reference
				// count decrement happens-after the CAS on the head.
				refs = prevHead->freeListRefs.fetch_sub(1, std::memory_order_acq_rel);
				if (refs == SHOULD_BE_ON_FREELIST + 1) {
					add_knowing_refcount_is_zero(prevHead);
				}
			}
			
			return nullptr;
		}
		
		// Useful for traversing the list when there's no contention (e.g. to destroy remaining nodes)
		N* head_unsafe() const { return freeListHead.load(std::memory_order_relaxed); }