Method enqueue

source/util/concurrentqueue.h:1849–1945

		
		template<AllocationMode allocMode, typename U>
		inline bool enqueue(U&& element)
		{
			index_t currentTailIndex = this->tailIndex.load(std::memory_order_relaxed);
			index_t newTailIndex = 1 + currentTailIndex;
			if ((currentTailIndex & static_cast<index_t>(BLOCK_SIZE - 1)) == 0) {
				// We reached the end of a block, start a new one
				auto startBlock = this->tailBlock;
				auto originalBlockIndexSlotsUsed = pr_blockIndexSlotsUsed;
				if (this->tailBlock != nullptr && this->tailBlock->next->ConcurrentQueue::Block::template is_empty<explicit_context>()) {
					// We can re-use the block ahead of us, it's empty!					
					this->tailBlock = this->tailBlock->next;
					this->tailBlock->ConcurrentQueue::Block::template reset_empty<explicit_context>();
					
					// We'll put the block on the block index (guaranteed to be room since we're conceptually removing the
					// last block from it first -- except instead of removing then adding, we can just overwrite).
					// Note that there must be a valid block index here, since even if allocation failed in the ctor,
					// it would have been re-attempted when adding the first block to the queue; since there is such
					// a block, a block index must have been successfully allocated.
				}
				else {
					// Whatever head value we see here is >= the last value we saw here (relatively),
					// and <= its current value. Since we have the most recent tail, the head must be
					// <= to it.
					auto head = this->headIndex.load(std::memory_order_relaxed);
					assert(!details::circular_less_than<index_t>(currentTailIndex, head));
					if (!details::circular_less_than<index_t>(head, currentTailIndex + BLOCK_SIZE)
						|| (MAX_SUBQUEUE_SIZE != details::const_numeric_max<size_t>::value && (MAX_SUBQUEUE_SIZE == 0 || MAX_SUBQUEUE_SIZE - BLOCK_SIZE < currentTailIndex - head))) {
						// We can't enqueue in another block because there's not enough leeway -- the
						// tail could surpass the head by the time the block fills up! (Or we'll exceed
						// the size limit, if the second part of the condition was true.)
						return false;
					}
					// We're going to need a new block; check that the block index has room
					if (pr_blockIndexRaw == nullptr || pr_blockIndexSlotsUsed == pr_blockIndexSize) {
						// Hmm, the circular block index is already full -- we'll need
						// to allocate a new index. Note pr_blockIndexRaw can only be nullptr if
						// the initial allocation failed in the constructor.
						
						MOODYCAMEL_CONSTEXPR_IF (allocMode == CannotAlloc) {
							return false;
						}
						else if (!new_block_index(pr_blockIndexSlotsUsed)) {
							return false;
						}
					}
					
					// Insert a new block in the circular linked list
					auto newBlock = this->parent->ConcurrentQueue::template requisition_block<allocMode>();
					if (newBlock == nullptr) {
						return false;
					}
#ifdef MCDBGQ_TRACKMEM
					newBlock->owner = this;
#endif
					newBlock->ConcurrentQueue::Block::template reset_empty<explicit_context>();
					if (this->tailBlock == nullptr) {
						newBlock->next = newBlock;
					}