Method pop

practice/itemPool.cpp:31–41

    }

    void pop() // 出队操作
    {
        if (empty())
            return;
        QueueItem *first = _front->_next;
        _front->_next = first->_next;
        if (_front->_next == nullptr) {
            _rear = _front;
        }
        delete first;
    }

    T front() const
    {