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队列

    队列简称队, 也是一种操作受限的线性表, 只允许在表的一端进行插入, 而在表的另一端进行删除.其特点为”先进先出(FIFO)”,故又称为先进先出的线性表,简单队列如图所示:

 

循环队列

    顺序队列有一个先天不足, 那就是空间利用率不高, 会产生”假溢出”现象,即:其实队列中还有空闲的空间以存储元素, 但我们在判断队列是否还有空间时, 队列告诉我们队列已经满了, 因此这种溢出并不是真正的溢出, 在data数组中依然存在可以放置元素的空位置, 所以说这是一种”假溢出”;

    于是我们就引入了循环队列的概念, 将顺序队列臆造为一个环状的空间, 即把存储队列元素的表从逻辑上看成一个环, 称为循环队列,其示意图如下:

注意:如图中所示,我们的循环队列为了在实现上的便利, 会有一个位置的空闲, m_front(如图中的front)指针总会指向一个元素值为空的位置,因此(m_front+1)%capacity才真正的指向队首元素, 而m_rear(图中为rear)才指向一个真实存在的队尾元素;

//循环队列的实现与解析
template <typename Type>
class MyQueue
{template <typename T>friend ostream &operator<<(std::ostream &os, const MyQueue<T> &queue);
public:MyQueue(int queueSize = 64);~MyQueue();void push(const Type &item);void pop() throw (std::range_error);const Type &front() const throw (std::range_error);const Type &rear() const throw (std::range_error);bool isEmpty() const;private:Type *m_queue;int m_front;    //队首指针(其实(m_front+1)%capacity才真正的指向队首元素)int m_rear;     //队尾指针int capacity;   //队列的内存大小, 但实际可用的大小为capacity-1
};

template <typename Type>
MyQueue<Type>::MyQueue(int queueSize): capacity(queueSize)
{if (queueSize < 1)throw std::range_error("queueSize must >= 1");m_queue = new Type[capacity];if (m_queue == NULL)throw std::bad_alloc();m_front = m_rear = 0;
}

template <typename Type>
MyQueue<Type>::~MyQueue()
{delete []m_queue;m_queue = NULL;m_front = m_rear = 0;capacity = -1;
}

template <typename Type>
inline bool MyQueue<Type>::isEmpty() const
{return m_front == m_rear;
}

template <typename Type>
inline void MyQueue<Type>::push(const Type &item)
{if ((m_rear+1)%capacity == m_front) //队列已满{Type *newQueue = new Type[2 * capacity];    //新队列的长度为原队列的2倍if (newQueue == NULL)throw std::bad_alloc();int start = (m_front+1)%capacity;   //数据序列的起始地址if (start <= 1) //队列指针尚未回绕{//只需拷贝一次:从start所指向的元素直到m_rear所指向的元素//std::copy(m_queue+start, m_queue+start+capacity-1, newQueue);std::copy(m_queue+start, m_queue+m_rear+1, newQueue);}else{//需要拷贝两次//1:从start所指向的元素直到数组(不是队列)末尾std::copy(m_queue+start, m_queue+capacity, newQueue);//2:从数组(不是队列)起始直到队列末尾std::copy(m_queue, m_queue+m_rear+1, newQueue+capacity-start);}//重新设置指针位置:详细信息请看下面图解m_front = 2*capacity-1;m_rear = capacity-2;capacity *= 2;delete []m_queue;m_queue = newQueue;}//队尾指针后移//注意:此处m_front+1可能需要回绕m_rear = (m_rear+1)%capacity;m_queue[m_rear] = item;
}

template <typename Type>
inline const Type &MyQueue<Type>::front() const
throw (std::range_error)
{if (isEmpty())throw range_error("queue is empty");//注意:此处m_front+1可能需要回绕return m_queue[(m_front+1)%capacity];
}template <typename Type>
inline const Type &MyQueue<Type>::rear() const
throw (std::range_error)
{if (isEmpty())throw range_error("queue is empty");return m_queue[m_rear];
}

template <typename Type>
inline void MyQueue<Type>::pop()
throw (std::range_error)
{if (isEmpty())throw range_error("queue is empty");//注意:此处m_front+1可能需要回绕m_front = (m_front+1)%capacity;m_queue[m_front].~Type();   //显示调用析构函数以销毁(析构)对象
}

//输出队列所有内容以做测试
template <typename Type>
ostream &operator<<(ostream &os, const MyQueue<Type> &queue)
{for (int i = (queue.m_front+1)%(queue.capacity);i <= queue.m_rear; /**空**/ ){os << queue.m_queue[i] << ' ';if (i == queue.m_rear)break;elsei = (i+1)%(queue.capacity);}return os;
}

补充说明

当队列已满时的两类扩充操作:

扩充之后的内存布局:

 

 

附-测试代码:

int main()
{MyQueue<char> cQueue(3);cQueue.push('A');cQueue.push('B');//因为cQueue实际能够用的大小为2, 所以此处会对数组进行放大cQueue.push('C');cout << cQueue << endl;cout << "front = " << cQueue.front() << ", rear = "<< cQueue.rear() << endl;cQueue.pop();cQueue.pop();cQueue.push('D');cQueue.push('E');cQueue.push('F');//此时queue的m_rear会进行回绕cQueue.push('G');cQueue.pop();cQueue.push('H');//此时队列已满, 再添加元素则会进行对队列扩张//此时m_rear已经回绕, 则会触发两次拷贝操作cQueue.push('I');//验证是否能够正常工作cout << cQueue << endl;cout << "front = " << cQueue.front() << ", rear = "<< cQueue.rear() << endl;for (char ch = '1'; ch <= '9'; ++ch)cQueue.push(ch);for (int i = 0; i < 4; ++i)cQueue.pop();cout << cQueue << endl;cout << "front = " << cQueue.front() << ", rear = "<< cQueue.rear() << endl;return 0;
}