c++ - 删除临时指针时出现段错误

Question

我使用 gdb 找到了出现段错误的确切行。它在 dequeue 函数中被标记为注释。

这是整个队列类。

当队列中有两个对象和哨兵时，我在调用 dequeue() 时出现错误。

template <typename T>
void Queue<T>::clear()
{
    while(!isEmpty())
        dequeue();
}

template <typename T>
void Queue<T>::enqueue(const T& x)
{

    if(isEmpty())
    {
        Queue<T>* temp = new Queue<T>;
        m_data = x;
        m_next = temp;
        return;

    }


    Queue<T>* temp = this;

    while(temp->m_next != NULL)
    {
        temp = temp->m_next;
    }
    Queue<T>* node = new Queue<T>();
    temp->m_data = x;
    node->m_next = temp->m_next;
    temp->m_next = node;
    return;
}

template <typename T>
void Queue<T>::dequeue()
{
    if(isEmpty())
        return;
    if(m_next != NULL)
    {
        Queue<T>* temp = m_next;
        m_data = temp->m_data;
        m_next = temp->m_next;
        delete temp; //Seg fault here
    }
    return;
}

template <typename T>
const T& Queue<T>::front() const throw (int)
{
    if(isEmpty())
        throw 0;
    return m_data;
}

template <typename T>
bool Queue<T>::isEmpty() const
{
    return (m_next==NULL);
}

template <typename T>
int Queue<T>::size() const
{
    int size = 0;
    const Queue<T>* temp = this;
    while(temp->m_next != NULL)
    {
        temp = temp->m_next;
        size++;
    }
    return size;
}

抱歉，我以为我已经发布了 Queue 类:

template <typename T>
class Queue : public AbstractQueue<T> {
public:
    Queue(){m_next = NULL;};

    virtual void clear();

    virtual void enqueue(const T& x);

    virtual void dequeue();

    virtual const T& front() const throw (int);

    virtual bool isEmpty() const;

    virtual int size() const;

    ~Queue(){
        clear();
        return;
    };
private:
    T m_data;
    Queue* m_next;
};

它继承自这个类:

template < typename T >
class AbstractQueue
{
public:

  // Purpose: clears the queue
  // Postconditions: the queue is now empty 
  // -- PURE VIRTUAL
  virtual void clear() = 0;

  // Purpose: enqueue an element into the queue
  // Parameters: x is the item to add to the queue
  // Postconditions: x is now the element at the end of the queue, 
  // -- PURE VIRTUAL
  virtual void enqueue(const T& x) = 0;

  // Purpose: dequeues 
  // Postconditions: the element formerly at the front of the queue has
  //     been removed
  // Dequeueing from an empty Queue produces no errors, queue remains empty.
  // -- PURE VIRTUAL
  virtual void dequeue() = 0;

  // Purpose: looks at the front of the queue
  // Returns: a reference to the element currently in front of the queue
  // Exception: if the queue is currently empty, throw SOMETHING!!
  // -- PURE VIRTUAL
  virtual const T& front() const = 0;  

  // Purpose: Checks if a queue is empty
  // Returns: 'true' if the queue is empty
  //     'false' otherwise  
  // -- PURE VIRTUAL
  virtual bool isEmpty() const = 0;

  // Purpose: Returns the size of a queue.
  // Returns: the number of elements in the Queue
  // -- PURE VIRTUAL
  virtual int size() const = 0;

  // ----------------

  // Purpose: Destructor
  // -- VIRTUAL
    virtual ~AbstractQueue() {};

};

Answer 1

在这段代码中:

Queue<T>* temp = m_next;
m_data = m_next->m_data;
m_next = m_next->m_next;

您没有检查 m_next 是否为非空(如果您在列表的末尾)，所以您开始取消引用空指针，此时所有赌注都关闭了。