c++ - C++ 中的继承和虚拟析构函数问题

Question

所以我有两个类，一个是抽象的，一个不是。

抽象类是Iterator，具体类是LinkedListIterator。两者的代码都在帖子的底部。

我遇到的问题是下面的代码，我的 LinkedListIterator 出现 1 个错误在析构函数的最后一行说

undefined reference to `Iterator<int>::~Iterator()'

现在我尝试注释掉虚拟的 ~Iterator() 析构函数，没有错误，但我收到一条警告:

Class '[C@800c1' has virtual method 'remove' but non-virtual destructor

所以我的问题是:抽象 Iterator 基类中是否需要虚拟析构函数？我读过你应该总是有一个，但在这种情况下，LinkedListIterator 中的析构函数只是设置值，它不会释放任何东西......

谢谢!

迭代器代码:

template<class T>
class Iterator {

    public:
        //~Constructors/Destructors------------//
        /*
         * Destroys necessary resources.
         */
        virtual ~Iterator() = 0;

        //~Methods-------------------//
        /*
         * Informs the user whether there are more elements to be iterated
         * over in a List.
         *
         * @return true if there are more elements to iterate over, false otherwise.
         */
        virtual bool hasNext() = 0;

        /*
         * Gets the next element to iterate over.
         *
         * @return the next element in the iteration.
         */
        virtual T next() = 0;

        /*
         * Adds an element to the List being iterated over.
         *
         * @param element the element to add.
         * @return true if successful, false otherwise.
         */
        virtual bool add(T element) = 0;

        /*
         * Removes the element last returned by next from
         * the List being iterated over.
         *
         * @return true if successful, false otherwise.
         */
        virtual bool remove() = 0;
};

相关的 LinkedListIterator 代码(比较长的类):

template<class T>
class LinkedListIterator : public Iterator<T> {

    private:
        //~Data Fields---------------------------------//
        /*
         * Pointer to the node that the iterator is currently at.
         */
        Node<T>* current;
        /*
         * Pointer to the LinkedList being iterated through.
         */
        LinkedList<T>* list;
        /*
         * Boolean value indicating whether next has been called since
         * the last remove operation.
         */
        bool nextCalled;

    public:
        //~Constructors/Destructors------------------//
        /*
         * Constructor for LinkedListIterator, takes in a pointer to a Node
         * to initialize current to point to (intended to be the head of the
         * the LinkedList).
         *
         * @param theList pointer to the LinkedList being iterated through.
         */
        LinkedListIterator(LinkedList<T>* theList) {

            current = theList->head;
            list = theList;
            nextCalled = false;
        }

        /*
         * Destructor, resets pointer values to 0.
         */
        ~LinkedListIterator() {

            current = 0;
            list = 0;
        }

Answer 1

你的基类应该有一个virtual 析构函数，但不是纯 virtual 析构函数 (*)。

纯虚函数(即标记为 virtual 并在其签名后附加 = 0 后缀的函数)没有实现。然而，基类的析构函数总是需要被子类的析构函数调用，你应该为它提供一个定义(可能是一个空的):

template<class T>
class Iterator {

    public:
        //~Constructors/Destructors------------//
        /*
         * Destroys necessary resources.
         */
        virtual ~Iterator() { }
    ...

另见 this Q&A on StackOverflow 获取相关信息。

(*) 如链接的问答中所述，可以有一个纯虚拟析构函数(仍然需要定义)，但我认为这不是一个特别好的编程实践。