c++ - 实现一个链表【C++】编译报错

Question

我有以下代码:

template <class T> 
class List {
    public:
        class Iterator;
        class ConstIterator;

        //Constructors and Destructors.
        List() : head(NULL), tail(NULL), size(0) {}
        List(const List& list);
        ~List();

        //Methods
        Iterator begin();
        Iterator end();
        void insert(const T& data);
        void insert(const T& data, const Iterator& iterator);
        void remove(const Iterator& iterator);
        int getSize() const;
        Iterator find();
        void sort();

        //Operators
        List operator = (const List& list);


    private:
        class Node;
        Node* head;
        Node* tail;
        int size;
};


template <class T>
class List<T>::Node
{
    public:
        //Constructors and destructors
        Node(const T& _data, const Node* _next) : data(_data), next(_next) {}       
        ~Node(); //Destructor 

        //Methods


        //Operators
        Node operator = (const Node& node);

    private:
        T data;
        Node* next;
};


template<class T> 
class List<T>::Iterator 
{
    public:
        Iterator() : list(NULL), node(NULL){} //Constructor
        Iterator(const Iterator& it) : list(it.list), node(it.node) {}
        ~Iterator(); //Destructor

        Iterator& operator=(const Iterator& it); 
        T& operator * ();
        T& operator ++ ();
        T operator ++ (int);
        T& operator -- ();
        T operator -- (int);
        bool operator == (const Iterator& iterator) const;
        bool operator != (const Iterator& iterator) const;


    private:
        List<T>* list;
        Node* node;
};


template<class T> 
class List<T>::ConstIterator 
{
    public:
        ConstIterator() : list(NULL), node(NULL){}
        ConstIterator(const ConstIterator& it) : list(it.list), node(it.node) {}
        ~ConstIterator(); //Destructor

        ConstIterator& operator=(const ConstIterator& it); 
        T& operator * ();
        T& operator ++ ();
        T operator ++ (int);
        T& operator -- ();
        T operator -- (int);
        bool operator == (const ConstIterator& iterator) const;
        bool operator != (const ConstIterator& iterator) const;


    private:
        const List<T>* list;
        const Node* node;
};

template<class T>
Iterator List<T>::begin() {
    return Iterator(this, head);
}

---

当我尝试编译时出现以下错误:

error: expected constructor, destructor, or type conversion before ‘List’

在线:

Iterator List<T>::begin() {

我不确定我做错了什么。

Answer 1

Iterator未定义，但 List<T>::Iterator是。您还需要添加 typename :

template<class T>
typename List<T>::Iterator List<T>::begin() { ... };

在这里，typename需要作为歧义器告诉编译器 List<T>::Iterator是一种类型(而不是静态成员)。这在模板化上下文中始终是必需的(参见 here )。