c++ - ' =' : cannot convert from ' CircularDoubleDirectedList::节点 *' to ' 节点 *'

Question

我有 Node* current ，我在其中存储指向列表“顶部”当前节点的指针。当我将一个新节点设置为当前节点时，出现错误:

'=' : cannot convert from 'CircularDoubleDirectedList<int>::Node *' to 'Node *'
while compiling class template member function 'void CircularDoubleDirectedList<int>::addAtCurrent(const T &)' with [ T=int ]

这是带有//Problem 注释的三行，如果把它们去掉一切正常，就会产生这些错误。

#include "ICircularDoubleDirectedList.h"

template <typename T> class CircularDoubleDirectedList;
class Node;

template <typename T>
class CircularDoubleDirectedList :
    public ICircularDoubleDirectedList<T>{
public:
    //Variables
    Node* current;
    int nrOfElements;
    direction currentDirection;

    //Functions
    CircularDoubleDirectedList();
    ~CircularDoubleDirectedList();
    void addAtCurrent(const T& element) override;

private:
    class Node
    {
    public:
        T data;
        Node* forward;
        Node* backward;

        Node(const T& element);
    };

};
template <typename T>
void CircularDoubleDirectedList<T>::addAtCurrent(const T& element){
    Node* newNode = new Node(element);
    newNode->data = element;
    if (this->nrOfElements == 0){
        newNode->forward = newNode;
        newNode->backward = newNode;
    }
    else{
        this->current->forward = newNode; // Problem
        this->current->forward->backward = newNode; // Problem
    }
    this->current = newNode; //Problem
}

Answer 1

当你转发声明Node因为不在这里上课:

template <typename T> class CircularDoubleDirectedList;
class Node;

那是声明一个类型 Node在全局命名空间中。是::Node .然后，在你的类声明中，current接受那个类型:

template <typename T>
class CircularDoubleDirectedList 
    : public ICircularDoubleDirectedList<T>
{
public:
    Node* current;   // this is a pointer to ::Node.
};

然后您提供 CircularDoubleDirectedList<T>::Node 的声明.这与 ::Node 类型不相同.它还首先通过名称解析规则进行查找。所以在这里:

template <typename T>
void CircularDoubleDirectedList<T>::addAtCurrent(const T& element){
    Node* newNode = new Node(element); // newNode is a pointer to
                                       // CircularDoubleDirectedList<T>::Node

但是current是指向仍然不完整类型的指针 ::Node .因此出现错误 - 您无意中创建了名为 Node 的 两个 类型.

如果你要转发声明Node ，你必须在类里面做这件事:

template <typename T>
class CircularDoubleDirectedList 
    : public ICircularDoubleDirectedList<T>
{
    class Node; // NOW it's CircularDoubleDirectedList<T>::Node
};