「=」：無法從「CircularDoubleDirectedList <int> ::節點*」轉換爲「節點*」

Question

我有節點*電流在那裏我存儲一個指針在列表的「頂部」當前哪個節點。當我設置一個新的節點作爲目前我得到的錯誤：

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

它是三排//產生這些錯誤的問題發表評論，如果把他們帶走一切工作正常。

#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

當你向前聲明Node爲這裏的課堂外：

template <typename T> class CircularDoubleDirectedList; 
class Node; 

即宣佈在全局命名空間類型Node。它是::Node。然後，你的類聲明中，current承擔是類型：

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

然後你提供的CircularDoubleDirectedList<T>::Node聲明。這是不同一類型::Node。它也會首先按名稱解析規則查找。所以在這裏：

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

但current是一個指針仍然不完全類型::Node。因此，錯誤 - 你在不經意間創造類型的命名Node。

如果你要前瞻性聲明Node，你必須這樣做內類：

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