声明了类，但将其作为空指针接收

Question

由于要查找和查找Null Pointer的所有问题，因此很难找到我想要的东西。我有一个用于Graph结构的类（邻接列表模型），其中有一个顶点的链接列表，而没有相邻顶点的链接列表。但是，当我尝试传递邻接列表（名为adjList，在此-> adjList-> add中传递）时，即使它应该是指向类的指针，也将其读取为空指针。有什么想法吗？

Graph::Vertex::Vertex(char lab) {
    this->next = 0;
    this->prev = 0;
    this->prior = 0;

    this->label = lab;
    this->dist = 0;
    this->index = 0;

    this->adjList = 0;
}

Graph::List::List() {
    this->head = 0;
}

Graph::List::Node::Node(Vertex *to, int dist) {
    this->dist = dist;
    this->next = 0;
    this->prev = 0;
    this->datum = 0;
}

void Graph::List::add(Vertex *to, int dist) {
    if (this == 0) {
        this->head = new Node(to, dist);
        return;
}

Node *temp = this->head;
    while (temp->next != 0) {
        temp = temp->next;
    }
    temp->next = new Node(to, dist);
    temp->next->prev = temp;

}

void Graph::addV(char vert) {
    if (this->vertices == 0) {
        this->vertices = new Vertex(vert);
        this->order++;
        return;
    }

    else {
        Vertex *temp = this->vertices;
        while (temp->next != 0) {
            temp = temp->next;
        }

    temp->next = new Vertex(vert);
    temp->next->prev = temp;
    this->order++;
}
void Graph::addE(char Fvert, char Tvert, int dist) {            //From Vert, To Vert, and Distance
    Vertex *tempT = this->vertices;
    Vertex *tempF = this->vertices;
    while (tempF != 0 && tempF->label != Fvert) {
        tempF = tempF->next;
    }

    while (tempT != 0 && tempT->label != Tvert) {
        tempT = tempT->next;
    }

    //Insertion start
    if (tempF->label == Fvert && tempT->label == Tvert) {
        tempF->adjList->add(tempT, dist);
    }
    //Insertion finished
    else if (tempF->label != Fvert)
        cout << "No Vertex to start from." << endl;

    else if (tempT->label != Tvert)
        cout << "No Vertex to go to." << endl;

    else
        cout << "Neither of these Vertices exist." << endl;
}

头文件

class Graph {
    class List;

    class Vertex {                          //Vertex is a list of Verticies that have a list of Edge pointer that point to other Vertecies
    public:                                 
        char label;
        int dist, index;
        List *adjList;                      //EdgeList //For Vertex Remove, a list of all Verticies that are connected to the Vertex
        Vertex *next, *prev, *prior;
        Vertex(char);
        ~Vertex();
    };

    class List {                            //List is a list of Verticies with a Distance that shows the distance between verticies
    public:                                 
        class Node {
        public:
            Vertex * datum;
            Node *next, *prev;
            int dist;
            Node(Vertex*, int);             //Input Vertex to compete edge for that list, and provide a distance
        };

        Node* head;
        void destr_helper(Node*);
        List();
        ~List();
        void add(Vertex*, int);
        void rem(char);
        bool contains(char);
    }; 

    Vertex* vertices;
    int order;
    void destr_helper(Vertex*);
public:
    Graph();                                    //bool can be added for directed and undirected
    //~Graph();
    void addV(char);
    void addE(char, char, int);
    void remV(char);
    void remE(char, char);

};