我得到「未處理的異常的類型'System.NullReferenceException'發生」如何解決它？

Question

我試過下面的代碼，我得到這個錯誤。

Linkedlist.exe中發生未處理的類型爲'System.NullReferenceException'的異常其他信息：未將對象引用設置爲對象的實例。

我認爲問題出在insertlast（），當我檢查類似問題的解決方案時，他們討論了實例化新節點。我的方法，即節點* q =新節點;錯了？

struct Node { 
    int data; 
    Node* next; 
}; 
int is_list_empty(struct Node*head){ 
    int count=0; 
    Node* p = head; 
    while (p!= NULL) 
{ 
    ++count; 
    p = p->next; 
    cout<<"go"; 
} 
    return count; 
} 

void insertlast(struct Node *head,int value) 
{ 
    Node *q = new Node; 
    q->data=value; 
    q->next=NULL; 
    Node *p=head; 
    while(p!=NULL) 
    { 
     p=p->next; 
    } 
    q=p->next; 
} 

void display(struct Node *head){ 
    Node*p = head; 
    while(p!=NULL){ 
     cout <<p->data<< " "; 
     p=p->next; 
    } 
} 

int main(){ 
    //Node *head = NULL; 
    Node *head; 
    Node *x ;    
    x = (Node*)malloc(sizeof(Node)); 
    x->data=112; 
    x->next = head; 
    head = x; 
    display(head); 
    //works fine upto here and 112 is displayed 
    insertlast(head,34); 
    insertlast(head,32); 
    insertlast(head,44); 
    display(head); 
    cout<< is_list_empty(head); 
    system("Pause"); 
    return 0; 
} 

Answer 1

您應該使頭爲空。接下來，在將q分配回p（它應該是p->next=q）時出現錯誤，您的while循環應該只檢查最多p->next!=NULL。
查看我所做的更改。

struct Node { 
    int data; 
    Node* next; 
}; 
int is_list_empty(struct Node*head){ 
    int count=0; 
    Node* p = head; 
    while (p!= NULL) 
{ 
    ++count; 
    p = p->next; 
    cout<<"go"; 
} 
    return count; 
} 

void insertlast(struct Node *head,int value) 
{ 
    Node *q = new Node; 
    q->data=value; 
    q->next=NULL; 
    Node *p=head; 
    while(p->next!=NULL) 
    { 
     p=p->next; 
    } 
    p->next=q; 
} 

void display(struct Node *head){ 
    Node*p = head; 
    while(p!=NULL){ 
     cout <<p->data<< " "; 
     p=p->next; 
    } 
} 

int main(){ 
    //Node *head = NULL; 
    Node *head=NULL; 
    Node *x ;    
    x = (Node*)malloc(sizeof(Node)); 
    x->data=112; 
    x->next = head; 
    head = x; 
    display(head); 
    //works fine upto here and 112 is displayed 
    insertlast(head,34); 
    insertlast(head,32); 
    insertlast(head,44); 
    display(head); 
    cout<< is_list_empty(head); 
    system("Pause"); 
    return 0; 
}