关于学生信息管理系统的问题C++

1. 为什么这个学生管理系统只有添加功能正确，但插入和其他功能都不能使用呢？
2. 该怎样修改
3. 如何让初始信息从文件中调用，更新后存入一新文件中

#include<iostream>
#include<stdlib.h>

using namespace std;
int k=1; 
struct student{
    int num;
    char name[30];
    struct student *next;
};

 class stu{
     private:
         struct student *head,*tail;
     public:
         stu(){
    head=new student;
    head->num=0;
    head->name[0]='\0';
         }; 
        void create(int n);
        void print();
        int Insert();
        void del();
        void destroy();
 }; 

void stu:: create(int n){
     
    struct student *p;
    head->next=NULL;
    tail=head; 
    for(int i=0;i<n;i++){
        p=new student;
        cout<<"input num,name"<<endl;
        cin>>p->num>>p->name;
        cout<<endl;
        p->next=NULL;
        tail->next=p;
        tail=p;
    }
}
void stu::print()
{
    struct student *p;
    p=head->next;
    if(p==NULL){
        cout<<"kongbiao"<<endl;
        return;
    } 
    while(p!=NULL){
        cout<<p->num<<'\t'<<p->name<<endl;
        p=p->next;
    }
}
int stu::Insert(){
    system("cls");
    cout<<"请输入要在哪个学号后面插入信息"<<endl;
    int sno;
    cin>>sno;
    struct student *newp=new student,*p;
    cout<<"请输入要插入的信息"<<endl; 
    cin>>newp->num>>newp->name;
    p=head->next;
    while(p!=NULL){
        if(p->num==sno){
            newp->next=p->next;
            p->next=newp;
            
            break;
        }
        p=p->next;
    }  
    return 0; 
    }
void stu::del(){
    cout<<"请输入要删除的学号"<<endl;
    int sno;
    cin>>sno;
    struct student *p,*pre;
    p=head->next;
    pre=head;
    while(p!=NULL){
        if(p->num==sno){
        pre->next=p->next;
        delete p;
        break;
        }
        pre=p;
        p=p->next;
    } }
void stu::destroy(){
    struct student *p,*ptr;
    p=head;
    while(p!=NULL){
        ptr=p;
        p=ptr->next;
        delete ptr;
    }
}
void menu()/* 界面*/
{
cout<<"1.添加"<<endl; 
cout<<"2.打印"<<endl;
cout<<"3.插入"<<endl;
cout<<"4.删除"<<endl;
cout<<"5.销毁"<<endl;
cout<<"6.退出"<<endl;
int num;
    stu STU;
cout<<"请输入你的选择"<<endl;
cin>>num;
switch(num){
case 1:
    int n;
    cout<<"请输入个数"<<endl;
    cin>>n;
    STU.create(n);
    STU.print();
    break;
case 2:
    STU.print();
    break;
case 3:
    STU.Insert();
    STU.print();
    break;
case 4:
    STU.del();
    STU.print();
    break;
case 5:
    STU.destroy();
    STU.print();
    break;
case 6:
    k=0;
        cout<<"即将退出程序!"<<endl;
        break;
    default:
    cout<<"请在1~6之间选择"<<endl;
    break;
}
}
int main(){
while(k){
menu();
}
system("pause");
return 0;}

    OK·1
   输入要在哪个学
  请输入要插入的信息
    wang
           exited after 18.23 seconds with retul
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1.

把void stu::create(int n) 函数修改下，改为从文件里载入链表就可以了，其余的修改如下，供参考：

#include<iostream>
#include<stdlib.h>
using namespace std;
int k = 1;
struct student { //修改
    int  num;
    char name[30];
    struct student* next;
};
class stu {
private:
    struct student* head, * tail;
public:
    stu() {
        head = new struct student; //修改
        head->num = 0;
        head->name[0] = '\0';
        head->next = NULL;  //修改
    };
    void create(int n);
    void print();
    int  Insert();
    void del();
    void destroy();
};

void stu::create(int n) {
    struct student* p;
             //head->next = NULL;  修改
    tail = head;
    for (int i = 0; i < n; i++) {
        p = new struct student;  //修改
        cout << "input num,name" << endl;
        cin >> p->num >> p->name;
        cout << endl;
        p->next = NULL;
        tail->next = p;
        tail = p;
    }
}
void stu::print()
{
    if (head->next == NULL) { //修改
        cout << "kongbiao" << endl;
        return;
    }
    struct student* p;
    p = head->next;
    while (p != NULL) {
        cout << p->num << '\t' << p->name << endl;
        p = p->next;
    }
}
int stu::Insert() {
    system("cls");
    if (head->next == NULL) { //修改
        cout << "kongbiao" << endl;
        return -1;
    }
    cout << "请输入要在哪个学号后面插入信息" << endl;
    int sno;
    cin >> sno;
    struct student* p = head->next;
    while (p != NULL){ 
        if (p->num == sno) {
            struct student* newp = new struct student; //修改 
            newp->next = NULL;                      
            cout << "请输入要插入的信息:num,name" << endl;
            cin >> newp->num >> newp->name;
            newp->next = p->next; 
            p->next = newp;
            break;
        }
        p = p->next;
    }
    if (p == NULL)                                   //修改
        printf("学号:%d 记录未找到，插入未成功！\n", sno);
    return 0;
}
void stu::del() {
    if (head->next == NULL) {       //修改
        cout << "kongbiao" << endl;
        return;
    }
    struct student* p, * pre;
    p = head->next;
    pre = head;
    cout << "请输入要删除的学号" << endl;
    int sno;
    cin >> sno;
    while (p != NULL) {
        if (p->num == sno) {
            pre->next = p->next;
            delete p;
            printf("学号:%d 记录删除成功！\n", sno);//修改
            return;  //break;                        //修改
        }
        pre = p;
        p = p->next;
    }
    if (p == NULL)                                  //修改
        printf("学号:%d 记录未找到，删除未成功！\n", sno);
}
void stu::destroy() {
    struct student* p, * ptr;
    //p = head;             //修改
    while (head->next != NULL) {
        ptr = head->next;
        head->next = ptr->next;
        delete ptr;
    }
}
void menu(stu STU)/* 界面*/   //修改
{
    cout << "1.添加" << endl;
    cout << "2.打印" << endl;
    cout << "3.插入" << endl;
    cout << "4.删除" << endl;
    cout << "5.销毁" << endl;
    cout << "6.退出" << endl;
    int num;
    //stu STU;               //修改
    cout << "请输入你的选择" << endl;
    cin >> num;
    switch (num) {
    case 1:
        int n;
        cout << "请输入个数" << endl;
        cin >> n;
        STU.create(n);
        STU.print();
        break;
    case 2:
        STU.print();
        break;
    case 3:
        STU.Insert();
        STU.print();
        break;
    case 4:
        STU.del();
        STU.print();
        break;
    case 5:
        STU.destroy();
        STU.print();
        break;
    case 6:
        k = 0;
        cout << "即将退出程序!" << endl;
        break;
    default:
        cout << "请在1~6之间选择" << endl;
        break;
    }
}
int main() 
{
    stu STU;    //修改
    while (k) {
        menu(STU);
    }
    system("pause");
    return 0;
}