请问图片那里引发了异常应该怎么处理遇到取消对nullptr指针的引用是哪里出了问题#include <iostream>

请问图片那里引发了异常应该怎么处理
遇到取消对nullptr 指针的引用是哪里出了问题

#include
#include
using namespace std;
 
typedef struct  { //结构体定义
    int num;
    char name[10];
}student;

typedef struct Lnode{
    student stu;
    Lnode* pre;
    Lnode* next;
}*lnode;

bool initlnode(lnode& l) {//链表初始化
    lnode p= new Lnode;
    p = nullptr;
    return true;
}

bool creatlnode(lnode& l, const int a) { //链表赋值
    lnode current = l;
    cout << "请输入链表信息" << endl;
    for (int i = 1; i <= a; i++) {
        lnode temp=new Lnode;
        cout << "请输入第" << i << "个学生的数据" << endl;
        cin >> temp->stu.num;
        cin >>temp->stu.name;
        current->next = temp;
        temp->next = nullptr;
        temp->pre = current;
        current = temp;
    }
    return true;
}

bool insert(lnode& l, const student a,const int b) {//插入到b后
    if (l = nullptr) return false;
    lnode temp=new Lnode;
    temp->stu= a;
    lnode current = l;
    int i = 0;
    for( i=0;inext!=nullptr;i++){
        current = current->next;//current指向b
        }
    if (i < b) return false;
    temp->next =nullptr;
    temp->pre = current;
    current->next = temp;
    return true;
}

bool eraselnode(lnode& l, const int a) {//删除第a个元素
    if (l == nullptr) {
        cout << "空链表，无法删除" << endl;
        return false;
    }
    int i = 0;
    lnode current = l;
    for (i; i < a&¤t!=nullptr; i++) {
        current = current->next;
    }
    if (i < a || a < 1) {
        cout << "删除位置错误" << endl;
        return false;
    }
    lnode p = current;
    current->pre->next = current->next;
    current->next->pre = current->pre;
    delete p;
    return true;

}
bool searchlnode(lnode& l, const int a,student* b) {//查找a处的元素
    if (l == nullptr) {
        cout << "空链表，无法查找" << endl;
        return false;
    }
    int i = 0;
    lnode current = l;
    for (i; i < a && current != nullptr; i++) {
        current = current->next;
    }
    if (i < a || a < 1) {
        cout << "查找位置错误" << endl;
        return false;
    }
    b = &(current->stu);
    return true;
}
bool findbeforeAndAfter(lnode& l, const int i, lnode m,const int j,lnode n,int num) {//查找第i个前驱，第j个后继
    lnode current = l;
    while (current->stu.num != num&¤t->next!=nullptr) {
        current = current->next;
    }
    if (current->stu.num != num && current->next == nullptr) {
        return false;
    }
    lnode temp = current;
    for (int t = i; t > 0; t--) {
        temp = temp->pre;
        if (temp->pre = nullptr) {
            m = nullptr;
            cout << "前驱查找失败，请检查您输入的数值是否在正确范围内" << endl;
            return false;
        }
    }
    m = temp;
    lnode temp2 = current;
    for (int t = j; t > 0; t--) {
        temp2 = temp2->next;
        if (temp2->next = nullptr) {
            n= nullptr;
            cout << "后继查找失败，请检查您输入的数值是否在正确范围内" << endl;
            return false;
        }    
    }
    n = temp2;
    return true;

}
void print(lnode& l) {
    lnode current = l;
    while (current) {
        cout << current->stu.num << "    " << current->stu.name;
        current = current->next;
    }

}
int main() {
    lnode stulist;
    if (initlnode(stulist)) {
        cout << " 请输入学生个数" << endl;
        int stnum;
        cin >> stnum;
        creatlnode(stulist, stnum);
    }
    else cout << "链表初始化错误，请检查程序" << endl;
    cout << "请选择您要实现的功能：" << endl;
    cout << "1.插入学生信息    2.删除学生信息    3.查找学生信息    4.查找前驱后继" << endl;
    int a;
    cin >> a;
    student* searchout = nullptr;
    switch (a) {
    case 1:
        student stinsert;
        cout << "请输入插入学生的学号" << endl;
        cin >> stinsert.num;
        cout << "请输入插入学生的姓名" << endl;
        cin >> stinsert.name;
        cout << "请输入要插入的位置（将在你输入的元素位置之后插入）" << endl;
        int locinsert;
        cin >> locinsert;
        if (insert(stulist, stinsert, locinsert)) {
            cout << "插入成功" << endl;
            print(stulist);
        }
        else cout << "插入失败" << endl;
        break;


    case 2:
        cout << "请输入你要删除学生的位置" << endl;
        int locerase;
        cin >> locerase;
        if (eraselnode(stulist, locerase)) {
            cout << "删除成功" << endl;
            print(stulist);
        }
        break;
    case 3:
        cout << "请输入你要查找学生的位置" << endl;
        
        int locsearch;
        cin >> locsearch;
        if (searchlnode(stulist, locsearch, searchout)) {
            cout << "查找成功，您查找的学生信息为：" << endl;
                cout << searchout->num<< "    " << searchout->name;
        }
        break;
    case 4:
        cout << "请输入学生学号" << endl;
            int flagstu;
            cin >> flagstu;
            cout << "请输入您向前查找的数目" << endl;
            int i;
            cin >> i;
        cout << "请输入您向后查找的数目" << endl;
        int j;
        cin >> j;
        lnode p=nullptr, q=nullptr;
        if (findbeforeAndAfter(stulist, i, p, j, q, flagstu)) {
            cout << "查找成功" << endl;
            cout << "第" << i << "个前驱为：" << endl;
            cout << p->stu.num << "    " << p->stu.name;
            cout << "第" << j << "个后继为：" << endl;
            cout << q->stu.num << "    " << q->stu.name;
            break;

        }
    }
    return 0;
    
}

你检查一下传入的l变量是不是空的啊，或者没有初始化

bool initlnode(lnode& l) {//链表初始化
    lnode p= new Lnode;
    p = nullptr;
    return true;
}

你这初始化函数和传入的变量毫无关系啊，根本没有创建啊，改为：

bool initlnode(lnode& l) {//链表初始化
    l = new Lnode;
    if(l==NULL)
        return false;
    l->next = NULL;
    return true;
}

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