C++有序链表实现学生名册时触发string下标越界断言错误求助

作业要求 #

本次作业需基于有序链表（sorted linked list）实现学生名册，每个学生记录包含name、SSN和grade，使用链表数据结构存储所有学生数据。测试程序需从输入文件读取数据，按SSN顺序插入链表，支持删除指定学生，每次增删操作后打印完整链表。

当前实现代码 #

StudentLinkedList.h #

#include <ostream>
#include <iostream>
using namespace std;

struct StudentData {
    string name;
    int ssn;
    char grade;
};

struct StudentNode {
    StudentData student;
    StudentNode* next;
};

class StudentLinkedList
{
public:
    StudentLinkedList();
    void addStudent(StudentData student);
    void deleteStudent(int);
    void printList(ostream&);

private:
    StudentNode* head;
    int length;
};

StudentLinkedList::StudentLinkedList()
{
    head = NULL;
}


void StudentLinkedList::addStudent(StudentData student)
{
    StudentNode* temp = new StudentNode();//Creating new node
    temp->student = student;
    temp->next = NULL;
    if (length == 0) {
        head = temp;
    }
    else {
        int i;
        StudentNode* p = head;
        StudentNode* p2 = head;
        for (i = 0; i < length; i++)
        {
            if (student.ssn < p->student.ssn)
                break;
            else if (p->next != NULL) {
                if (i == 0)
                    p = p->next;
                else {
                    p = p->next;
                    p2 = p2->next;//Making p2 stay before p
                }
            }
        }
        if (i == 0) {//If SSN is lesser than head SSN
            temp->next = head;
            head = temp;
        }

        else {
            int i;
            StudentNode* p = head;
            StudentNode* p2 = head;
            for (i = 0; i < length; i++)
            {
                if (student.ssn < p->student.ssn)
                    break;
                else if (p->next != NULL) {
                    if (i == 0)
                        p = p->next;
                    else {
                        p = p->next;
                        p2 = p2->next;//Making p2 stay before p
                    }
                }
            }
            if (i == 0) {//If SSN is lesser than head SSN
                temp->next = head;
                head = temp;
            }
            else if (p->next == NULL && p->student.ssn < student.ssn) {//Adding student at the end
                p->next = temp;
            }
            else {//Inserting student in the middle
                temp->next = p;
                p2->next = temp;
            }
        }
        length++;

    }
}


    void StudentLinkedList::deleteStudent(int ssn)
    {
        StudentNode* p = head;
        while (1)
        {
            if (p->student.ssn == ssn)//Deleting head
            {
                head = p->next;
                break;
            }
            else if (p->next == NULL)//If student not found when there is only 1 node
            {
                std::cout << "Student not found!" << std::endl;
                break;
            }
            else {
                if (p->next->next == NULL)
                {
                    if (p->next->student.ssn == ssn)//Deleting last node
                        p->next = NULL;
                    else
                        std::cout << "Student not found!" << std::endl;
                    break;
                }
                else
                {
                    if (p->next->student.ssn == ssn) {//Deleting node in the middle
                        p->next = p->next->next;
                        break;
                    }
                    else {
                        p = p->next;//Looping until node is found
                    }
                }

            }
        }
        length--;
    }


    void StudentLinkedList::printList(ostream&)
    {
        StudentNode* p = head;
        while (p != NULL)
        {
            cout << p->student.name << " " << p->student.ssn << " " << p->student.grade << std::endl;
            p = p->next;
        }
    }

Driver.cpp #

#include "StudentLinkedList.h"
#include <fstream>
#include <string>

int main()
{
    StudentLinkedList slist;
    StudentData student;
    fstream infile;
    infile.open("Student.txt");
    int ssn;
    string str;

    //Read each student from the file and insert into linked list using while loop

    while (!infile.eof())
    {
        getline(infile, str);
        int i = 0;
        char c = str[i++];
        std::string word = "";
        while (c != '\t')
        {
            word += c;
            c = str[i++];
        }
        student.name = word;
        word = "";
        c = str[i++];
        while (c != '\t')//Extracting student SSN
        {
            word += c;
            c = str[i++];
        }
        student.ssn = std::stoi(word);//Converting string to integer
        word = "";
        c = str[i++];
        student.grade = c;//Student Grade
        //add a student to the student list
        slist.addStudent(student);
    }

    slist.printList(cout);

    cout << "-----------------------------------------------------" << endl;

    //Deleting one student from the student linked list!!!
    cout << "Enter SSN to be deleted : ";
    int sno;
    cin >> sno;
    slist.deleteStudent(sno);
    slist.printList(cout);
    return 0;

}

问题分析与修复 #

触发Debug Assertion Failed (string subscript out of range)的核心原因是字符串访问时下标超出范围，结合代码来看，具体问题及修复方案如下：

1. 文件读取逻辑错误（直接导致越界） #

• 问题：使用while (!infile.eof())循环读取文件，会导致最后一次循环读取空字符串（因为eof标志在读取失败后才会触发），此时访问str[i++]（i=0）会直接越界。同时，读取字符时没有检查i是否超出字符串长度，若文件行格式错误（缺少制表符），循环会持续i++直到越界。
• 修复：
  1. 把循环改为while (getline(infile, str))，仅当成功读取一行时进入循环。
  2. 读取字符时添加i < str.size()的判断，防止下标越界。
  3. 跳过空行，避免处理无效数据。
     修改后的读取代码：

  while (getline(infile, str))
  {
      if (str.empty()) continue; // 跳过空行
      int i = 0;
      std::string word = "";
      // 读取name
      while (i < str.size() && str[i] != '\t')
      {
          word += str[i++];
      }
      if (i >= str.size()) continue; // 格式错误跳过
      student.name = word;
      i++; // 跳过制表符
      word = "";
      // 读取SSN
      while (i < str.size() && str[i] != '\t')
      {
          word += str[i++];
      }
      if (i >= str.size()) continue; // 格式错误跳过
      student.ssn = std::stoi(word);
      i++; // 跳过制表符
      // 读取grade
      if (i >= str.size()) continue; // 格式错误跳过
      student.grade = str[i];
      slist.addStudent(student);
  }
  

2. 链表类初始化遗漏 #

• 问题：StudentLinkedList构造函数中未初始化length成员变量，初始值为随机垃圾值，导致addStudent中if (length == 0)判断逻辑错误，可能引发后续链表操作异常。
• 修复：在构造函数中初始化length为0：

  StudentLinkedList::StudentLinkedList()
  {
      head = NULL;
      length = 0;
  }
  

3. deleteStudent函数的潜在问题 #

• 问题：
  1. 若链表为空（head为NULL），直接访问p->student.ssn会崩溃。
  2. 无论是否找到并删除学生，都会执行length--，导致length值与实际链表长度不符。
• 修复：

  void StudentLinkedList::deleteStudent(int ssn)
  {
      if (head == NULL)
      {
          std::cout << "Student not found!" << std::endl;
          return;
      }
  
      StudentNode* p = head;
      StudentNode* prev = NULL;
      bool found = false;
  
      while (p != NULL)
      {
          if (p->student.ssn == ssn)
          {
              found = true;
              if (prev == NULL) // 删除头节点
              {
                  head = p->next;
              }
              else // 删除中间或尾节点
              {
                  prev->next = p->next;
              }
              delete p; // 释放内存，避免内存泄漏
              break;
          }
          prev = p;
          p = p->next;
      }
  
      if (found)
      {
          length--;
      }
      else
      {
          std::cout << "Student not found!" << std::endl;
      }
  }
  

4. addStudent函数冗余代码 #

• 问题：函数内的else块重复执行了一遍for循环，属于冗余代码，不仅浪费资源还容易引发逻辑不一致。
• 修复：删除重复的for循环，直接复用第一次循环的p、p2、i值：

  void StudentLinkedList::addStudent(StudentData student)
  {
      StudentNode* temp = new StudentNode();
      temp->student = student;
      temp->next = NULL;
  
      if (length == 0) {
          head = temp;
      }
      else {
          int i = 0;
          StudentNode* p = head;
          StudentNode* p2 = head;
          for (; i < length; i++)
          {
              if (student.ssn < p->student.ssn)
                  break;
              if (p->next != NULL) {
                  if (i != 0) {
                      p2 = p2->next;
                  }
                  p = p->next;
              }
              else {
                  break; // 已到链表末尾
              }
          }
  
          if (i == 0) { // 插入到头部
              temp->next = head;
              head = temp;
          }
          else if (p->next == NULL && p->student.ssn < student.ssn) { // 插入到尾部
              p->next = temp;
          }
          else { // 插入到中间
              temp->next = p;
              p2->next = temp;
          }
      }
      length++;
  }
  

5. printList函数参数未使用 #

• 问题：函数声明了ostream&参数，但实际直接使用cout，不符合设计意图。
• 修复：使用传入的ostream对象输出：

  void StudentLinkedList::printList(ostream& os)
  {
      StudentNode* p = head;
      while (p != NULL)
      {
          os << p->student.name << " " << p->student.ssn << " " << p->student.grade << std::endl;
          p = p->next;
      }
  }
  

内容的提问来源于stack exchange，提问作者oStarZ