详解C++编程中对二进制文件的读写操作

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

struct student {
  char name[20];
  int num;
  int age;
  char sex;
};

int main() {
  // 初始化学生数据
  student stud[3] = {"Li", 1001, 18, 'f', "Fun", 1002, 19, 'm', "Wang", 1004, 17, 'f'};
  
  // 以二进制模式打开输出文件
  ofstream outfile("stud.dat", ios::binary);
  if (!outfile) {
    cerr << "open error!" << endl;
    return 1; // 建议使用return代替abort
  }
  
  // 方式A：循环写入每个元素
  for(int i = 0; i < 3; i++) {
    outfile.write((char*)&stud[i], sizeof(stud[i]));
  }
  
  // 方式B：一次性写入全部数据（推荐）
  // outfile.write((char*)&stud[0], sizeof(stud));
  
  outfile.close();
  return 0;
}

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

// 注意：这里结构与写入时一致，若写入时使用静态数组，读取也应匹配
struct student {
  char name[20];
  int num;
  int age;
  char sex;
};

int main() {
  student stud[3];
  ifstream infile("stud.dat", ios::binary);
  
  if (!infile) {
    cerr << "open error!" << endl;
    return 1;
  }
  
  // 逐条读取
  for(int i = 0; i < 3; i++) {
    infile.read((char*)&stud[i], sizeof(stud[i]));
    cout << "Name: " << stud[i].name
         << " Num: " << stud[i].num
         << " Age: " << stud[i].age
         << " Sex: " << stud[i].sex << endl;
  }
  
  infile.close();
  return 0;
}

#include <iostream>
#include <fstream>
#include <cstring>
using namespace std;

struct student {
  int num;
  char name[20];
  float score;
};

int main() {
  student stud[5] = {
    1001, "Li", 85.0, 1002, "Fun", 97.5,
    1004, "Wang", 54.0, 1006, "Tan", 76.5,
    1010, "ling", 96.0
  };
  
  // 使用 fstream 允许同时读写
  fstream iofile("stud.dat", ios::in | ios::out | ios::binary);
  if (!iofile) {
    cerr << "open error!" << endl;
    return 1;
  }

  // 1. 首先将所有学生数据写入文件
  for (int i = 0; i < 5; i++) {
    iofile.write((char*)&stud[i], sizeof(stud[i]));
  }

  // 2. 随机读取：读取第1个和第3个学生的数据
  cout << "--- Read Odd Students ---" << endl;
  for(int i = 0; i < 5; i += 2) {
    // seekg用于移动输入文件指针
    iofile.seekg(i * sizeof(student), ios::beg); 
    student temp;
    iofile.read((char*)&temp, sizeof(temp));
    cout << "Num:" << temp.num << " Name:" << temp.name << " Score:" << temp.score << endl;
  }

  // 3. 修改数据：更新第3个学生（索引为2）的信息
  student updateRec = {1012, "Wu", 60.0};
  
  // seekp用于移动输出文件指针
  iofile.seekp(2 * sizeof(student), ios::beg); // 定位到第三个元素的开头
  iofile.write((char*)&updateRec, sizeof(updateRec));

  // 4. 验证修改：从头读取所有数据
  cout << "--- Verify Update ---" << endl;
  iofile.seekg(0, ios::beg); // 重置输入指针
  for(int i = 0; i < 5; i++) {
    iofile.read((char*)&stud[i], sizeof(stud[i]));
    cout << "Num:" << stud[i].num << " Name:" << stud[i].name 
         << " Score:" << stud[i].score << endl;
  }

  iofile.close();
  return 0;
}