用c++设计一个简单的学生成绩管理系统。设有学生成绩文件student.txt，每位学生成绩信息包含学号（num）、姓名（name）、性别（sex）、出生日期（birthday，包括年、月、日，分别表示为year,month,day）、3门功课的成绩（高数、C语言、大学物理）、总分和平均分。设计程序，要求从文件student.txt中读出学生成绩表，之后参对成绩表进行如下操作： （1）建立学生成绩信息结构数组（从student.txt读入数据） （2）显示。即输出出所有成绩。 （3）添加。可添加一条或多条记录。 （4）排序。分为两个方面，按学号递增排序和按总分递减排序。 （5）查找。分为三方面：按学号查找、按姓名查找和按分数查找。 （6）计算。计算各门课的平均分数。 （7）保存。退出程序前将结果保存到studeng.txt。 2）要求 （1）设计一个简单的界面 （2）每一个功能设为一个模块

这是一个比较大的项目，需要涉及到读写文件、结构体、数组、排序、查找等多个知识点，下面是一个简单的实现：

#include <iostream>
#include <fstream>
#include <string>
#include <algorithm>

using namespace std;

struct Student {
    string num;
    string name;
    string sex;
    int year;
    int month;
    int day;
    int math;
    int cprog;
    int phy;
    int total;
    float avg;
};

int count = 0;
Student students[100];

void readData() {
    ifstream fin("student.txt");
    while (!fin.eof()) {
        fin >> students[count].num;
        fin >> students[count].name;
        fin >> students[count].sex;
        fin >> students[count].year;
        fin >> students[count].month;
        fin >> students[count].day;
        fin >> students[count].math;
        fin >> students[count].cprog;
        fin >> students[count].phy;
        students[count].total = students[count].math + students[count].cprog + students[count].phy;
        students[count].avg = students[count].total / 3.0;
        count++;
    }
    fin.close();
}

void display() {
    cout << "学号\t姓名\t性别\t出生日期\t高数\tC语言\t大学物理\t总分\t平均分" << endl;
    for (int i = 0; i < count; i++) {
        cout << students[i].num << "\t" << students[i].name << "\t" << students[i].sex << "\t" << students[i].year << "-" << students[i].month << "-" << students[i].day << "\t";
        cout << students[i].math << "\t" << students[i].cprog << "\t" << students[i].phy << "\t" << students[i].total << "\t" << students[i].avg << endl;
    }
}

void add() {
    cout << "请输入要添加的学生个数：";
    int num;
    cin >> num;
    for (int i = 0; i < num; i++) {
        cout << "请输入第" << i + 1 << "个学生的信息：" << endl;
        cout << "学号：";
        cin >> students[count].num;
        cout << "姓名：";
        cin >> students[count].name;
        cout << "性别：";
        cin >> students[count].sex;
        cout << "出生日期（格式为YYYY MM DD）：";
        cin >> students[count].year >> students[count].month >> students[count].day;
        cout << "高数成绩：";
        cin >> students[count].math;
        cout << "C语言成绩：";
        cin >> students[count].cprog;
        cout << "大学物理成绩：";
        cin >> students[count].phy;
        students[count].total = students[count].math + students[count].cprog + students[count].phy;
        students[count].avg = students[count].total / 3.0;
        count++;
    }
}

bool cmpNum(Student a, Student b) {
    return a.num < b.num;
}

bool cmpTotal(Student a, Student b) {
    return a.total > b.total;
}

void sort() {
    int choice;
    cout << "请选择排序方式（1.按学号递增排序 2.按总分递减排序）：";
    cin >> choice;
    if (choice == 1) {
        std::sort(students, students + count, cmpNum);
    } else if (choice == 2) {
        std::sort(students, students + count, cmpTotal);
    } else {
        cout << "输入有误，请重新选择！" << endl;
        sort();
    }
}

void searchNum() {
    string num;
    cout << "请输入要查找的学生学号：";
    cin >> num;
    for (int i = 0; i < count; i++) {
        if (students[i].num == num) {
            cout << "学号\t姓名\t性别\t出生日期\t高数\tC语言\t大学物理\t总分\t平均分" << endl;
            cout << students[i].num << "\t" << students[i].name << "\t" << students[i].sex << "\t" << students[i].year << "-" << students[i].month << "-" << students[i].day << "\t";
            cout << students[i].math << "\t" << students[i].cprog << "\t" << students[i].phy << "\t" << students[i].total << "\t" << students[i].avg << endl;
            return;
        }
    }
    cout << "未找到该学生！" << endl;
}

void searchName() {
    string name;
    cout << "请输入要查找的学生姓名：";
    cin >> name;
    for (int i = 0; i < count; i++) {
        if (students[i].name == name) {
            cout << "学号\t姓名\t性别\t出生日期\t高数\tC语言\t大学物理\t总分\t平均分" << endl;
            cout << students[i].num << "\t" << students[i].name << "\t" << students[i].sex << "\t" << students[i].year << "-" << students[i].month << "-" << students[i].day << "\t";
            cout << students[i].math << "\t" << students[i].cprog << "\t" << students[i].phy << "\t" << students[i].total << "\t" << students[i].avg << endl;
            return;
        }
    }
    cout << "未找到该学生！" << endl;
}

void searchScore() {
    int score;
    cout << "请输入要查找的成绩：";
    cin >> score;
    bool found = false;
    cout << "学号\t姓名\t性别\t出生日期\t高数\tC语言\t大学物理\t总分\t平均分" << endl;
    for (int i = 0; i < count; i++) {
        if (students[i].math == score || students[i].cprog == score || students[i].phy == score) {
            cout << students[i].num << "\t" << students[i].name << "\t" << students[i].sex << "\t" << students[i].year << "-" << students[i].month << "-" << students[i].day << "\t";
            cout << students[i].math << "\t" << students[i].cprog << "\t" << students[i].phy << "\t" << students[i].total << "\t" << students[i].avg << endl;
            found = true;
        }
    }
    if (!found) {
        cout << "未找到该成绩！" << endl;
    }
}

void calcAvg() {
    int mathSum = 0;
    int cprogSum = 0;
    int phySum = 0;
    for (int i = 0; i < count; i++) {
        mathSum += students[i].math;
        cprogSum += students[i].cprog;
        phySum += students[i].phy;
    }
    float mathAvg = mathSum / (float)count;
    float cprogAvg = cprogSum / (float)count;
    float phyAvg = phySum / (float)count;
    cout << "高数平均分：" << mathAvg << endl;
    cout << "C语言平均分：" << cprogAvg << endl;
    cout << "大学物理平均分：" << phyAvg << endl;
}

void saveData() {
    ofstream fout("student.txt");
    for (int i = 0; i < count; i++) {
        fout << students[i].num << " ";
        fout << students[i].name << " ";
        fout << students[i].sex << " ";
        fout << students[i].year << " ";
        fout << students[i].month << " ";
        fout << students[i].day << " ";
        fout << students[i].math << " ";
        fout << students[i].cprog << " ";
        fout << students[i].phy << endl;
    }
    fout.close();
}

void showMenu() {
    cout << "1. 显示成绩" << endl;
    cout << "2. 添加学生成绩" << endl;
    cout << "3. 排序" << endl;
    cout << "4. 查找" << endl;
    cout << "5. 计算平均分" << endl;
    cout << "6. 保存数据" << endl;
    cout << "0. 退出程序" << endl;
}

int main() {
    readData();
    while (true) {
        showMenu();
        int choice;
        cout << "请输入要执行的操作：";
        cin >> choice;
        if (choice == 0) {
            break;
        } else if (choice == 1) {
            display();
        } else if (choice == 2) {
            add();
        } else if (choice == 3) {
            sort();
        } else if (choice == 4) {
            int searchChoice;
            cout << "请选择查找方式（1.按学号查找 2.按姓名查找 3.按分数查找）：";
            cin >> searchChoice;
            if (searchChoice == 1) {
                searchNum();
            } else if (searchChoice == 2) {
                searchName();
            } else if (searchChoice == 3) {
                searchScore();
            } else {
                cout << "输入有误，请重新选择！" << endl;
            }
        } else if (choice == 5) {
            calcAvg();
        } else if (choice == 6) {
            saveData();
        } else {
            cout << "输入有误，请重新选择！" << endl;
        }
    }
    return 0;
}

这个程序中，我们首先定义了一个结构体 `Student`，表示学生的基本信息和成绩，然后用一个数组 `students` 存储所有学生的信息。`readData()` 函数从文件中读取数据，`display()` 函数显示所有成绩，`add()` 函数添加一条或多条记录，`sort()` 函数按照学号或总分进行排序，`searchNum()`、`searchName()` 和 `searchScore()` 函数分别按照学号、姓名和分数进行查找，`calcAvg()` 函数计算各门课程的平均分数，`saveData()` 函数将结果保存到文件中。

在主函数中，我们通过一个循环来不断读取用户的输入，根据用户输入的操作来调用相应的函数。