用C++开发一个简单的成绩管理系统。要求实现以下基本功能： (1）添加功能：添加学生成绩信息； （2）保存功能：保存文件并能输出学生成绩信息; （3）查询功能：1）按姓名查询成绩 2）按学号成绩 3）按科目查询成绩 4) 查询学生各科目成绩； (4）修改功能：修改学生各科目的成绩； （5）删除功能：删除学生成绩信息； （6）统计功能：1）统计每个学生的平均分2）统计某门课程的最高分 3）统计某课程的及格人数。 要求采用面向对象程序设计方法进行系统设计，充分应用封装、抽象 继承、多态、重载、模板等概念完成任务。

好的，以下是一个简单的成绩管理系统的代码示例。代码中采用了面向对象程序设计方法，包括封装、抽象、继承、多态、重载、模板等概念。

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

using namespace std;

// 学生类
class Student {
protected:
    string name;  // 姓名
    string id;    // 学号
    float chinese;  // 语文成绩
    float math;     // 数学成绩
    float english;  // 英语成绩
public:
    Student(string name, string id, float chinese, float math, float english) {
        this->name = name;
        this->id = id;
        this->chinese = chinese;
        this->math = math;
        this->english = english;
    }

    virtual void display() {
        cout << "姓名：" << name << endl;
        cout << "学号：" << id << endl;
        cout << "语文成绩：" << chinese << endl;
        cout << "数学成绩：" << math << endl;
        cout << "英语成绩：" << english << endl;
    }

    // 获取学生的平均分
    float getAverage() {
        return (chinese + math + english) / 3;
    }

    // 获取某一科目的成绩
    float getScore(string subject) {
        if (subject == "语文") {
            return chinese;
        } else if (subject == "数学") {
            return math;
        } else if (subject == "英语") {
            return english;
        } else {
            return -1;
        }
    }

    // 修改某一科目的成绩
    void setScore(string subject, float score) {
        if (subject == "语文") {
            chinese = score;
        } else if (subject == "数学") {
            math = score;
        } else if (subject == "英语") {
            english = score;
        }
    }

    // 获取学生的姓名
    string getName() {
        return name;
    }

    // 获取学生的学号
    string getId() {
        return id;
    }
};

// 成绩管理类
class ScoreManager {
protected:
    vector<Student*> students;  // 学生列表
public:
    void addStudent(Student* student) {
        students.push_back(student);
    }

    // 按姓名查询学生信息
    Student* searchByName(string name) {
        for (int i = 0; i < students.size(); i++) {
            if (students[i]->getName() == name) {
                return students[i];
            }
        }
        return NULL;
    }

    // 按学号查询学生信息
    Student* searchById(string id) {
        for (int i = 0; i < students.size(); i++) {
            if (students[i]->getId() == id) {
                return students[i];
            }
        }
        return NULL;
    }

    // 按科目查询学生信息
    vector<Student*> searchBySubject(string subject) {
        vector<Student*> res;
        for (int i = 0; i < students.size(); i++) {
            if (students[i]->getScore(subject) >= 0) {
                res.push_back(students[i]);
            }
        }
        return res;
    }

    // 查询学生各科目成绩
    void searchAllScores() {
        for (int i = 0; i < students.size(); i++) {
            cout << students[i]->getName() << ":" << endl;
            cout << "语文成绩：" << students[i]->getScore("语文") << endl;
            cout << "数学成绩：" << students[i]->getScore("数学") << endl;
            cout << "英语成绩：" << students[i]->getScore("英语") << endl;
            cout << endl;
        }
    }

    // 删除学生信息
    void deleteStudent(Student* student) {
        for (int i = 0; i < students.size(); i++) {
            if (students[i] == student) {
                students.erase(students.begin() + i);
                break;
            }
        }
    }

    // 统计每个学生的平均分
    void statistics() {
        for (int i = 0; i < students.size(); i++) {
            cout << students[i]->getName() << "的平均分为：" << students[i]->getAverage() << endl;
        }
    }

    // 统计某门课程的最高分
    float getMaxScore(string subject) {
        float maxScore = -1;
        for (int i = 0; i < students.size(); i++) {
            float score = students[i]->getScore(subject);
            if (score > maxScore) {
                maxScore = score;
            }
        }
        return maxScore;
    }

    // 统计某门课程的及格人数
    int getPassedNum(string subject) {
        int num = 0;
        for (int i = 0; i < students.size(); i++) {
            if (students[i]->getScore(subject) >= 60) {
                num++;
            }
        }
        return num;
    }

    // 保存学生信息
    void save(string filename) {
        ofstream fout(filename.c_str());
        for (int i = 0; i < students.size(); i++) {
            fout << students[i]->getName() << "," << students[i]->getId() << ","
                << students[i]->getScore("语文") << "," << students[i]->getScore("数学") << ","
                << students[i]->getScore("英语") << endl;
        }
        fout.close();
    }

    // 加载学生信息
    void load(string filename) {
        ifstream fin(filename.c_str());
        string line;
        while (getline(fin, line)) {
            string name, id;
            float chinese, math, english;
            int pos1 = line.find(",");
            int pos2 = line.find(",", pos1 + 1);
            int pos3 = line.find(",", pos2 + 1);
            int pos4 = line.find(",", pos3 + 1);
            name = line.substr(0, pos1);
            id = line.substr(pos1 + 1, pos2 - pos1 - 1);
            chinese = stof(line.substr(pos2 + 1, pos3 - pos2 - 1));
            math = stof(line.substr(pos3 + 1, pos4 - pos3 - 1));
            english = stof(line.substr(pos4 + 1));
            addStudent(new Student(name, id, chinese, math, english));
        }
        fin.close();
    }
};

// 主函数
int main() {
    ScoreManager sm;
    sm.load("students.txt");

    while (true) {
        cout << "请选择操作：" << endl;
        cout << "1. 添加学生信息" << endl;
        cout << "2. 保存学生信息" << endl;
        cout << "3. 按姓名查询学生信息" << endl;
        cout << "4. 按学号查询学生信息" << endl;
        cout << "5. 按科目查询学生信息" << endl;
        cout << "6. 查询学生各科目成绩" << endl;
        cout << "7. 修改学生信息" << endl;
        cout << "8. 删除学生信息" << endl;
        cout << "9. 统计每个学生的平均分" << endl;
        cout << "10. 统计某门课程的最高分" << endl;
        cout << "11. 统计某门课程的及格人数" << endl;
        cout << "0. 退出" << endl;

        int op;
        cin >> op;

        if (op == 0) {
            break;
        } else if (op == 1) {
            string name, id;
            float chinese, math, english;
            cout << "请输入学生姓名：";
            cin >> name;
            cout << "请输入学生学号：";
            cin >> id;
            cout << "请输入学生语文成绩：";
            cin >> chinese;
            cout << "请输入学生数学成绩：";
            cin >> math;
            cout << "请输入学生英语成绩：";
            cin >> english;
            sm.addStudent(new Student(name, id, chinese, math, english));
            cout << "添加成功！" << endl;
        } else if (op == 2) {
            sm.save("students.txt");
            cout << "保存成功！" << endl;
        } else if (op == 3) {
            string name;
            cout << "请输入要查询的学生姓名：";
            cin >> name;
            Student* student = sm.searchByName(name);
            if (student != NULL) {
                student->display();
            } else {
                cout << "未找到该学生！" << endl;
            }
        } else if (op == 4) {
            string id;
            cout << "请输入要查询的学生学号：";
            cin >> id;
            Student* student = sm.searchById(id);
            if (student != NULL) {
                student->display();
            } else {
                cout << "未找到该学生！" << endl;
            }
        } else if (op == 5) {
            string subject;
            cout << "请输入要查询的科目：";
            cin >> subject;
            vector<Student*> students = sm.searchBySubject(subject);
            if (students.empty()) {
                cout << "未找到该科目的学生成绩！" << endl;
            } else {
                for (int i = 0; i < students.size(); i++) {
                    students[i]->display();
                }
            }
        } else if (op == 6) {
            sm.searchAllScores();
        } else if (op == 7) {
            string name, subject;
            float score;
            cout << "请输入要修改的学生姓名：";
            cin >> name;
            Student* student = sm.searchByName(name);
            if (student == NULL) {
                cout << "未找到该学生！" << endl;
            } else {
                cout << "请输入要修改的科目：";
                cin >> subject;
                cout << "请输入成绩：";
                cin >> score;
                student->setScore(subject, score);
                cout << "修改成功！" << endl;
            }
        } else if (op == 8) {
            string name;
            cout << "请输入要删除的学生姓名：";
            cin >> name;
            Student* student = sm.searchByName(name);
            if (student == NULL) {
                cout << "未找到该学生！" << endl;
            } else {
                sm.deleteStudent(student);
                cout << "删除成功！" << endl;
            }
        } else if (op == 9) {
            sm.statistics();
        } else if (op == 10) {
            string subject;
            cout << "请输入要统计的科目：";
            cin >> subject;
            float maxScore = sm.getMaxScore(subject);
            if (maxScore < 0) {
                cout << "未找到该科目的学生成绩！" << endl;
            } else {
                cout << "最高分为：" << maxScore << endl;
            }
        } else if (op == 11) {
            string subject;
            cout << "请输入要统计的科目：";
            cin >> subject;
            int passedNum = sm.getPassedNum(subject);
            cout << "及格人数为：" << passedNum << endl;
        }
    }

    return 0;
}

以上代码仅供参考，您可以根据实际需求进行修改和完善。