学生成绩管理系统 设计一个学生成绩管理程序，实现按班级完成对学生成绩信息的录入和修改，并用文件保存。实现按班级输出学生的成绩单；实现按学号和姓名进行查询，按平均成绩进行排序功能。C++

好的，以下是一个简单的 C++ 代码示例：

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

using namespace std;

// 学生类
class Student {
public:
    Student(string id, string name, double grade) :
        m_id(id), m_name(name), m_grade(grade) {}

    string getId() const {
        return m_id;
    }

    string getName() const {
        return m_name;
    }

    double getGrade() const {
        return m_grade;
    }

    void setGrade(double grade) {
        m_grade = grade;
    }

private:
    string m_id;   // 学号
    string m_name; // 姓名
    double m_grade; // 成绩
};

// 班级类
class Class {
public:
    Class(string name) :
        m_name(name) {}

    string getName() const {
        return m_name;
    }

    void addStudent(const Student& student) {
        m_students.push_back(student);
    }

    vector<Student>& getStudents() {
        return m_students;
    }

private:
    string m_name; // 班级名称
    vector<Student> m_students; // 学生列表
};

// 成绩管理系统类
class GradeManager {
public:
    void addStudent(const string& className, const Student& student) {
        bool found = false;
        for (auto& c : m_classes) {
            if (c.getName() == className) {
                c.addStudent(student);
                found = true;
                break;
            }
        }
        if (!found) {
            Class newClass(className);
            newClass.addStudent(student);
            m_classes.push_back(newClass);
        }
    }

    void saveToFile(const string& filename) const {
        ofstream outfile(filename);
        if (outfile.is_open()) {
            for (auto& c : m_classes) {
                outfile << c.getName() << endl;
                for (auto& s : c.getStudents()) {
                    outfile << s.getId() << " " << s.getName() << " " << s.getGrade() << endl;
                }
            }
            outfile.close();
        }
    }

    void loadFromFile(const string& filename) {
        ifstream infile(filename);
        if (infile.is_open()) {
            string className;
            while (getline(infile, className)) {
                Class newClass(className);
                string line;
                while (getline(infile, line)) {
                    string id, name;
                    double grade;
                    istringstream iss(line);
                    iss >> id >> name >> grade;
                    Student newStudent(id, name, grade);
                    newClass.addStudent(newStudent);
                }
                m_classes.push_back(newClass);
            }
            infile.close();
        }
    }

    void printClassGrades(const string& className) const {
        for (auto& c : m_classes) {
            if (c.getName() == className) {
                cout << c.getName() << endl;
                for (auto& s : c.getStudents()) {
                    cout << s.getId() << " " << s.getName() << " " << s.getGrade() << endl;
                }
                break;
            }
        }
    }

    void searchStudent(const string& keyword) const {
        for (auto& c : m_classes) {
            for (auto& s : c.getStudents()) {
                if (s.getId() == keyword || s.getName() == keyword) {
                    cout << c.getName() << " " << s.getId() << " " << s.getName() << " " << s.getGrade() << endl;
                }
            }
        }
    }

    void sortByAverageGrade() {
        vector<pair<string, double>> classGrades;
        for (auto& c : m_classes) {
            double sum = 0.0;
            for (auto& s : c.getStudents()) {
                sum += s.getGrade();
            }
            double averageGrade = sum / c.getStudents().size();
            classGrades.push_back(make_pair(c.getName(), averageGrade));
        }
        sort(classGrades.begin(), classGrades.end(), [](const pair<string, double>& a, const pair<string, double>& b) {
            return a.second > b.second;
        });
        for (auto& cg : classGrades) {
            cout << cg.first << " " << cg.second << endl;
        }
    }

private:
    vector<Class> m_classes; // 班级列表
};

int main() {
    // 使用示例
    GradeManager gm;

    // 添加学生信息
    gm.addStudent("class1", Student("001", "张三", 80));
    gm.addStudent("class1", Student("002", "李四", 90));
    gm.addStudent("class2", Student("003", "王五", 70));

    // 保存学生信息到文件
    gm.saveToFile("grades.txt");

    // 从文件中加载学生信息
    gm.loadFromFile("grades.txt");

    // 按班级输出学生成绩单
    gm.printClassGrades("class1");

    // 按学号和姓名查询学生信息
    gm.searchStudent("001");
    gm.searchStudent("李四");

    // 按平均成绩进行排序
    gm.sortByAverageGrade();

    return 0;
}

需要注意的是，如果您使用的是 C++11 以前的版本，可能需要将 lambda 表达式改写成函数对象的形式。