求解答整行排序有关问题

#include <map>
#include <string>
#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;

// 学生类
class Student
{
private:
    int id;                    // 学号
    string name;            // 姓名
    double chinese;            // 语文成绩
    double english;            // 英语成绩
    double math;            // 数学成绩
    double computer;        // 计算机成绩
public:
    Student()
    {
        id = -1;
        name = "";
        chinese = 0.0;
        english = 0.0;
        math = 0.0;
        computer = 0.0;
    }

    Student(int id, string name, double chinese, double english, double math, double computer)
    {
        this->id = id;
        this->name = name;
        this->chinese = chinese;
        this->english = english;
        this->math = math;
        this->computer = computer;
    }

    int get_id() const
    {
        return id;
    }

    string get_name() const
    {
        return name;
    }

    void set_name(string name)
    {
        this->name = name;
    }

    double get_chinese() const
    {
        return chinese;
    }

    void set_chinese(double chinese)
    {
        this->chinese = chinese;
    }

    double get_english() const
    {
        return english;
    }

    void set_english(double english)
    {
        this->english = english;
    }

    double get_math() const
    {
        return math;
    }

    void set_math(double math)
    {
        this->math = math;
    }

    double get_computer() const
    {
        return computer;
    }

    void set_computer(double computer)
    {
        this->computer = computer;
    }

    // 获取总分
    double get_sum() const
    {
        return (chinese + english + math + computer);
    }

    void show_info()
    {
        cout << id << "\t" << name << "\t" << chinese << "\t" << english << "\t" << math << "\t" << computer << "\t" << get_sum() << endl;
    }
};


// 按语文分数排序
bool compare_chinese(const Student& stu1, const Student& stu2)
{
    return (stu1.get_chinese() < stu2.get_chinese());
}

// 按英语分数排序
bool compare_english(const Student& stu1, const Student& stu2)
{
    return (stu1.get_english() < stu2.get_english());
}

// 按数学分数排序
bool compare_math(const Student& stu1, const Student& stu2)
{
    return (stu1.get_math() < stu2.get_math());
}

// 按计算机分数排序
bool compare_computer(const Student& stu1, const Student& stu2)
{
    return (stu1.get_computer() < stu2.get_computer());
}

// 按总分排序
bool compare_sum(const Student& stu1, const Student& stu2)
{
    return (stu1.get_sum() < stu2.get_sum());
}


// 数据库类。包含增、删、改、查等操作
class Database
{
private:
    // 模拟数据库。学号作为主键
    map<int, Student> students;
public:
    // 增加一个学生
    void add_student(Student student)
    {
        students.insert(pair<int, Student>(student.get_id(), student));
    }

    // 删除一个学生
    void delete_student(int id)
    {
        map<int, Student>::iterator iter;
        iter = students.find(id);
        students.erase(iter);
    }

    // 更改一个学生
    void update_student(Student student)
    {
        map<int, Student>::iterator iter;
        iter = students.find(student.get_id());
        if(iter != students.end())
        {
            iter->second.set_name(student.get_name());
            iter->second.set_chinese(student.get_chinese());
            iter->second.set_english(student.get_english());
            iter->second.set_math(student.get_math());
            iter->second.set_computer(student.get_computer());
        }
        else
        {
            cout << "This student does not exist in database, update failed." << endl;
        }
    }

    // 根据学号查询学生
    Student select_student(int id)
    {
        map<int, Student>::iterator iter;
        iter = students.find(id);
        return Student(iter->second.get_id(), iter->second.get_name(), 
                       iter->second.get_chinese(), iter->second.get_english(),
                       iter->second.get_math(), iter->second.get_computer());
    }

    // 计算某门课程的平均成绩
    double average(string course)
    {
        double sum = 0.0;
        for(map<int, Student>::iterator iter = students.begin(); iter != students.end(); ++iter)
        {
            if(course == "chinese")
            {
                sum += iter->second.get_chinese();
            }

            if(course == "english")
            {
                sum += iter->second.get_english();
            }

            if(course == "math")
            {
                sum += iter->second.get_math();
            }

            if(course == "computer")
            {
                sum += iter->second.get_computer();
            }
        }

        return sum / students.size();
    }

    vector<Student> sortBy(string course)
    {
        vector<Student> vec;

        for(map<int, Student>::iterator iter = students.begin(); iter != students.end(); ++iter)
        {
            vec.push_back(iter->second);
        }
        if(course == "chinese")
            sort(vec.begin(), vec.end(), compare_chinese);
        if(course == "english")
            sort(vec.begin(), vec.end(), compare_english);
        if(course == "math")
            sort(vec.begin(), vec.end(), compare_math);
        if(course == "computer")
            sort(vec.begin(), vec.end(), compare_computer);

        return vec;
    }

    // 获取数据库中的所有学生
    vector<Student> select_all()
    {
        vector<Student> vec;
        map<int, Student>::iterator iter;
        for(iter = students.begin(); iter != students.end(); ++iter)
        {
            vec.push_back(iter->second);
        }

        return vec;
    }
};

int main(int argc, char* argv[])
{
    Student student1(1, "Andy", 100.0, 100.0, 100.0, 100.0);
    Student student2(2, "Bob", 99.0, 99.0, 99.0, 99.0);
    Student student3(3, "Chris", 98.0, 98.0, 98.0, 98.0);

    Database db;

    // 增加数据
    db.add_student(student1);
    db.add_student(student2);
    db.add_student(student3);

    // 显示增加数据后，数据库中的所有数据记录
    vector<Student> vec = db.select_all();
    for(vector<Student>::iterator iter = vec.begin(); iter != vec.end(); ++iter)
    {
        iter->show_info();
    }

    cout << "++++++++++++++++++++++++++++++++++++++++++++++++++++" << endl;

    // 删除student2
    db.delete_student(student2.get_id());
    // 显示删除数据后，数据库中的所有数据记录
    vec = db.select_all();
    for(vector<Student>::iterator iter = vec.begin(); iter != vec.end(); ++iter)
    {
        iter->show_info();
    }

    cout << "++++++++++++++++++++++++++++++++++++++++++++++++++++" << endl;

    // 更改数据
    Student update_student(1, "Andrew", 120.0, 120.0, 120.0, 120.0);
    db.update_student(update_student);

    // 显示更改数据后，数据库中的所有数据记录
    vec = db.select_all();
    for(vector<Student>::iterator iter = vec.begin(); iter != vec.end(); ++iter)
    {
        iter->show_info();
    }

    cout << "++++++++++++++++++++++++++++++++++++++++++++++++++++" << endl;

    // 查询数据
    Student student = db.select_student(1);

    // 显示查询结果
    student.show_info();

    cout << "++++++++++++++++++++++++++++++++++++++++++++++++++++" << endl;
    // 平均成绩
    cout << "average of chinese:\t" << db.average("chinese") << endl;    // 语文
    cout << "average of english:\t" << db.average("english") << endl;    // 英语
    cout << "average of maths:\t" << db.average("math") << endl;        // 数学
    cout << "average of computer:\t" << db.average("computer") << endl;    // 计算机

    cout << "++++++++++++++++++++++++++++++++++++++++++++++++++++" << endl;
    // 俺总分排名
    vector<Student> sorted_vec = db.sortBy("");
    for(vector<Student>::iterator iter = sorted_vec.begin(); iter != sorted_vec.end(); ++iter)
    {
        iter->show_info();
    }

    return 0;
}