如何自定义map模板排序

我先定义了一个Stu类：

class Stu {
private:
    string name;
    int id;
    string sex;
    int type;
    float mathscore;
    float engscore;
    float cppscore;
public:
    Stu()=default;
    Stu(string n,int i,string s,float math,float eng,float cpp):name(n),id(i),sex(s),mathscore(math),engscore(eng),cppscore(cpp){};
    ~Stu();

主程序中定义了map<int,Stu>Stu_map;
默认是按id排序，如何实现按mathscore（数学成绩）排序并输出？

map默认是使用key排序的，因为mathScore是value中的值，需要借助vector和sort函数来实现对map的排序。
调用add函数，在map中添加数据及排序后的运行结果：

代码如下：

#include <iostream>
#include <string>
#include <map>
#include <vector>
#include <algorithm>
using namespace std;
class Stu 
{
private:
    string name;
    int id;
    string sex;
    int type;
    float mathscore;
    float engscore;
    float cppscore;
public:
    Stu(){}
    Stu(string n,int i,string s,float math,float eng,float cpp):name(n),id(i),sex(s),mathscore(math),engscore(eng),cppscore(cpp){}
    ~Stu(){}
    //暂时写这几个
    string getname(){return name;}
    int get_id(){return id;};
    float getmathscore() const {return mathscore;}  //不要漏了这里的const 否则cmp函数会报错


    int get_type(){return type;}
    void set_type(int t){type = t;}

    void change_id(int d){id = d;}
    void change_name(char* nn){name = nn;}
    void change_sex(char* sx){sex = sx;}

    void set_mathscore(float f){mathscore = f;}
    void set_engscore(float f){engscore = f;}
    void set_cppscore(float f){cppscore = f;}
};

map<int,Stu>Stu_map;  //声明全局变量，因为add函数中没有参数，所以把这个作为全局变量


bool cmp(const pair<int,Stu> p1,const pair<int,Stu> p2)//要用常数，不然编译错误 
{
    return p1.second.getmathscore() > p2.second.getmathscore();
}

istream &operator>>(istream &is,Stu &stu)
{
    char temp[50];
    double te;
    is.getline(temp,50);
    cout<<"请输入学号："<<endl;
    is.getline(temp,50);
    stu.change_id(atoi(temp)); 
    cout<<"请输入姓名："<<endl; 
    is.getline(temp,20);
    stu.change_name(temp);
    cout<<"请输入性别："<<endl;
    is.getline(temp,10);
    if(!strcmp(temp,"男")||!strcmp(temp,"女"))
    {
        stu.change_sex(temp);
    } else
    {
        cout<<"性别输入错误  Σ( ° △ °|||)︴"<<endl;
    }

    cout<<"科目成绩："<<endl;
    cout<<"输入数学成绩："<<endl;
    cin>>te;
    stu.set_mathscore(te);
    cout<<"输入英语成绩："<<endl;
    cin>>te;
    stu.set_engscore(te);
    cout<<"输入C++成绩："<<endl;
    cin>>te;
    stu.set_cppscore(te);
    return is;
}
void add_stu()
{
    int choice=0;
    Stu stu;
    cout<<"1.添加大一学生"<<endl;
    cout<<"2.添加大二学生"<<endl;
    cout<<"请输入你的选择："<<endl;
    cin>>choice;
    switch (choice) 
    {
    case 1: 
        {
            cin >> stu;
            stu.set_type(1);
            Stu_map.insert(map<int, Stu>::value_type(stu.get_id(), stu));
            cout<<"添加成功"<<endl;
            break;
        }

    case 2: 
        {

            cin >> stu;
            stu.set_type(2);
            Stu_map.insert(map<int, Stu>::value_type(stu.get_id(), stu));
            cout<<"添加成功"<<endl;
            break;
        }

    default:
        break;
    }
}




int main()
{
    //调用函数添加4个数据
    for(int i=0;i<4;i++)
        add_stu();
    map<int,Stu>::iterator it = Stu_map.begin();
    vector< pair<int,Stu> > arr;
    for (; it != Stu_map.end(); it++)
    {
        arr.push_back(make_pair(it->first,it->second) );
    }
    //排序
    sort(arr.begin(),arr.end(),cmp);
    //输出
    vector< pair<int,Stu> >::iterator is = arr.begin();
    for (; is != arr.end(); is++)
    {
        cout << is->first << " " << is->second.getname() << " " << is->second.getmathscore()<<endl;
    }
    return 0;
}

再用map<float, Stu>中间变量保存一份，然后输出？

你没有定义type变量， 这里就没考虑了。有用请采纳

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

class Stu {
private:
    string name;
    int id;
    string sex;
    int type;
    float mathscore;
    float engscore;
    float cppscore;

public:
    Stu() = default;
    Stu(string n, int i, string s, float math, float eng, float cpp) :name(n), id(i), sex(s), mathscore(math), engscore(eng), cppscore(cpp) {};
    ~Stu() {};
    float getMathScore () const;
    void showInfo();

};

float Stu::getMathScore() const{
    return mathscore;
}

void Stu::showInfo() {
    cout << "name: " << name << "   ";
    cout << "id: " << id << "   ";
    cout << "sex: " << sex << "   ";
    cout << "mathscore: " << mathscore << "   ";
    cout << "engscore: " << engscore << "   ";
    cout << "cppscore: " << cppscore << "   ";
    cout << endl;
}


/*
* 对自定义字典排序： 比较函数
* 对字典中的Value排序： map默认不可对Value排序， 因此得转换成Vecotr进行排序
*/

// 比较函数
bool cmp(const pair<int, Stu>& a, const pair<int, Stu>& b) {
    return a.second.getMathScore() > b.second.getMathScore();
}

void sortMap(map<int, Stu>& Stu_map) {
    vector<pair<int, Stu>> sortAsMathScore(Stu_map.begin(), Stu_map.end());
    sort(sortAsMathScore.begin(), sortAsMathScore.end(), cmp);//用sort排序

    for (vector<pair<int, Stu>>::iterator it = sortAsMathScore.begin(); it != sortAsMathScore.end(); it++) {
        it->second.showInfo();
    }
}



int main() {
    // 你自己定义的没有用到type参数
    Stu stu1 = Stu("小红", 1, "女",90.0f, 92.5f, 85.0f);
    Stu stu2 = Stu("小明", 2, "男", 94.0f, 92.5f, 85.0f);
    Stu stu3 = Stu("小军", 3, "男", 85.0f, 92.5f, 85.0f);

    map<int, Stu> Stu_map;
    Stu_map.insert(make_pair(1, stu1));
    Stu_map.insert(make_pair(2, stu2));
    Stu_map.insert(make_pair(3, stu3));


    sortMap(Stu_map);
}

#include <iostream>         
#include <map>
using namespace std;
class B {
    int n;
public:       
  
    B(int t = 0) :n(t) {}
    friend bool operator<(const B& lh, const B& rh);
  
   

};
bool operator<(const B& lh, const B& rh) {
    return lh.n < rh.n;
}
int main()
{
    map<B,string> m;
    m.insert(make_pair(B(3), "hello"));
    m.insert(make_pair(B(1), "foo"));

   
}

给你一个简单例子，参考一下

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