关于#c++#的问题：假如map类m的key值是一个Student类，现在已经重载比较运算符了，如果给出name或者id或者age，该怎么根据给出的关键字来在m中进行哈希查找

Question

#include <iostream>
#include <string>
#include <map>
using namespace std;
class Student
{
public:
    string name;
    string id;
    int age;
    bool operator()(const Student& s1, const Student& s2)
    {
        if (s1.name == s2.name)
        {
            if (s1.id == s2.id)
            {
                return s1.age > s2.age;
            }
            else
            {
                return s1.id > s2.id;
            }
        }
        else
        {
            return s1.name > s2.name;
        }
    }
};
int main()
{
    map<Student, int>m;
    return 0;
}

假如map类m的key值是一个Student类，现在已经重载比较运算符了，如果给出name或者id或者age，该怎么根据给出的关键字来在m中进行哈希查找？

Answer 1

#include <iostream>
#include <string>
#include <map>
#include <algorithm>
using namespace std;
class Student
{
public:
    string name;
    string id;
    int age;
    bool operator()(const Student& s1, const Student& s2)
    {
        if (s1.name == s2.name)
        {
            if (s1.id == s2.id)
            {
                return s1.age > s2.age;
            }
            else
            {
                return s1.id > s2.id;
            }
        }
        else
        {
            return s1.name > s2.name;
        }
    }
};
// 定义比较函数
bool compareName(const pair<Student, int>& p, const string& name)
{
    return p.first.name == name;
}
bool compareId(const pair<Student, int>& p, const string& id)
{
    return p.first.id == id;
}
bool compareAge(const pair<Student, int>& p, const int& age)
{
    return p.first.age == age;
}
int main()
{
    map<Student, int>m;
    // 在map中添加元素
    Student s1 = { "Tom", "001", 20 };
    Student s2 = { "Jack", "002", 22 };
    Student s3 = { "Lucy", "003", 21 };
    m[s1] = 100;
    m[s2] = 90;
    m[s3] = 80;

    // 根据名字查找
    string name = "Jack";
    auto it1 = find_if(m.begin(), m.end(), [&](const pair<Student, int>& p) {return compareName(p, name); });
    if (it1 != m.end())
    {
        cout << "找到了，名字为" << it1->first.name << ",成绩为" << it1->second << endl;
    }
    else
    {
        cout << "未找到" << endl;
    }

    // 根据id查找
    string id = "002";
    auto it2 = find_if(m.begin(), m.end(), [&](const pair<Student, int>& p) {return compareId(p, id); });
    if (it2 != m.end())
    {
        cout << "找到了，id为" << it2->first.id << ",成绩为" << it2->second << endl;
    }
    else
    {
        cout << "未找到" << endl;
    }

    // 根据年龄查找
    int age = 21;
    auto it3 = find_if(m.begin(), m.end(), [&](const pair<Student, int>& p) {return compareAge(p, age); });
    if (it3 != m.end())
    {
        cout << "找到了，年龄为" << it3->first.age << ",成绩为" << it3->second << endl;
    }
    else
    {
        cout << "未找到" << endl;
    }
    return 0;
}