这个unordered_map错在哪里呢？

Question

#第一次尝试哈希结构，写了一个代码，立刻就错了！
代码如下：

#include <iostream> 
#include <unordered_map> 

// Data Structure to store a Student Information
struct Student 
{ 
    int id; 
    const char* name; 
    Student(int Id, const char* Name) : id(Id), name(Name) {}
}; 

// Function to print the student details 
void print_student(Student s) 
{ 
    std::cout << "ID: " << s.id << ", Name: " << s.name << std::endl; 
} 

// Driver Code 
int main() 
{ 
    // Creating a unordered map with Student structure 
    // as key and marks as value 
    std::unordered_map<Student, int> studentMarksMap; 

    // Inserting values in studentMarksMap 
    studentMarksMap.insert({ { 1, "Martin" }, 84}); 
    studentMarksMap.insert({ { 2, "Andrew" }, 87}); 
    studentMarksMap.insert({ { 3, "John" }, 98}); 

    // Traversing an unordered map 
    for (auto itr = studentMarksMap.begin(); itr != studentMarksMap.end(); itr++) 
    { 
        print_student(itr->first); 
        std::cout << "\tMarks: " << itr->second << std::endl; 
    } 

    return 0; 
} 

报错信息：

Answer 1

如楼上所说, C++无序容器对于非基本类型需要定义hash函数以及operator==, 补充一个方法, 对于不想用仿函数的可用lambda方法

#include <iostream>
#include <unordered_map>

// Data Structure to store a Student Information
struct Student
{
    Student(int Id, const char *Name)
        : id(Id)
        , name(Name)
    {}

    // Function to print the student details
    void print_student() const
    {
        std::cout << "ID: " << id << ", Name: " << name << std::endl;
    }

    auto operator==(const Student &rhs) const -> bool
    {
        if (this != &rhs)
        {
            return id == rhs.id && (strcmp(name, rhs.name) == 0);
        }
        return true;
    }

    [[nodiscard]] auto getid() const -> int
    {
        return id;
    }

    [[nodiscard]] auto getname() const -> const char *
    {
        return name;
    }

  private:
    int id = 0;
    const char *name = nullptr;
    friend std::hash<Student>;
};

// namespace std
//{
// template <>
// struct hash<Student>
//{
//     auto operator()(const Student &stu) const -> size_t
//     {
//         return std::hash<int>()(stu.id) ^
//                std::hash<std::string>()(std::string(stu.name));
//     }
// };
// } // namespace std

// Driver Code
int main()
{
    auto hash = [](Student stu) -> size_t {
        return std::hash<int>()(stu.getid()) ^
               std::hash<std::string>()(std::string(stu.getname()));
    };
    // Creating a unordered map with Student structure
    // as key and marks as value
    std::unordered_map<Student, int, decltype(hash)> studentMarksMap(10, hash);

    // Inserting values in studentMarksMap
    studentMarksMap.insert({{1, "Martin"}, 84});
    studentMarksMap.insert({{2, "Andrew"}, 87});
    studentMarksMap.insert({{3, "John"}, 98});

    // Traversing an unordered map
    for (auto &itr : studentMarksMap)
    {
        itr.first.print_student();
        std::cout << "\tMarks: " << itr.second << std::endl;
    }

    return 0;
}