如何将临时对象存储在地图中，以访问其创建上下文之外的对象

Question

我对 C++ 比较陌生。我试图分解问题。

所有代码也在这里：https://onlinegdb.com/wAmLAONkF（可以执行）

基本上，我有一个创建临时对象的函数。对象被推送到 std::vector:

vector<Student> GetStudents(int some_params)
{
    vector<Student> students;
    //logic where students are returned 
    Student s1 { "John", 42 };
    Student s2 { "Bill", 12 };
    students.push_back(s1);
    students.push_back(s2);
    
    return students;
}

为了维护对象，它们存储在外部 std::map。

map 通过指针将每个 Student 存储在 vector<Student*> 中（必须是 vector<Student*>！）：

map<int, vector<Student*>> m {};
    
{ //start of new context, other class context in another file, map is passed by reference
    int some_params = 5;
    vector<Student> students = GetStudents(some_params);

为了将它们存储在地图中，我将引用复制到新的 vector:

    vector<Student*> tmpV;
    for(auto & student : students)
        tmpV.push_back( &student);

    //store them in the map
    m.emplace(some_params, tmpV);

}//leave context

//data loss - Students are lost

我知道，问题是因为指向对象的指针存储在本地向量中。在我离开上下文后，指针没有指向有效的地方。

但是我该如何解决呢？

Answer 1

由GetStudents()编辑的vector<Student> return需要存储在与[=14=相同范围（或更高范围）的变量中] 指的是它的 Students，例如：

map<int, vector<Student*>> m;
vector<Student> students; // <-- move it up here!

{ //start of new context

    int some_params = 5;
    students = GetStudents(some_params); 

    vector<Student*> tmpV;
    for(auto & student : students)
        tmpV.push_back( &student);

    //store them in the map
    m.emplace(some_params, tmpV);

} //leave context

// no data loss - Students are valid!

否则，GetStudents() 将不得不 return 一个 vector<Student*>，其中每个 Student 都是通过 new 动态创建的，然后通过 new 销毁delete 在 map 完成后使用它们：

vector<Student*> GetStudents(int some_params)
{
    vector<Student*> students;
    //logic where students are returned 
    students.push_back(new Student{ "John", 42 });
    students.push_back(new Student{ "Bill", 12 });
    
    return students;
}

...

map<int, vector<Student*>> m;

{ //start of new context

    int some_params = 5;
    vector<Student*> students = GetStudents(some_params); 

    //store them in the map
    m.emplace(some_params, students);

} //leave context

// no data loss - Students are valid!

...

for(auto &entry : m) {
    for (auto *student : entry.second) {
        delete student;
    }
}

理想情况下，您的 map 应该存储 vector<Student> 或 vector<unique_ptr<Student>>，但您的要求“ 必须是 vector<Student*>！" 阻止了这种情况。

如何将临时对象存储在地图中，以访问其创建上下文之外的对象

How do I store the tmp object in a map, to acess the object outside of it's creation context

c++

pointers