如何保持指向存储在向量中的结构的指针有效？

Question

I am learning C++ and I have trouble with pointers to structures stored in vector. The problem is that I need to keep the structure Student sorted twice. Once by student's id and another time by student's name, so it is easy to search the values in it. Because of this I created two vectors of pointers:

vector<Student *> sortedByID;    
vector<Student *> sortedByName;

The structure looks like this and I keep it in vector as well (even though it is probably not a good idea):

struct Student {
    int id;
    string name;
};

vector <Student> students;

I am creating the new struct with push_back and filling it with parameters from a function (yes, I have a constructor). To keep the vector of pointers sorted I am using lower_bound as shown below:

students.push_back(Student(id, name));
it = lower_bound(sortedByID.begin(), sortedByID.end(), id, cmp());
sortedByID.insert(it, &(students.back()));
//the same for name

The problem is that everytime I add the structure with push_back it reallocs new vector and destroys the address of previous objects, so pointers in vector sortedByID point to invalid value. I think it would be the same with an array of structs, because once the array is full, there is no other way (as far as I know) to resize it, than to create a new array and copy all the data from previous one (so the address will change again).

Is there any clever way to solve this problem? Please note that I am only allowed to use vector and not any other containers from STL.

Answer 1

There are three options to solve this issue using only vectors and no other containers:

1) Avoid the reallocation. This can only be achieved if you know the maximum M of expected number of elements to be inserted in your vector. In this case you canstudents.reserve(M); .

2) Forget the the pointers for sortedByID and sortedByName . Use integers (or better said size_t ) store the index of a student in students instead of a pointer. This supposes of course that the order of items in students is never changed.

3) Do not store the students themselves in a vector, but make students a vector of pointers to students (unsorted) that are allocated from free store. If this alternative meets all your criteria, i'd suggest to go fore shared_ptr<Student> instead of raw pointers.