为什么矢量不改变它的元素？

Question

Have a vector which contains pointers to a Student class. I need to selection sort the vector elements only changing the pointers, not the content. Here is my caller code:

StudentsFileReader* sfr = new StudentsFileReader(filepath);

cout << "Calling sortPositionExchange..." << endl;

SelectionSort::sortPositionExchange(&(sfr->getStudents()));

cout << "Done!" << endl;

Here is my calling code:

void SelectionSort::sortPositionExchange(vector<Student*>* students)
{
    int i, j, min, aux, tam = students->size();

    for (i = 0; i < tam - 1; i++)
    {
        min = i;
        for (j = (i + 1); j < tam; j++)
        {
            if (students->at(j)->getCode() < students->at(min)->getCode())
            {
                min = j;
            }
        }

        if (i != min)
        {
            Student* s = students->at(i);
            *(&students->at(i)) = *(&students->at(min));
            *(&students->at(min)) = s;
        }
    }
}

The selection sort executes perfectly inside sortPositionExchange function, but when it returns, the vector keeps unchanged.

What can I do to persist the changes between calling functions?

Thanks!

Answer 1

Your method getStudents() returns a vector, the returned vector is not the original vector, it is a copy of it.
In order to fix your issue, make the method getStudents() return a reference to the original vector.
so your method should look something like this:

vector<Student*>* getStudents() {

    return &studentsVector;
}

and your call should look something like this:

SelectionSort::sortPositionExchange(sfr->getStudents());

Another solution would be to make a method inside StudentsFileReader that uses SelectionSort::sortPositionExchange to sort the vector. This would look something like:

void StudentsFileReader::sortPositionExchange() {

    SelectionSort::sortPositionExchange(students);
}