Vector of a class resetting member variables after for loop-CodePudding

I have an assignment where we need to use this basic structure of vectors and classes to learn about parent and child classes and polymorphism. Here is the code of the function I'm supposed to write:

void assignStudents(vector<Student*>& v) {
    for (int i = 0; i < 5; i  )
    {
        cout << "Enter a study level: ";
        string input;
        cin >> input;
        if (input == "graduate")
        {
            Graduate inputClass;
            Student* inputParentClassPtr = &inputClass;
            v.push_back(inputParentClassPtr);
            v[i]->addToVector(input);
            inputParentClassPtr = nullptr;

        }
        else if (input == "undergraduate")
        {
            Undergraduate inputClass;
            Student* inputParentClassPtr = &inputClass;
            inputParentClassPtr->addToVector(input);
            v.push_back(inputParentClassPtr);
        }
        else
        {
            cout << "Please enter a valid response, either graduate or undergraduate" << endl;
            i--;
        }

    }
    for (size_t i = 0; i < v.size(); i  )
    {
        vector<string> studyLevels = v[i]->getStudyLevels();
        size_t size = studyLevels.size();
        for (int j = 0; j < size; j  )
        {
            cout << studyLevels[j];
        }
    }
}

I debug the program and every time the first for loop moves on to the next iteration, every member variable inside each object in my vector goes blank, but then when I add a new object into the vector, then call the addToVector() function they come back.

I added the bottom for loop to check if any editing is happening, and once I get to that bottom for loop, every member variable is empty again.

I have the Student class vector where I am adding Undergraduate and Graduate classes to. Every Student class has a protected vector inside called levels. I need to add the class to vector that holds all my objects, then edit the member variable vector to include the string representing the type of class it is.

Why do the member variables (levels) go blank every time it finishes an iteration of the for loop?

CodePudding user response：

I'll just focus on one part, as the same issue appears twice in your code.

{
    Graduate inputClass; // create local student "on the stack"
    Student* inputParentClassPtr = &inputClass;
    v.push_back(inputParentClassPtr); // store address of student
    v[i]->addToVector(input);
    inputParentClassPtr = nullptr; // has no real effect
} // inputClass goes out of scope and is destroyed here

When the block ends, the local "stack" variables from that block are destroyed. That means the Graduate object is no longer valid, and the pointer you stored in v is now pointing at something unusable.

To fix that, you need to create the objects in dynamic memory.

You should change your vector to store std::unique_ptr<Student>, and create the objects using std::make_unique(), like this:

auto inputParentClassPtr = std::make_unique<Graduate>();
v.push_back(std::move(inputParentClassPtr));

But, if you can't do that, you will need to use new instead, like this:

Student* inputParentClassPtr = new Graduate();
v.push_back(inputParentClassPtr);

Either way, even though inputParentClassPtr is still destroyed at the end of the block, it is only a pointer and the Graduate object it pointed to is still alive.

If you use new, you'll then need to delete all the objects in the vector when you are done using them, or you'll have a memory leak. Using std::unique_ptr will handle that for you.