Question

header file:

private:
vector<int*>* nums;

public slots:
void buttonClicked();

cpp file:

NewWindow(){
    int one = 1;
    int* pone = &one;
    int two = 2;
    int* ptwo = &two;

    vector<int*> numbers;
    numbers.push_back(pone);
    numbers.push_back(ptwo);
    nums = &numbers;

    test();
}

//Prints size of nums vector
void NewWindow::test(){
    stringstream woo;
    woo << nums->size()<<endl;
    cout << woo.str();
}

//I just had one button on my gui
void NewWindow::buttonClicked(){
    test();
}

When I run this, the first test method executes in the contructor and outputs 2, as expected. However, after clicking the button (i.e executing buttonClicked() method), the test method outputs a seemingly arbitrary number (e.g. 4292719658). After messing around a bit I discovered that if I make the numbers vector an attribute, the test() method will output 2 each time. Why was I getting a weird output with numbers as a non-attribute?

Answer 1

If numbers is a local variable, as it is here:

NewWindow(){
  vector<int*> numbers;
  nums = &numbers;
}

then it is destroyed, and ceases to exist, when it goes out of scope. In this case, when the NewWindow function returns. Referencing a non-existent object (as you do through the nums pointer), results in undefined behavior.

If numbers is a member variable, on the other hand, it is destroyed when the object of which it is a member is destroyed.