Do Vectors resize automatically?

Question

I am very sorry that I am asking such a beginner question but I am finding contradictory information online. I would ask at University but it is out until February next year.

Do Vectors resize automatically? Or do you need to check the current size periodically and resize when you need more room. It looks to be resizing for me automatically but I'm not sure if that is a feature or the compiler waving a magic wand.

Answer 1

If you use push_back or insert, yes vector resizes itself. Here is a demo:

#include<iostream>
#include<vector>

using namespace std;

int main() {
    vector < int > a;
    a.push_back(1);
    a.push_back(2);
    a.push_back(3);
    for (int value : a) {
        cout << value << " ";
    }
    cout << endl << "Current size " << a.size() << endl;
    return 0;
}

It gives output as:

1 2 3
Current size 3

Remember now if you do a[3] = 5. It will not resize your vector automatically.
Also you can manually resize vector if you want. For demo append following code to above code.

a.resize(6);
for (int value : a) {
    cout << a << " ";
}
cout << endl << "Current size " << a.size() << endl;

Now it will output:

1 2 3
Current size 3
1 2 3 0 0 0
Current size 6

It think you got your answer.