Why aren't these vectors equal?

Question

I've created two vectors and filled the other one with push_back and the other one with indices. I would expect these to be equal but ther aren't. Can someone explain me why is this?

#include <vector>
#include <iostream>

using namespace std;

int main() {

    vector<int> v0;
    v0.push_back(0);
    v0.push_back(1);
    v0.push_back(2);

    vector<int> v1;
    v1.reserve(3);
    v1[0] = 0;
    v1[1] = 1;
    v1[2] = 2;

    if (v0 != v1) {
            cout << "why aren't they equal?" << endl;
    }

    return 0;
}

Answer 1

vector<int> v1;
v1.reserve(3);
v1[0] = 0;
v1[1] = 1;
v1[2] = 2;

This is probably an undefined behavior ( although not sure if it's implementation dependent).

You cannot use operator[] for filling up the vector as it's returning the reference to the underlying object which in your case is nothing other than bunch of bits.

You should either use push_back() OR just resize your vector.Using latter:-

vector<int> v1;
v1.resize(3);
v1[0] = 0;
v1[1] = 1;
v1[2] = 2;