[C++] Reference demo

#include <vector>
#include <iostream>

using std::vector;
using std::cout;
using std::endl;

int sum(vector<int> &);

int main() {
    vector<int> vec {1, 2, 3};

    // Pass the reference of vec into sum
    int s = sum(vec);

    cout << s << endl;
    cout << vec[1] << endl;
    return 0;
}

int sum(vector<int> &vec) {
    int sum = 0;

    for (auto ptr = vec.begin(); ptr != vec.end(); ptr++) {
        sum += *ptr;
    }

    return sum;
}