What is different between two codes of for loop?

Question

#include <vector>
#include <iostream>
using namespace std;

int main(void)
{
    vector<int> a = {1, 2, 3, 4, 5};
    for (auto &x : a)
        cout << x << endl;
}

#include <vector>
#include <iostream>
using namespace std;

int main(void)
{
    vector<int> a = {1, 2, 3, 4, 5};
    for (auto x : a)
        cout << x << endl;
}

Two codes above prints same values (1, 2, 3, 4, 5). But is there different thing between initializing &x and x? Thanks for reading!

Answer 1

There is no difference in the output for the code you wrote. However, if you tried to change the value of x when in the loop, there would be a difference.

#include <vector>
#include <iostream>
using namespace std;

int main(void)
{
    vector<int> a = {1, 2, 3, 4, 5};
    for (auto x : a)
        x = 0;
    for (auto x : a)
        cout << x << endl;
}

is very different to:

#include <vector>
#include <iostream>
using namespace std;

int main(void)
{
    vector<int> a = {1, 2, 3, 4, 5};
    for (auto & x : a)
        x = 0;

    for (auto x : a)
        cout << x << endl;
}

In the second, the vector a will be all zeros at the end of the program. This is because auto by itself copies each element to a temporary value inside the loop, whereas auto & takes a reference to an element of the vector, which means that if you assign something to the reference it overwrites wherever the reference is pointing.