Insert a character in all positions of a given string using C++

#include <bits/stdc++.h>

using namespace std;

int main() {
    string s;
    cin >> s;
    char ch = 'x';
    // don't forget the '=' in the for-loop condition
    for (int i = 0; i <= (int) s.size(); i++) {
        cout << s.substr(0, i) + ch + s.substr(i) << '\n';
    }
    return 0;
}