cartesian product in C++

cart_product.cpp

#include <iostream>
#include <vector>
#include <algorithm>

using namespace std;

vector<vector<int> > cart_product (const vector<vector<int>>& v) {
    vector<vector<int>> s = {{}};
    for (auto& u : v) {
        vector<vector<int>> r;
        for (auto& x : s) {
            for (auto y : u) {
                r.push_back(x);
                r.back().push_back(y);
            }
        }
        s.swap(r);
    }
    return s;
}

int main () {
	vector<vector<int> > test{{1}, {4,5,6}, {8,9}};
	vector<vector<int> > res = cart_product(test);
	for(size_t i = 0; i < res.size();i++){
		for (size_t j = 0; j < res[i].size(); j++){
			cout << res[i][j] << "\t";
		}
		cout << std::endl;
	}
    return 0;
}