#include <iostream>
#include <string>
#include <vector>

bool increase(const std::string& s, std::vector<std::size_t>& it)
{
    for (std::size_t i = 0, size = it.size(); i != size; ++i) {
        const std::size_t index = size - 1 - i;
        ++it[index];
        if (it[index] >= s.size()) {
            it[index] = 0;
        } else {
            return true;
        }
    }
    return false;
}

void do_job(const std::string& s,
            const std::vector<std::size_t>& it)
{
    for (std::size_t i = 0; i != it.size(); ++i) {
        std::cout << s[it[i]] << " ";
    }
    std::cout << std::endl;
}

void cartesian_product(const std::string& s, std::size_t n)
{
    std::vector<std::size_t> it(n, 0u);

    do {
        do_job(s, it);
    } while (increase(s, it));
}

int main()
{
    cartesian_product("abcde", 3);
}