#include <utility>
#include <cstddef>
#include <vector>

using std::size_t;

namespace details {
template<typename Lambda>
    void for_each_multiset_combo_worker( std::vector<size_t> const& counts, Lambda&& lambda, size_t depth, std::vector<size_t>& current )
    {
        if (depth >= counts.size()) {
            lambda( current );
            return;
        }
        for (size_t i = 0; i <= counts[depth]; ++i) {
            current.push_back(i);
            for_each_multiset_combo_worker( counts, lambda, depth+1, current );
            current.pop_back();
        }
    }
}
template<typename Lambda>
void for_each_multiset_combo( std::vector<size_t> const& counts, Lambda&& lambda )
{
    std::vector<size_t> current;
    current.reserve( counts.size() );
    details::for_each_multiset_combo_worker( counts, std::forward<Lambda>(lambda), 0, current );
}
#include <iostream>

int main() {
    std::vector<size_t> multiset = {3, 2, 1, 1};
    size_t counter = 0;
    for_each_multiset_combo( multiset, [&]( std::vector<size_t> const& counts ){
        std::cout << counter << ": [";
        for(auto it = counts.begin(); it != counts.end(); ++it) {
            if (it != counts.begin()) {
                std::cout << ", ";
            }
            std::cout << *it;
        }
        std::cout << "]\n";
        ++counter;
    });
}