TMP: how to generalize a Cartesian Product of Vectors?

Question

There is an excellent C++ solution (actually 2 solutions: a recursive and a non-recursive), to a Cartesian Product of a vector of integer vectors. For purposes of illustrat

Answer 1

Simpler recursive solution. It takes vectors as function arguments, not as a tuple. This version doesn't build temporary tuples, but uses lambdas instead. Now it makes no unnecessary copies/moves and seems to get optimized successfully.

#include
#include

// cross_imp(f, v...) means "do `f` for each element of cartesian product of v..."
template
inline void cross_imp(F f) {
    f();
}
template
inline void cross_imp(F f, std::vector const& h,
                           std::vector const&... t) {
    for(H const& he: h)
        cross_imp([&](Ts const&... ts){
                      f(he, ts...);
                  }, t...);
}

template
std::vector> cross(std::vector const&... in) {
    std::vector> res;
    cross_imp([&](Ts const&... ts){
                  res.emplace_back(ts...);
              }, in...);
    return res;
}

#include

int main() {
    std::vector is = {2,5,9};
    std::vector cps = {"foo","bar"};
    std::vector ds = {1.5, 3.14, 2.71};
    auto res = cross(is, cps, ds);
    for(auto& a: res) {
        std::cout << '{' << std::get<0>(a) << ',' <<
                            std::get<1>(a) << ',' <<
                            std::get<2>(a) << "}\n";
    }
}