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 illustration/simplicity, let us just focus on the non-recursive version.

My question is, how can one generalize this code with templates to take a std::tuple of homogeneous vectors that looks like this:

{{2,5,9},{"foo","bar"}}

and generate a homogeneous vector of tuple

{{2,"foo"},{2,"bar"},{5,"foo"},{5,"bar"},{9,"foo"},{9,"bar"}}

If it makes life any easier, let us assume that the internal vectors in the input are each homogeneous. So inputs like this are not allowed: {{5,"baz"}{'c',-2}}

EDIT changed input from jagged vector to a tuple

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<tuple>
#include<vector>

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

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

#include<iostream>

int main() {
    std::vector<int> is = {2,5,9};
    std::vector<char const*> cps = {"foo","bar"};
    std::vector<double> 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";
    }
}