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";     } }