How can I create a n way Cartesian product of type lists in C++?

Question

Self explanatory.

Basically, say I have type lists like so:

using type_list_1 = type_list;
using type_list_2 = type_list

        

Answer 1

Fold expressions to the rescue again

template
typelist...> layered(typelist);

template
auto operator+(typelist, typelist)
    -> typelist;

template
auto operator*(typelist, typelist)
    -> typelist;

template
auto operator^(typelist, TL tl)
    -> decltype(((typelist{} * tl) + ...));

template
using product_t = decltype((layered(TLs{}) ^ ...));

And you're done. This has the additional benefit over recursion of having O(1) instantiation depth.

struct A0;
struct A1;
struct B0;
struct B1;
struct C0;
struct C1;
struct C2;

using t1 = typelist;
using t2 = typelist;
using t3 = typelist; 

using p1 = product_t;
using p2 = product_t;

using expect1 = typelist,
                         typelist,
                         typelist,
                         typelist>;

using expect2 = typelist,
                         typelist,
                         typelist,
                         typelist,
                         typelist,
                         typelist,
                         typelist,
                         typelist,
                         typelist,
                         typelist,
                         typelist,
                         typelist>;

static_assert(std::is_same_v);
static_assert(std::is_same_v);