How to create the Cartesian product of a type list?

Question

I\'d like to create the cross product of a list of types using variadic templates.

Here\'s what I have so far:

#include 
#include <         


        

Answer 1

C++17

Working Demo

Logic to concatenate type_lists to avoid nested type_list like you are asking for:

// base case: 2 type_lists
template
auto concat(type_list, type_list) -> type_list;

// recursive case: more than 2 type_lists
template
auto concat(type_list, type_list, Rest...) -> decltype(concat(type_list{}, Rest{}...));

Note that these functions don't have (or need) implementations; this is a trick to avoid class template specialization (I learned it from Hana Dusikova's compile time regular expressions)

Then, simplifying your row and cross_product impls as pairs and cross_product_impl, respectively:

template
using pairs = type_list...>;

template
auto cross_product_impl()
{
    if constexpr(sizeof...(T) == 0)
        return type_list<> {};
    if constexpr(sizeof...(T) == 1)
        return type_list...>{};
    if constexpr(sizeof...(T) > 1)
        return concat(pairs{}...);
}

if constexpr allows us to more easily express the logic, I think.

Finally a type alias for cross_product that gives us what the type would be if we theoretically invoked cross_product_impl:

template
using cross_product = decltype(cross_product_impl());

Usage basically the same as before:

cross_product result;