Call template function with typename combinations

February 23, 2023

I want to loop over a "list of typenames" and look at all combinations of them. I was thinking of using a tuple for this, compare:

template<typename T, typename R>
void foo() {
    std::cout << std::numeric_limits<T>::max() << ", " << std::numeric_limits<R>::max();
}

void call() {
    using int_types = std::tuple<int8_t, uint8_t, int16_t, uint16_t, int32_t, uint32_t, int64_t, uint64_t>;
    constexpr auto indizes = std::make_integer_sequence<size_t, 8>();
    for (auto i: indizes) {
        using T = std::tuple_element_t<i, int_types>;
        for (auto j: indizes) {
            using R = std::tuple_element_t<j, int_types>;
            foo<T, R>();
        }
    }
}

Of course for loops are not consteval, thus this approach does not work. I think it might work using recursion or some consteval stuff from the STL but I can’t quite figure it out.

>Solution :

You can expand pack that way

    using int_types = std::tuple<int8_t, uint8_t, int16_t, uint16_t, int32_t, uint32_t, int64_t, uint64_t>;
    
    []<typename... Ts>(std::tuple<Ts...>){
        ([]<typename T, typename... Us>(std::type_identity<T>, std::tuple<Us...>){
            (foo<T, Us>(), ...);
        }(std::type_identity<Ts>{}, int_types{}), ...);
    }(int_types{});

Demo