271.md

Info

• Did you know that C++20 added support for floating point values as non-type template parameters?

  • https://wg21.link/P0127

Example

template<double Value> constexpr auto value = Value;

int main() {
    std::cout << value<4.2>; // prints 4.2
}

https://godbolt.org/z/4cfMPY686

Puzzle

• Can you implement function calc which adds values from Vl1s... multipled by all values from Vls2...? For example : Vl1s = { 1., 2. }, Vl2 { 3., 4.}; calc = (1. * 3. * 4.) + (2. * 3. * 4.)

template<double... Values> struct values {};

template<double... Vl1s, double... Vl2s>
[[nodiscard]] consteval auto calc(values<Vl1s...>, values<Vl2s...>) // TODO

template<double Epsilon = 0.1>
[[nodiscard]] consteval auto eq(double a, double b) {
    return std::fabs(a - b) < Epsilon;
}

static_assert(eq(1. * 2., calc(values<1.>{}, values<2.>{})));
static_assert(eq(2.*3.*4., calc(values<2.>{}, values<3., 4.>{})));
static_assert(eq(4.2 * 2. * 3. + .1 * 2. * 3., calc(values<4.2, .1>{}, values<2., 3.>{})));
static_assert(eq(1. * 4. * 5. + 2. * 4. * 5., calc(values<1., 2.>{}, values<4., 5.>{})));

https://godbolt.org/z/7KhKaK8jT

Solutions

template<double... Vl1s, double... Vl2s>
[[nodiscard]] consteval auto calc(values<Vl1s...>, values<Vl2s...>) {
    constexpr auto mul = (Vl2s * ...);
    return ((Vl1s * mul) + ...);
}

https://godbolt.org/z/K7T9czezG

template <double... List1, double... List2>
[[nodiscard]] consteval auto calc(values<List1...>, values<List2...>) {
    return (0 + ... + (List1 * (1 * ... * List2)));
}

https://godbolt.org/z/fGsjTMnab

template<double... Values> struct values {};

template<double... Vl1s, double... Vl2s>
[[nodiscard]] consteval auto calc(values<Vl1s...>, values<Vl2s...>) {
    return ((Vl1s * (Vl2s * ...)) + ...);
}

template<double Epsilon = 0.1>
[[nodiscard]] consteval auto eq(double a, double b) {
    return std::fabs(a - b) < Epsilon;
}

https://godbolt.org/z/71KaKc4fM

template<double... Values> struct values {};

template<double... Vl1s, double... Vl2s>
[[nodiscard]] consteval auto calc(values<Vl1s...>, values<Vl2s...>) {
    constexpr auto c = (Vl2s * ...);
    return ((Vl1s * c) + ...);
}

template<double Epsilon = 0.1>
[[nodiscard]] consteval auto eq(double a, double b) {
    return std::fabs(a - b) < Epsilon;
}

https://godbolt.org/z/s6r8xKEe6

template<double... Values> struct values {};

template<double... Vl1s, double... Vl2s>
[[nodiscard]] consteval auto calc(values<Vl1s...>, values<Vl2s...>) {
  return (Vl1s + ...) * (Vl2s * ...);
}

template<double Epsilon = 0.1>
[[nodiscard]] consteval auto eq(double a, double b) {
    return std::fabs(a - b) < Epsilon;
}

https://godbolt.org/z/nE79Ya6z3

template<double... Values> struct values{};

template<double Vl1, double... Vl2s>
[[nodiscard]] consteval auto mult(){
  return  (Vl1 * ... * Vl2s);
}

template<double... Vl1s, double... Vl2s>
[[nodiscard]] consteval auto calc(values<Vl1s...>, values<Vl2s...>){
  return (... + mult<Vl1s, Vl2s...>());
}

template<double Epsilon = 0.1>
[[nodiscard]] consteval auto eq(double a, double b) {
    return std::fabs(a - b) < Epsilon;
}

https://godbolt.org/z/KfPefTPbd

template<double... Values> struct values {};

template<double... Vl1s, double... Vl2s>
[[nodiscard]] consteval auto calc(values<Vl1s...>, values<Vl2s...>){
    return (0.0 + ... + (Vl1s * (Vl2s * ... )));
}

template<double Epsilon = 0.1>
[[nodiscard]] consteval auto eq(double a, double b) {
    return std::fabs(a - b) < Epsilon;
}

https://godbolt.org/z/M5Y5dnfvY