251.md

Info

• Did you know that C++20 added type_identity which implements the identity metafunction?

  • http://wg21.link/p0887

Example

template <class T>
void print1() {
  std::cout << typeid(T).name() << std::endl;
}

template <class T>
void print2() {
  std::cout << typeid(std::type_identity<T>).name() << std::endl;
}

int main() {
  print1<int const&>();  // 'int'
  print2<int const&>();  // 'type_identity<int const &>'
}

https://godbolt.org/z/WWKh3673v

Puzzle

• Can you add required changes to tests and implement overload_args function which returns sum of sizes of passed arguments?

template<class...>
struct not_constructible {
  not_constructible() = delete;
};

constexpr auto overload_args_sum(...); // TODO

static_assert(0u == overload_args_sum(not_constructible{}/*TODO*/));
static_assert(sizeof(int) == overload_args_sum(not_constructible<int>{}/*TODO*/));
static_assert(sizeof(int) + sizeof(float) == overload_args_sum(not_constructible<int, float>{}/*TODO*/));
static_assert(sizeof(int) + sizeof(float) + sizeof(char) == overload_args_sum(not_constructible<int, float, char>{}/*TODO*/));

https://godbolt.org/z/8Y1qPrKMj

Solutions

template<template<typename ...> class whatever, typename ... Ts>
constexpr auto overload_args_sum(whatever<not_constructible<Ts ...>>){
  return (sizeof(Ts) + ... + 0);
}

static_assert(0u == overload_args_sum(std::type_identity<not_constructible<>>{}));
static_assert(sizeof(int) == overload_args_sum(std::type_identity<not_constructible<int>>{}));
static_assert(sizeof(int) + sizeof(float) == overload_args_sum(std::type_identity<not_constructible<int, float>>{}));
static_assert(sizeof(int) + sizeof(float) + sizeof(char) == overload_args_sum(std::type_identity<not_constructible<int, float, char>>{}));

https://godbolt.org/z/WWExM6Pfo

template <class... Ts>
constexpr auto overload_args_sum(std::type_identity<not_constructible<Ts...>>) -> std::size_t {
    return (0 + ... + sizeof(Ts));
}

static_assert(0u == overload_args_sum(std::type_identity<not_constructible<>>{}));
static_assert(sizeof(int) == overload_args_sum(std::type_identity<not_constructible<int>>{}));
static_assert(sizeof(int) + sizeof(float) == overload_args_sum(std::type_identity<not_constructible<int, float>>{}));
static_assert(sizeof(int) + sizeof(float) + sizeof(char) == overload_args_sum(std::type_identity<not_constructible<int, float, char>>{}));

https://godbolt.org/z/Wjnnh3n1b

template <class... Ts>
using type_list_t = std::type_identity<not_constructible<Ts...>>;

template <class... Ts>
[[nodiscard]] constexpr auto overload_args_sum(
    const type_list_t<Ts...>) noexcept {
    return (0 + ... + sizeof(Ts));
}

static_assert(0u ==
              overload_args_sum(std::type_identity<not_constructible<>>{}));
static_assert(sizeof(int) ==
              overload_args_sum(std::type_identity<not_constructible<int>>{}));
static_assert(
    sizeof(int) + sizeof(float) ==
    overload_args_sum(std::type_identity<not_constructible<int, float>>{}));
static_assert(sizeof(int) + sizeof(float) + sizeof(char) ==
              overload_args_sum(
                  std::type_identity<not_constructible<int, float, char>>{}));

https://godbolt.org/z/sdhrEqKf6

template<template<typename ... Args> class C, typename ... Args>
constexpr auto overload_args_sum(C<Args...>* c) { return (sizeof(Args)+...+0);  }

static_assert(0u == overload_args_sum((not_constructible<>*)(nullptr) ));
static_assert(sizeof(int) == overload_args_sum((not_constructible<int>*)(nullptr) ));
static_assert(sizeof(int) + sizeof(float) == overload_args_sum((not_constructible<int, float>*)(nullptr)));
static_assert(sizeof(int) + sizeof(float) + sizeof(char) == overload_args_sum((not_constructible<int, float, char>*)(nullptr)));

https://godbolt.org/z/oqaMf7fxa