365.md

<details open><summary>Info</summary><p>

* **Did you know about C++26 static reflection proposal (5/N)?**

  * https://wg21.link/P2996

</p></details><details open><summary>Example</summary><p>

```cpp
struct foo;
[[maybe_unused]] constexpr auto _ =
  define_class(^foo, {std::meta::nsdm_description(^int, {.name = "bar"})});
foo f{.bar = 42}; // has bar int member
```

> https://godbolt.org/z/aqxanTe3r

</p></details><details open><summary>Puzzle</summary><p>

* **Can you implement template alias `pack` which will pack given struct by sorting members by their size?**

```cpp
template<class T> using pack; // TODO

struct unpacked {
  short s;
  int i;
  bool b;
};
static_assert(12 == sizeof(unpacked));

using packed = pack<unpacked>;
static_assert(8 == sizeof(packed));
static_assert(requires(packed p) { p.i; p.s; p.b; });
```

> https://godbolt.org/z/hnPvsE9h3

</p></details>

</p></details><details><summary>Solutions</summary><p>

```cpp
namespace detail {
template<class T> struct packed;
template<class T> [[nodiscard]] consteval auto pack() {
  std::vector members = std::meta::nonstatic_data_members_of(^T);
  sort(members, [](auto lhs, auto rhs) consteval { return std::meta::size_of(lhs) < std::meta::size_of(rhs); });
  std::vector<std::meta::nsdm_description> new_members{};
  for (const auto& member: members) {
      new_members.push_back({std::meta::type_of(member), {.name = std::meta::name_of(member)}});
  }
  return define_class(^packed<T>, new_members);
}
} // namespace detail
template<class T> using pack = [:detail::pack<T>():];

struct unpacked {
  short s;
  int i;
  bool b;
};
static_assert(12 == sizeof(unpacked));

using packed = pack<unpacked>;
static_assert(8 == sizeof(packed));
static_assert(requires(packed p) { p.i; p.s; p.b; });
```

> https://godbolt.org/z/ExYfTv4nK

</p></details>