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

README.md

@@ -18,6 +18,7 @@
 * [Did you know about C++26 static reflection proposal (2/N)?](https://github.com/tip-of-the-week/cpp/blob/master/tips/362.md)
 * [Did you know about C++26 static reflection proposal (3/N)?](https://github.com/tip-of-the-week/cpp/blob/master/tips/363.md)
 * [Did you know about C++26 static reflection proposal (4/N)?](https://github.com/tip-of-the-week/cpp/blob/master/tips/364.md)
+* [Did you know about C++26 static reflection proposal (5/N)?](https://github.com/tip-of-the-week/cpp/blob/master/tips/365.md)
 * [Did you know that C++26 added `Pack Indexing`?](https://github.com/tip-of-the-week/cpp/blob/master/tips/358.md)
 * [Did you know about C++26 proposal - `variadic friends`?](https://github.com/tip-of-the-week/cpp/blob/master/tips/352.md)
 * [Did you know about C++26 proposal - `inplace_vector`?](https://github.com/tip-of-the-week/cpp/blob/master/tips/351.md)

tips/365.md

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+<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>