[354] - Did you know that C++23 added range string_view constructor?

README.md

@@ -31,6 +31,7 @@
 
 > C++23
 
+* [Did you know that C++23 added range `string_view` constructor?**](https://github.com/tip-of-the-week/cpp/blob/master/tips/354.md)
 * [Did you know that C++23 added `std::invoke_r`?](https://github.com/tip-of-the-week/cpp/blob/master/tips/334.md)
 * [Did you know that C++23 extended floating-point types?](https://github.com/tip-of-the-week/cpp/blob/master/tips/328.md)
 * [Did you know that C++23 deprecated std::aligned_storage and std::aligned_union?](https://github.com/tip-of-the-week/cpp/blob/master/tips/326.md)

tips/353.md

@@ -60,4 +60,40 @@ constexpr auto visit(auto&& f, auto&& v); // TODO
 </p></details>
 
 </p></details><details><summary>Solutions</summary><p>
+
+```cpp
+template<class F, class V, class R, std::size_t N, std::size_t Size>
+constexpr auto visit_impl([[maybe_unused]] F&& f, [[maybe_unused]] V&& v) -> R {
+  if constexpr (N < Size) {
+    switch (v.index()) {
+      default:
+        return visit_impl<F, V, R, N+1, Size>(std::forward<F>(f), std::forward<V>(v));
+      case N:
+        return std::forward<F>(f)(std::get<N>(std::forward<V>(v)));
+    }
+  } else {
+    __builtin_unreachable();
+  }
+}
+
+template<class F, class V, template<class...> class T, class... Ts>
+auto result_type(T<Ts...>&&) ->
+  std::common_type_t<decltype(std::declval<F>()(std::get<Ts>(std::declval<V>())))...>;
+
+template<class F, class V, template<class...> class T, class... Ts>
+auto result_type(T<std::monostate, Ts...>&&) ->
+  std::common_type_t<decltype(std::declval<F>()(std::get<Ts>(std::declval<V>())))...>;
+
+template<class F, class V>
+constexpr decltype(auto) visit(F&& f, V&& v) {
+  using variant_t = std::remove_const_t<std::remove_reference_t<V>>;
+  constexpr auto size = std::variant_size_v<variant_t>;
+  static_assert(size > 0, "Empty variant is not supported!");
+  using result_t = decltype(result_type<F, V>(std::declval<variant_t>()));
+  return visit_impl<F, V, result_t, 0u, size>(std::forward<F>(f), std::forward<V>(v));
+}
+```
+
+> https://godbolt.org/z/YPqGzKP95
+
 </p></details>

tips/354.md

@@ -0,0 +1,36 @@
+<details open><summary>Info</summary><p>
+
+* **Did you know that C++23 added range `string_view` constructor?**
+
+  * https://wg21.link/P1989
+
+</p></details><details open><summary>Example</summary><p>
+
+```cpp
+constexpr std::string text = "Hello, World!";
+constexpr std::string_view view(text.c_str() + 7, 5);
+static_assert(view == std::string_view{"World"});
+```
+
+> https://godbolt.org/z/GYfWxPc17
+
+</p></details><details open><summary>Puzzle</summary><p>
+
+* **Can you implement `buffer` with explicit conversion to string_view?**
+
+```cpp
+template<auto N>
+struct buffer; // TODO
+
+constexpr buffer<42> b;
+static_assert(42 == std::string_view{b}.size());
+static_assert(&b.data[0] == std::string_view{b}.begin());
+static_assert(&b.data[42] == std::string_view{b}.end());
+```
+
+> https://godbolt.org/z/hKjo8Pr88
+
+</p></details>
+
+</p></details><details><summary>Solutions</summary><p>
+</p></details>