[370] - Did you know that C++26 added span.at?

README.md

@@ -14,6 +14,7 @@
 
 > C++26
 
+* [Did you know that C++26 added `span.at`?](https://github.com/tip-of-the-week/cpp/blob/master/tips/370.md)
 * [Did you know about C++26 simd proposal (1/N)?](https://github.com/tip-of-the-week/cpp/blob/master/tips/367.md)
 * [Did you know about C++26 static reflection proposal (1/N)?](https://github.com/tip-of-the-week/cpp/blob/master/tips/361.md)
 * [Did you know about C++26 static reflection proposal (2/N)?](https://github.com/tip-of-the-week/cpp/blob/master/tips/362.md)

tips/370.md

@@ -0,0 +1,74 @@
+<details open><summary>Info</summary><p>
+
+* **Did you know that C++26 added `span.at`?
+
+  * https://wg21.link/P2821
+
+</p></details><details open><summary>Example</summary><p>
+
+```cpp
+std::array<int, 42> storage{};
+const auto span = std::span(storage);
+(void)span.at(0); // okay
+(void)span.at(storage.size()); // throws
+```
+
+> https://godbolt.org/z/ezvMvraW1
+
+</p></details><details open><summary>Puzzle</summary><p>
+
+* **Can you implement `span_at` which will use `[]` if `fn` is noexcept and `at` otherwise?
+
+```cpp
+struct span_at; // TODO
+
+int main() {
+    using namespace boost::ut;
+
+    std::array<int, 42> storage{};
+    for (auto i = 0; i < storage.size(); ++i) { storage[i] = i; }
+
+    "span[index]"_test = [&] {
+        span_at t{storage};
+        expect(0_i == t([](auto i) noexcept(true) { return i; }, 0));
+        expect(2_i == t([](auto i) noexcept(true) { return i; }, 2));
+        expect(41_i == t([](auto i) noexcept(true) { return i; }, 41));
+        // expect(not throws([&] { (void)t([](auto i) noexcept(true) { return i; }, 42); }));
+    };
+
+    "span.at"_test = [&] {
+        span_at t{storage};
+        expect(0_i == t([](auto i) noexcept(false) { return i; }, 0));
+        expect(2_i == t([](auto i) noexcept(false) { return i; }, 2));
+        expect(41_i == t([](auto i) noexcept(false) { return i; }, 41));
+        expect(throws([&] { (void)t([](auto i) noexcept(false) { return i; }, 42); }));
+    };
+}
+```
+
+> https://godbolt.org/z/oTxo9aaz4
+
+</p></details>
+
+</p></details><details><summary>Solutions</summary><p>
+
+```cpp
+struct span_at {
+  constexpr explicit span_at(std::span<const int> span) : span_{span} { }
+
+  [[nodiscard]] constexpr auto operator()(auto&& fn, auto index) {
+    if constexpr(noexcept(fn(span_[index]))) {
+      return fn(span_[index]);
+    } else {
+      return fn(span_.at(index));
+    }
+  }
+
+ private:
+  std::span<const int> span_{};
+};
+```
+
+> https://godbolt.org/z/hcnWjWodM
+
+</p></details>