[371] - Did you know that C++26 added `= delete("should have a reason…

…")`?

README.md

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 > C++26
 
+* [Did you know that C++26 added `= delete("should have a reason")`?](https://github.com/tip-of-the-week/cpp/blob/master/tips/371.md)
 * [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)

tips/371.md

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+<details open><summary>Info</summary><p>
+
+* **Did you know that C++26 added `= delete("should have a reason")`?**
+
+  * https://wg21.link/P2573
+
+</p></details><details open><summary>Example</summary><p>
+
+```cpp
+void newapi();
+void oldapi() = delete("This old API is outdated and already been removed. Please use newapi() instead.");
+
+int main () {
+  oldapi();
+}
+```
+
+> https://godbolt.org/z/aPz1bM4x6
+
+</p></details><details open><summary>Puzzle</summary><p>
+
+* **Can you implment simplified addressof with `delete("Cannot take address of rvalue.")` when used incorreclty?**
+
+```cpp
+// TODO addressof
+
+int main() {
+  int i{};
+  addressof(i);     // okay
+  addressof(int{}); // error: "Cannot take address of rvalue."
+}
+```
+
+> https://godbolt.org/z/M6881vY65
+
+</p></details>
+
+</p></details><details><summary>Solutions</summary><p>
+
+```cpp
+template<class T> constexpr T* addressof(T& r) noexcept { return &r; }
+template<class T> const T* addressof(const T&&) = delete("Cannot take address of rvalue.");
+
+int main() {
+  int i{};
+  addressof(i);     // okay
+  addressof(int{}); // error: "Cannot take address of rvalue."
+}
+```
+
+> https://godbolt.org/z/hMYWThGxj
+
+</p></details>