[351] - Did you know about C++26 proposal - variadic friends?

README.md

@@ -14,6 +14,7 @@
 
 > C++26
 
+* [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)
 * [Did you know about C++26 proposal - Aggregates are named tuples?](https://github.com/tip-of-the-week/cpp/blob/master/tips/350.md)
 * [Did you know that C++26 added new SI prefixes?](https://github.com/tip-of-the-week/cpp/blob/master/tips/349.md)

tips/351.md

@@ -105,3 +105,5 @@ class inplace_vector {
 ```
 
 > https://godbolt.org/z/scz4EfTa5
+
+</p></details>

tips/352.md

@@ -0,0 +1,73 @@
+<details open><summary>Info</summary><p>
+
+* **Did you know about C++26 proposal - `variadic friends*`*?
+
+ * https://wg21.link/P2893
+
+</p></details><details open><summary>Example</summary><p>
+
+```cpp
+class FriendClass;
+
+class MyClass {
+ friend class FriendClass;
+
+public:
+ constexpr MyClass(int data) : privateData(data) {}
+
+private:
+ int privateData;
+};
+
+struct FriendClass {
+ constexpr auto accessPrivateData(const MyClass& obj) const {
+ return obj.privateData;
+ }
+};
+
+constexpr MyClass obj{42};
+constexpr FriendClass friend_obj{};
+static_assert(42 == friend_obj.accessPrivateData(obj));
+```
+
+> https://godbolt.org/z/Te8zzEoPd
+
+</p></details><details open><summary>Puzzle</summary><p>
+
+* **Can you extend `MyClass` so that all `Ts...` will have access to `privateData`?**
+
+```cpp
+template<class... Ts>
+class MyClass {
+ // TODO
+
+ public:
+ constexpr explicit MyClass(int data) : privateData(data) {}
+
+ private:
+ int privateData{};
+};
+
+template<auto>
+struct FriendClass {
+ constexpr auto accessPrivateData(const auto& obj) const {
+ return obj.privateData;
+ }
+};
+
+constexpr MyClass<FriendClass<0>, FriendClass<1>, FriendClass<2>> obj{42};
+constexpr FriendClass<0> friend_obj0{};
+constexpr FriendClass<1> friend_obj1{};
+constexpr FriendClass<2> friend_obj2{};
+
+static_assert(42 == friend_obj0.accessPrivateData(obj));
+static_assert(42 == friend_obj1.accessPrivateData(obj));
+static_assert(42 == friend_obj2.accessPrivateData(obj));
+```
+
+> https://godbolt.org/z/81dn33e41
+
+</p></details>
+
+</p></details><details><summary>Solutions</summary><p>
+</p></details>