add example for R.24 (#1127)

* add example for R.24

taken from https://cpppatterns.com/patterns/weak-reference.html, where
it's published under CC0 Public Domain Dedication

* improve example for R.24

CppCoreGuidelines.md

@@ -9151,7 +9151,35 @@ be able to destroy a cyclic structure.
 
 ##### Example
 
-    ???
+    #include <memory>
+
+    class bar;
+
+    class foo
+    {
+    public:
+      explicit foo(const std::shared_ptr<bar>& forward_reference)
+        : forward_reference_(forward_reference)
+      { }
+    private:
+      std::shared_ptr<bar> forward_reference_;
+    };
+
+    class bar
+    {
+    public:
+      explicit bar(const std::weak_ptr<foo>& back_reference)
+        : back_reference_(back_reference)
+      { }
+      void do_something()
+      {
+        if (auto shared_back_reference = back_reference_.lock()) {
+          // Use *shared_back_reference
+        }
+      }
+    private:
+      std::weak_ptr<foo> back_reference_;
+    };
 
 ##### Note