[util.smartptr.atomic] Add example from Matthew Butler.

Author: zygoloid

source/atomics.tex

@@ -2736,6 +2736,35 @@
 resulting from this is performed.
 \end{note}
 
+\pnum
+\begin{example}
+\begin{codeblock}
+template<typename T> class atomic_list {
+  struct node {
+    T t;
+    shared_ptr<node> next;
+  };
+  atomic<shared_ptr<node>> head;
+
+public:
+  auto find(T t) const {
+    auto p = head.load();
+    while (p && p->t != t)
+      p = p->next;
+
+    return shared_ptr<node>(move(p));
+  }
+
+  void push_front(T t) {
+    auto p = make_shared<node>();
+    p->t = t;
+    p->next = head;
+    while (!head.compare_exchange_weak(p->next, p)) {}
+  }
+};
+\end{codeblock}
+\end{example}
+
 \rSec3[util.smartptr.atomic.shared]{Partial specialization for \tcode{shared_ptr}}
 \indexlibraryglobal{atomic<shared_ptr<T>>}%
 \begin{codeblock}