merge(3p/immer): Subtree merge at 'ad3e3556d' as 'third_party/immer'

Change-Id: I9636a41ad44b4218293833fd3e9456d9b07c731b

third_party/immer/example/box/box.cpp

@@ -0,0 +1,25 @@
+//
+// immer: immutable data structures for C++
+// Copyright (C) 2016, 2017, 2018 Juan Pedro Bolivar Puente
+//
+// This software is distributed under the Boost Software License, Version 1.0.
+// See accompanying file LICENSE or copy at http://boost.org/LICENSE_1_0.txt
+//
+
+#include <immer/box.hpp>
+
+#include <cassert>
+#include <string>
+
+int main()
+{
+    {
+        // include:update/start
+        auto v1 = immer::box<std::string>{"hello"};
+        auto v2 = v1.update([&](auto l) { return l + ", world!"; });
+
+        assert((v1 == immer::box<std::string>{"hello"}));
+        assert((v2 == immer::box<std::string>{"hello, world!"}));
+        // include:update/end
+    }
+}