Backport std::exchange

Summary: [Folly] Backport `std::exchange` to `folly/Utility.h`.

Reviewed By: aary

Differential Revision: D7722731

fbshipit-source-id: a4a68bfb9ec8b356b77f4a73bdadb7f2807d517f

folly/Utility.h

@@ -101,6 +101,23 @@ void as_const(T const&&) = delete;
 
 #endif
 
+#if __cpp_lib_exchange_function || _LIBCPP_STD_VER > 11 || _MSC_VER
+
+/* using override */ using std::exchange;
+
+#else
+
+//  mimic: std::exchange, C++14
+//  from: http://en.cppreference.com/w/cpp/utility/exchange, CC-BY-SA
+template <class T, class U = T>
+T exchange(T& obj, U&& new_value) {
+  T old_value = std::move(obj);
+  obj = std::forward<U>(new_value);
+  return old_value;
+}
+
+#endif
+
 namespace utility_detail {
 template <typename...>
 struct make_seq_cat;

folly/test/UtilityTest.cpp

@@ -77,6 +77,13 @@ TEST_F(UtilityTest, as_const) {
   EXPECT_TRUE(noexcept(folly::as_const(s)));
 }
 
+TEST_F(UtilityTest, exchange) {
+  auto obj = std::map<std::string, int>{{"hello", 3}};
+  auto old = exchange(obj, {{"world", 4}});
+  EXPECT_EQ((std::map<std::string, int>{{"world", 4}}), obj);
+  EXPECT_EQ((std::map<std::string, int>{{"hello", 3}}), old);
+}
+
 TEST(FollyIntegerSequence, core) {
   constexpr auto seq = folly::integer_sequence<int, 0, 3, 2>();
   static_assert(seq.size() == 3, "");