- 2f6a3c92ce0f64e0923ad326ec5d07eae03061fd Add C++14's std::exchange to absl/utility/utility.h. by Abseil Team <absl-team@google.com>

  - 9f2c9adbf998dd9062c8cc3da1076bde87081d71 Fix comment typo. by Greg Falcon <gfalcon@google.com>

GitOrigin-RevId: 2f6a3c92ce0f64e0923ad326ec5d07eae03061fd
Change-Id: I7b9db94ad315092baefe56b02f250acadafbe312

2 files changed, 30 insertions, 0 deletions

diff --git a/absl/utility/utility.h b/absl/utility/utility.h
index 5b9b84e05d84..d73602c47d3b 100644
--- a/absl/utility/utility.h
+++ b/absl/utility/utility.h
@@ -24,6 +24,7 @@
 //   * make_index_sequence<N>        == std::make_index_sequence<N>
 //   * index_sequence_for<Ts...>     == std::index_sequence_for<Ts...>
 //   * apply<Functor, Tuple>         == std::apply<Functor, Tuple>
+//   * exchange<T>                   == std::exchange<T>
 //
 // This header file also provides the tag types `in_place_t`, `in_place_type_t`,
 // and `in_place_index_t`, as well as the constant `in_place`, and
@@ -264,6 +265,27 @@ auto apply(Functor&& functor, Tuple&& t)
       absl::make_index_sequence<std::tuple_size<
           typename std::remove_reference<Tuple>::type>::value>{});
 }
+
+// exchange
+//
+// Replaces the value of `obj` with `new_value` and returns the old value of
+// `obj`.  `absl::exchange` is designed to be a drop-in replacement for C++14's
+// `std::exchange`.
+//
+// Example:
+//
+//   Foo& operator=(Foo&& other) {
+//     ptr1_ = absl::exchange(other.ptr1_, nullptr);
+//     int1_ = absl::exchange(other.int1_, -1);
+//     return *this;
+//   }
+template <typename T, typename U = T>
+T exchange(T& obj, U&& new_value) {
+  T old_value = absl::move(obj);
+  obj = absl::forward<U>(new_value);
+  return old_value;
+}
+
 }  // namespace absl
 
 #endif  // ABSL_UTILITY_UTILITY_H_
diff --git a/absl/utility/utility_test.cc b/absl/utility/utility_test.cc
index 342165ed5240..3c447b20bb04 100644
--- a/absl/utility/utility_test.cc
+++ b/absl/utility/utility_test.cc
@@ -333,5 +333,13 @@ TEST(ApplyTest, FlipFlop) {
   EXPECT_EQ(42, absl::apply(&FlipFlop::member, std::make_tuple(obj)));
 }
 
+TEST(ExchangeTest, MoveOnly) {
+  auto a = Factory(1);
+  EXPECT_EQ(1, *a);
+  auto b = absl::exchange(a, Factory(2));
+  EXPECT_EQ(2, *a);
+  EXPECT_EQ(1, *b);
+}
+
 }  // namespace