Add atomic_ref::exchange

Reviewed By: ot

Differential Revision: D33857528

fbshipit-source-id: 265457c5771684d6830a16a3b7390e9bd99b277d

folly/synchronization/AtomicRef.h

@@ -43,6 +43,11 @@ struct atomic_ref_base {
     return atomic().load(order);
   }
 
+  T exchange(T desired, std::memory_order order = std::memory_order_seq_cst)
+      const noexcept {
+    return atomic().exchange(desired, order);
+  }
+
   bool compare_exchange_weak(
       T& expected,
       T desired,

folly/synchronization/test/AtomicRefTest.cpp

@@ -32,7 +32,8 @@ TEST_F(AtomicRefTest, integer) {
     auto ref = folly::make_atomic_ref(value);
     EXPECT_EQ(17, ref.load(std::memory_order_relaxed));
     ref.store(55, std::memory_order_relaxed);
-    EXPECT_EQ(55, ref.load(std::memory_order_relaxed));
+    EXPECT_EQ(55, ref.exchange(42, std::memory_order_relaxed));
+    EXPECT_EQ(42, ref.load(std::memory_order_relaxed));
   }
 
   {