Add implicit default c-tor for PrimaryPtrRef

Summary: For to passing over empty refs and default constructing non-owning references to the owning PrimaryPtr.

Reviewed By: ot

Differential Revision: D48320199

fbshipit-source-id: 3ab09a1b4281b65989accd633214377d043cd5b8

folly/experimental/PrimaryPtr.h

@@ -312,6 +312,8 @@ PrimaryPtr<T> exchange(PrimaryPtr<T>& x, PrimaryPtr<T>&& newVal) noexcept {
 template <typename T>
 class PrimaryPtrRef {
  public:
+  PrimaryPtrRef() = default;
+
   // Attempts to lock a pointer. Returns null if pointer is not set or if
   // join() was called or cleanup() work was started (even if the call to join()
   // hasn't returned yet or the cleanup() work has not completed yet).

folly/experimental/test/PrimaryPtrTest.cpp

@@ -285,3 +285,19 @@ TEST(PrimaryPtrTest, Moves) {
   b.join();
   c.join();
 }
+
+TEST(PrimaryPtrTest, DefaultConstructPtrRef) {
+  auto ptr = std::make_unique<Primed>();
+  auto primaryPtr = folly::PrimaryPtr<Primed>{std::move(ptr)};
+
+  folly::PrimaryPtrRef<Primed> primaryPtrRef;
+  EXPECT_FALSE((bool)primaryPtrRef.lock());
+
+  primaryPtrRef = primaryPtr.ref();
+  EXPECT_TRUE((bool)primaryPtrRef.lock());
+  EXPECT_EQ(primaryPtrRef.lock(), primaryPtr.lock());
+
+  primaryPtr.join();
+  EXPECT_FALSE((bool)primaryPtr);
+  EXPECT_EQ(primaryPtrRef.lock().get(), nullptr);
+}