Make SimpleObservable<T> default-constructible if T is default-constructible

Summary: This simplifies generic code.

Reviewed By: andriigrynenko

Differential Revision: D25164246

fbshipit-source-id: 4f42a2360f7ce32978360ddad63d1ca9512e6135

folly/experimental/observer/SimpleObservable-inl.h

@@ -21,6 +21,11 @@
 namespace folly {
 namespace observer {
 
+template <typename T>
+template <typename, typename>
+SimpleObservable<T>::SimpleObservable()
+    : context_(std::make_shared<Context>(std::make_shared<T>())) {}
+
 template <typename T>
 SimpleObservable<T>::SimpleObservable(T value)
     : SimpleObservable(std::make_shared<const T>(std::move(value))) {}

folly/experimental/observer/SimpleObservable.h

@@ -27,6 +27,11 @@ namespace observer {
 template <typename T>
 class SimpleObservable {
  public:
+  template <
+      typename U = T,
+      typename = std::enable_if_t<std::is_default_constructible<U>::value>>
+  SimpleObservable();
+
   explicit SimpleObservable(T value);
   explicit SimpleObservable(std::shared_ptr<const T> value);
 

folly/experimental/observer/test/ObserverTest.cpp

@@ -734,3 +734,14 @@ TEST(Observer, WithJitterNoEarlyRefresh) {
   EXPECT_EQ(0, **laggingObserver);
   EXPECT_EQ(42, **delta);
 }
+
+TEST(SimpleObservable, DefaultConstructible) {
+  struct Data {
+    int i = 42;
+  };
+  static_assert(std::is_default_constructible<Data>::value);
+  static_assert(std::is_default_constructible<SimpleObservable<Data>>::value);
+
+  SimpleObservable<Data> observable;
+  EXPECT_EQ((**observable.getObserver()).i, 42);
+}