let EventBaseLocal::getOrCreateFn function return value

Summary:
Let the function passed to `EventBaseLocal::getOrCreateFn` directly return the value to be stored, rather than allocating it and returning a pointer.

Details of its storage should be internal to `EventBaseLocal` and should not leak out into the creator functions.

Differential Revision: D26246341

fbshipit-source-id: d72a62998985e42ef517eede3654324dc9f48709

folly/io/async/EventBaseLocal.h

@@ -62,7 +62,7 @@ class EventBaseLocalBase : public EventBaseLocalBaseBase {
  *   Foo& foo = myFoo.getOrCreate(evb, 1, 2); // ctor
  *   Foo& foo = myFoo.getOrCreate(evb, 1, 2); // no ctor
  *   myFoo.erase(evb);
- *   Foo& foo = myFoo.getOrCreateFn(evb, [] () { return new Foo(3, 4); })
+ *   Foo& foo = myFoo.getOrCreateFn(evb, [] { return Foo(3, 4); })
  *
  * The objects will be deleted when the EventBaseLocal or the EventBase is
  * destructed (whichever comes first). All methods must be called from the
@@ -112,8 +112,7 @@ class EventBaseLocal : public detail::EventBaseLocalBase {
     if (auto ptr = getVoid(evb)) {
       return *static_cast<T*>(ptr);
     }
-    std::shared_ptr<T> smartPtr(fn());
-    DCHECK(smartPtr.get() != nullptr);
+    auto smartPtr = std::shared_ptr<T>(new T(fn()));
     auto& ref = *smartPtr;
     setVoid(evb, std::move(smartPtr));
     return ref;

folly/io/async/test/EventBaseLocalTest.cpp

@@ -68,7 +68,7 @@ TEST(EventBaseLocalTest, getOrCreate) {
   folly::EventBase evb2;
   EXPECT_EQ(ints.getOrCreate(evb2, 5), 5);
   ints.erase(evb2);
-  EXPECT_EQ(4, ints.getOrCreateFn(evb2, []() { return new int(4); }));
+  EXPECT_EQ(4, ints.getOrCreateFn(evb2, [] { return 4; }));
 }
 
 using IntPtr = std::unique_ptr<int>;