Make *Observer::getUnderlyingObserver return a reference

Summary: Returning by reference is safe and is cheaper. Observer API is richer (e.g. needsRefresh) and if someone needs access to it it's useful to avoid a copy.

Reviewed By: wangyishuo123

Differential Revision: D70140149

fbshipit-source-id: 2aba0bf34606207aadbc5740910c0b1902fd5a3b

Author: Andrii Grynenko

folly/observer/Observer.h

@@ -360,7 +360,7 @@ class AtomicObserver {
   T get() const;
   T operator*() const { return get(); }
 
-  Observer<T> getUnderlyingObserver() const { return observer_; }
+  const Observer<T>& getUnderlyingObserver() const { return observer_; }
 
  private:
   mutable std::atomic<T> cachedValue_{};
@@ -379,7 +379,7 @@ class TLObserver {
   const Snapshot<T>& getSnapshotRef() const;
   const Snapshot<T>& operator*() const { return getSnapshotRef(); }
 
-  Observer<T> getUnderlyingObserver() const { return observer_; }
+  const Observer<T>& getUnderlyingObserver() const { return observer_; }
 
  private:
   Observer<T> observer_;
@@ -397,7 +397,7 @@ class ReadMostlyAtomicObserver {
   T get() const;
   T operator*() const { return get(); }
 
-  Observer<T> getUnderlyingObserver() const { return observer_; }
+  const Observer<T>& getUnderlyingObserver() const { return observer_; }
 
  private:
   Observer<T> observer_;