Return reference in small_vector::emplace_back

Summary: Since C++17 `vector::emplace_back()` returns a reference to the new element, support that in `small_vector` too.

Reviewed By: terrelln, luciang

Differential Revision: D15007059

fbshipit-source-id: 3873fd0097e3d50f7967f47fbecfa6ab20cc72c5

folly/small_vector.h

@@ -779,7 +779,7 @@ class small_vector : public detail::small_vector_base<
   }
 
   template <class... Args>
-  void emplace_back(Args&&... args) {
+  reference emplace_back(Args&&... args) {
     if (capacity() == size()) {
       // Any of args may be references into the vector.
       // When we are reallocating, we have to be careful to construct the new
@@ -792,10 +792,11 @@ class small_vector : public detail::small_vector_base<
       new (end()) value_type(std::forward<Args>(args)...);
     }
     this->setSize(size() + 1);
+    return back();
   }
 
   void push_back(value_type&& t) {
-    return emplace_back(std::move(t));
+    emplace_back(std::move(t));
   }
 
   void push_back(value_type const& t) {

folly/test/small_vector_test.cpp

@@ -435,17 +435,24 @@ TEST(small_vector, Emplace) {
 
   folly::small_vector<NontrivialType> vec;
   vec.reserve(1024);
-  vec.emplace_back(12);
+  {
+    auto& emplaced = vec.emplace_back(12);
     EXPECT_EQ(NontrivialType::ctored, 1);
     EXPECT_EQ(vec.front().a, 12);
-  vec.emplace_back(13);
+    EXPECT_TRUE(std::addressof(emplaced) == std::addressof(vec.back()));
+  }
+  {
+    auto& emplaced = vec.emplace_back(13);
     EXPECT_EQ(vec.front().a, 12);
     EXPECT_EQ(vec.back().a, 13);
     EXPECT_EQ(NontrivialType::ctored, 2);
+    EXPECT_TRUE(std::addressof(emplaced) == std::addressof(vec.back()));
+  }
 
   NontrivialType::ctored = 0;
   for (int i = 0; i < 120; ++i) {
-    vec.emplace_back(i);
+    auto& emplaced = vec.emplace_back(i);
+    EXPECT_TRUE(std::addressof(emplaced) == std::addressof(vec.back()));
   }
   EXPECT_EQ(NontrivialType::ctored, 120);
   EXPECT_EQ(vec[0].a, 12);