dynamic try_emplace

Summary: Expose `try_emplace` to give clients signal on whether an element was actually inserted. This will allow for more efficient code without needing to check for existence before inserting.

Reviewed By: yfeldblum

Differential Revision: D34006997

fbshipit-source-id: 3bdb7d4d81c19d0fcd7c82f194f1f695b79cf57b

folly/dynamic-inl.h

@@ -843,6 +843,20 @@ inline dynamic::IfNotIterator<K, void> dynamic::insert(K&& key, V&& val) {
   obj[std::forward<K>(key)] = std::forward<V>(val);
 }
 
+template <class... Args>
+inline std::pair<dynamic::item_iterator, bool> dynamic::emplace(
+    Args&&... args) {
+  auto& obj = get<ObjectImpl>();
+  return obj.emplace(std::forward<Args>(args)...);
+}
+
+template <class K, class... Args>
+inline std::pair<dynamic::item_iterator, bool> dynamic::try_emplace(
+    K&& key, Args&&... args) {
+  auto& obj = get<ObjectImpl>();
+  return obj.try_emplace(std::forward<K>(key), std::forward<Args>(args)...);
+}
+
 template <class T>
 inline dynamic::iterator dynamic::insert(const_iterator pos, T&& value) {
   auto& arr = get<Array>();

folly/dynamic.h

@@ -682,6 +682,30 @@ struct dynamic {
   template <class K, class V>
   IfNotIterator<K, void> insert(K&&, V&& val);
 
+  /**
+   * Inserts an element into an object constructed in-place with the given args
+   * if there is no existing element with the key, or throws if it's not an
+   * object. Returns a pair consisting of an iterator to the inserted element,
+   * or the already existing element if no insertion happened, and a bool
+   * denoting whether the insertion took place.
+   *
+   * Invalidates iterators.
+   */
+  template <class... Args>
+  std::pair<item_iterator, bool> emplace(Args&&... args);
+
+  /**
+   * Inserts an element into an object with the given key and value constructed
+   * in-place with the given args if there is no existing element with the key,
+   * or throws if it's not an object. Returns a pair consisting of an iterator
+   * to the inserted element, or the already existing element if no insertion
+   * happened, and a bool denoting whether the insertion took place.
+   *
+   * Invalidates iterators.
+   */
+  template <class K, class... Args>
+  std::pair<item_iterator, bool> try_emplace(K&& key, Args&&... args);
+
   /*
    * Inserts the supplied value into array, or throw if not array
    * Shifts existing values in the array to the right

folly/test/DynamicTest.cpp

@@ -103,6 +103,34 @@ TEST(Dynamic, ObjectBasics) {
   dynamic obj2 = dynamic::object;
   EXPECT_TRUE(obj2.isObject());
 
+  dynamic obj3 = dynamic::object("a", false);
+  EXPECT_EQ(obj3.at("a"), false);
+  EXPECT_EQ(obj3.size(), 1);
+  {
+    const auto [it, inserted] = obj3.emplace("a", true);
+    EXPECT_EQ(obj3.size(), 1);
+    EXPECT_FALSE(inserted);
+    EXPECT_EQ(it->second, false);
+  }
+  {
+    const auto [it, inserted] = obj3.emplace("b", true);
+    EXPECT_EQ(obj3.size(), 2);
+    EXPECT_TRUE(inserted);
+    EXPECT_EQ(it->second, true);
+  }
+  {
+    const auto [it, inserted] = obj3.try_emplace("a", true);
+    EXPECT_EQ(obj3.size(), 2);
+    EXPECT_FALSE(inserted);
+    EXPECT_EQ(it->second, false);
+  }
+  {
+    const auto [it, inserted] = obj3.try_emplace("c", true);
+    EXPECT_EQ(obj3.size(), 3);
+    EXPECT_TRUE(inserted);
+    EXPECT_EQ(it->second, true);
+  }
+
   dynamic d3 = nullptr;
   EXPECT_TRUE(d3 == nullptr);
   d3 = dynamic::object;