Path variants for get_(ref_)default()

Summary: Useful for the same reason as the `get_ptr()` path function, but when you want to use a default value.

Reviewed By: luciang, yfeldblum

Differential Revision: D4785728

fbshipit-source-id: 70fd56f9ffa7a9edd6740f6dd712d3a251bf9fb0

folly/MapUtil.h

@@ -18,6 +18,7 @@
 
 #include <folly/Conv.h>
 #include <folly/Optional.h>
+#include <tuple>
 
 namespace folly {
 
@@ -166,6 +167,8 @@ typename Map::mapped_type* get_ptr(
   return (pos != map.end() ? &pos->second : nullptr);
 }
 
+// TODO: Remove the return type computations when clang 3.5 and gcc 5.1 are
+// the minimum supported versions.
 namespace detail {
 template <
     class T,
@@ -179,6 +182,25 @@ template <class T>
 struct NestedMapType<T, 1> {
   using type = typename T::mapped_type;
 };
+
+template <typename... KeysDefault>
+struct DefaultType;
+
+template <typename Default>
+struct DefaultType<Default> {
+  using type = Default;
+};
+
+template <typename Key, typename... KeysDefault>
+struct DefaultType<Key, KeysDefault...> {
+  using type = typename DefaultType<KeysDefault...>::type;
+};
+
+template <class... KeysDefault>
+auto extract_default(const KeysDefault&... keysDefault) ->
+    typename DefaultType<KeysDefault...>::type const& {
+  return std::get<sizeof...(KeysDefault)-1>(std::tie(keysDefault...));
+}
 }
 
 /**
@@ -203,4 +225,53 @@ auto get_ptr(Map& map, const Key1& key1, const Key2& key2, const Keys&... keys)
   return pos != map.end() ? get_ptr(pos->second, key2, keys...) : nullptr;
 }
 
+/**
+ * Given a map and a path of keys, return the value corresponding to the nested
+ * value, or a given default value if the path doesn't exist in the map.
+ * The default value is the last parameter, and is copied when returned.
+ */
+template <
+    class Map,
+    class Key1,
+    class Key2,
+    class... KeysDefault,
+    typename = typename std::enable_if<sizeof...(KeysDefault) != 0>::type>
+auto get_default(
+    const Map& map,
+    const Key1& key1,
+    const Key2& key2,
+    const KeysDefault&... keysDefault) ->
+    typename detail::NestedMapType<Map, 1 + sizeof...(KeysDefault)>::type {
+  if (const auto* ptr = get_ptr(map, key1)) {
+    return get_default(*ptr, key2, keysDefault...);
+  }
+  return detail::extract_default(keysDefault...);
+}
+
+/**
+ * Given a map and a path of keys, return a reference to the value corresponding
+ * to the nested value, or the given default reference if the path doesn't exist
+ * in the map.
+ * The default value is the last parameter, and must be a lvalue reference.
+ */
+template <
+    class Map,
+    class Key1,
+    class Key2,
+    class... KeysDefault,
+    typename = typename std::enable_if<sizeof...(KeysDefault) != 0>::type,
+    typename = typename std::enable_if<std::is_lvalue_reference<
+        typename detail::DefaultType<KeysDefault...>::type>::value>::type>
+auto get_ref_default(
+    const Map& map,
+    const Key1& key1,
+    const Key2& key2,
+    KeysDefault&&... keysDefault) ->
+    typename detail::NestedMapType<Map, 1 + sizeof...(KeysDefault)>::type
+    const& {
+  if (const auto* ptr = get_ptr(map, key1)) {
+    return get_ref_default(*ptr, key2, keysDefault...);
+  }
+  return detail::extract_default(keysDefault...);
+}
 }  // namespace folly

folly/test/MapUtilTest.cpp

@@ -141,6 +141,19 @@ TEST(MapUtil, get_ptr_path_mixed) {
 }
 
 namespace {
+template <typename T>
+struct element_type {
+  using type = typename std::decay<T>::type;
+};
+
+template <typename T>
+struct element_type<T()> {
+  using type = T;
+};
+
+template <typename T>
+using element_type_t = typename element_type<T>::type;
+
 template <typename T, typename = void>
 struct Compiles : std::false_type {};
 
@@ -148,7 +161,7 @@ template <typename T>
 struct Compiles<
     T,
     void_t<decltype(get_ref_default(
-        std::declval<std::map<int, typename std::decay<T>::type>>(),
+        std::declval<std::map<int, element_type_t<T>>>(),
         std::declval<int>(),
         std::declval<T>()))>> : std::true_type {};
 }
@@ -158,4 +171,78 @@ TEST(MapUtil, get_default_temporary) {
   EXPECT_TRUE(Compiles<int&>::value);
   EXPECT_FALSE(Compiles<const int&&>::value);
   EXPECT_FALSE(Compiles<int&&>::value);
+
+  EXPECT_TRUE(Compiles<const int&()>::value);
+  EXPECT_TRUE(Compiles<int&()>::value);
+  EXPECT_FALSE(Compiles<int()>::value);
+}
+
+TEST(MapUtil, get_default_path) {
+  using std::map;
+  map<int, map<int, int>> m;
+  m[4][2] = 42;
+  EXPECT_EQ(42, get_default(m, 4, 2, 42));
+  EXPECT_EQ(42, get_default(m, 1, 3, 42));
+}
+
+TEST(MapUtil, get_default_path_mixed) {
+  using std::map;
+  using std::unordered_map;
+  using std::string;
+  map<int, unordered_map<string, StringPiece>> m;
+  int key1 = 42;
+  const string key2 = "hello";
+  constexpr StringPiece value = "world";
+  constexpr StringPiece dflt = "default";
+  m[key1][key2] = value;
+  EXPECT_EQ(value, get_default(m, 42, key2, dflt));
+  EXPECT_EQ(value, get_default(m, key1, "hello", dflt));
+  EXPECT_EQ(dflt, get_default(m, 0, key2, dflt));
+  EXPECT_EQ(dflt, get_default(m, key1, "bad", "default"));
+}
+
+TEST(MapUtil, get_ref_default_path) {
+  using std::map;
+  map<int, map<int, int>> m;
+  m[4][2] = 42;
+  const int dflt = 13;
+  EXPECT_EQ(42, get_ref_default(m, 4, 2, dflt));
+  EXPECT_EQ(dflt, get_ref_default(m, 1, 3, dflt));
+}
+
+TEST(MapUtil, get_ref_default_path_mixed) {
+  using std::map;
+  using std::unordered_map;
+  using std::string;
+  map<int, unordered_map<string, StringPiece>> m;
+  int key1 = 42;
+  const string key2 = "hello";
+  constexpr StringPiece value = "world";
+  constexpr StringPiece dflt = "default";
+  m[key1][key2] = value;
+  EXPECT_EQ(value, get_ref_default(m, 42, key2, dflt));
+  EXPECT_EQ(value, get_ref_default(m, key1, "hello", dflt));
+  EXPECT_EQ(dflt, get_ref_default(m, 0, key2, dflt));
+  EXPECT_EQ(dflt, get_ref_default(m, key1, "bad", dflt));
+}
+
+namespace {
+template <typename T, typename = void>
+struct GetRefDefaultPathCompiles : std::false_type {};
+
+template <typename T>
+struct GetRefDefaultPathCompiles<
+    T,
+    void_t<decltype(get_ref_default(
+        std::declval<std::map<int, std::map<int, element_type_t<T>>>>(),
+        std::declval<int>(),
+        std::declval<int>(),
+        std::declval<T>()))>> : std::true_type {};
+}
+
+TEST(MapUtil, get_ref_default_path_temporary) {
+  EXPECT_TRUE(GetRefDefaultPathCompiles<const int&>::value);
+  EXPECT_TRUE(GetRefDefaultPathCompiles<int&>::value);
+  EXPECT_FALSE(GetRefDefaultPathCompiles<const int&&>::value);
+  EXPECT_FALSE(GetRefDefaultPathCompiles<int&&>::value);
 }