X-Git-Url: http://plrg.eecs.uci.edu/git/?a=blobdiff_plain;f=folly%2FMapUtil.h;h=d6be57115c7d55d884abf49360f1a7372ec6145b;hb=6f027b290a0ee1754b655b5e739ecb98e11f2f3d;hp=77e092f6ab5b766aedf2d8e1374f2c2d4c44b3b1;hpb=136c9fba2f6b520946edb2556e1acb14d59ac660;p=folly.git

diff --git a/folly/MapUtil.h b/folly/MapUtil.h
index 77e092f6..d6be5711 100644
--- a/folly/MapUtil.h
+++ b/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