Bug 1041822 - Add LookupOrAdd to nsClassHashtable. r=bsmedberg

xpcom/glue/nsClassHashtable.h

@@ -35,6 +35,13 @@ public:
   {
   }
 
+  /**
+   * Looks up aKey in the hash table. If it doesn't exist a new object of
+   * KeyClass will be created (using its default constructor) and then
+   * returned.
+   */
+  UserDataType LookupOrAdd(KeyType aKey);
+
   /**
    * @copydoc nsBaseHashtable::Get
    * @param aData if the key doesn't exist, pData will be set to nullptr.
@@ -65,6 +72,17 @@ public:
 // nsClassHashtable definitions
 //
 
+template<class KeyClass, class T>
+T*
+nsClassHashtable<KeyClass, T>::LookupOrAdd(KeyType aKey)
+{
+  typename base_type::EntryType* ent = this->PutEntry(aKey);
+  if (!ent->mData) {
+    ent->mData = new T();
+  }
+  return ent->mData;
+}
+
 template<class KeyClass, class T>
 bool
 nsClassHashtable<KeyClass, T>::Get(KeyType aKey, T** aRetVal) const