1 files changed, 48 insertions, 0 deletions
diff --git a/events/event.h b/events/event.h
index d9316747..a66f5e68 100644
--- a/events/event.h
+++ b/events/event.h
@@ -83,6 +83,54 @@ namespace QMatrixClient
             }
         );
     }
+
+    /**
+     * @brief Lookup a value by a key in a varargs list
+     *
+     * The below overloaded function template takes the value of its first
+     * argument (selector) as a key and searches for it in the key-value map
+     * passed in a varargs list (every next pair of arguments forms a key-value
+     * pair). If a match is found, the respective value is returned; otherwise,
+     * the last value (fallback) is returned.
+     *
+     * All options should be of the same type or implicitly castable to the
+     * type of the first option. Note that pointers to methods of different
+     * classes are of different object types, in particular.
+     *
+     * Below is an example of usage to select a parser depending on contents of
+     * a JSON object:
+     * {@code
+     *  auto parser = lookup(obj.value["type"].toString(),
+     *                      "type1", fn1,
+     *                      "type2", fn2,
+     *                      fallbackFn);
+     *  parser(obj);
+     * }
+     *
+     * The implementation is based on tail recursion; every recursion step
+     * removes 2 arguments (match and option). There's no selector value for the
+     * fallback option (the last one); therefore, the total number of lookup()
+     * arguments should be even: selector + n key-value pairs + fallback
+     *
+     * @note Beware of calling lookup() with a <code>const char*</code> selector
+     * (the first parameter) - most likely it won't do what you expect because
+     * of shallow comparison.
+     */
+    template <typename ValueT, typename SelectorT, typename KeyT, typename... Ts>
+    ValueT lookup(SelectorT selector, KeyT key, ValueT value, Ts... remainingMapping)
+    {
+        if( selector == key )
+            return value;
+
+        // Drop the failed key-value pair and recurse with 2 arguments less.
+        return lookup(selector, remainingMapping...);
+    }
+
+    template <typename SelectorT, typename ValueT>
+    ValueT lookup(SelectorT/*unused*/, ValueT fallback)
+    {
+        return fallback;
+    }
 }
 
 #endif // QMATRIXCLIENT_EVENT_H