Use the newly-added try_emplace() function, to work around Qt's associative container's broken insert() behavior (which is equivalent to STL's insert_or_assign()), forcing either a double lookup: auto it = c.find(key); // lookup 1 if (it == c.end()) c.insert(key, ~~~); // lookup 2 or the usual size-check trick: const auto oldSize = c.size(); auto &e = c[key]; if (c.size() != oldSize) { // newly inserted onto any lookup-or-initialize-like code. try_emplace beats them both in terms of efficiency and readability. Pick-to: 6.3 Change-Id: I4c52d2f2d5175991db4931d29c688de125f5b032 Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com> Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>