QFlatMap, like its public brethren, features the broken Qt-style insert() behavior (what the STL calls insert_or_assign()), which makes its insert() unusable for actual STL-style insert() work, with no replacement except the size-check-and-index-operator trick: const auto oldSize = c.size(); auto &e = c[key]; if (c.size() != oldSize) { // inserted } Even though QFlatMap::insert() appears to return the correct info, it's useless, because the old value has been assigned over by the time insert() returns. Pick-to: 6.3 6.2 Change-Id: If4173c42523a128dfd22ab496dde0089ba73f41c Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org> Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>