The typical usage of a QDuplicateTracker should be to pass the guesstimate for the number of entries as a constructor argument, or, failing that, default-construct the object. In those cases, everything is peachy: QDuplicateTracker<int> seen(n); QDuplicateTracker<int> seen; But for users that subscribe to a AAA view and use auto seen = QDuplicateTracker<int>{}; or that emplace() a QDuplicateTracker in e.g. std::optional, we need to make sure that value-initializing QDuplicateTracker doesn't value-initialize the internal buffer (which can be several KiB in size). So don't = default the default ctor, but implement it as empty. Amends f21a6d409ea0504c64cd72861fc16b6f3e080086. Pick-to: 6.9 6.8 6.5 6.2 5.15 Change-Id: I930f40d2bb85b74d2216378d8b8ca854a53a49e4 Reviewed-by: Dennis Oberst <dennis.oberst@qt.io> Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com> Reviewed-by: Thiago Macieira <thiago.macieira@intel.com> Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>