Some QDataSets are be mutable as well, meaning their values can be assigned as well as read. In addition to the value() method they have putValue() methods. These datasets cannot be written to once they are made immutable by calling the makeImmutable() of MutablePropertyDataSet, and clients must check the isImmutable flag or call Ops.maybeCopy when they need write access to the data. Mutable datasets warning: No dataset should be mutable once it is accessible to the rest of the system. This would require clients make defensive copies which would seriously degrade performance.