Thread-Support in Qt Modules

Threads and Implicitly Shared Classes

Qt uses an optimization called implicit sharing for many of its value class, notably QImage and QString. Beginning with Qt 4, implicit shared classes can safely be copied across threads, like any other value classes. They are fully reentrant. The implicit sharing is really implicit.

In many people's minds, implicit sharing and multithreading are incompatible concepts, because of the way the reference counting is typically done. Qt, however, uses atomic reference counting to ensure the integrity of the shared data, avoiding potential corruption of the reference counter.

Note that atomic reference counting does not guarantee thread-safety. Proper locking should be used when sharing an instance of an implicitly shared class between threads. This is the same requirement placed on all reentrant classes, shared or not. Atomic reference counting does, however, guarantee that a thread working on its own, local instance of an implicitly shared class is safe. We recommend using signals and slots to pass data between threads, as this can be done without the need for any explicit locking.

To sum it up, implicitly shared classes in Qt 4 are really implicitly shared. Even in multithreaded applications, you can safely use them as if they were plain, non-shared, reentrant value-based classes.