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| -rw-r--r-- | src/corelib/doc/src/qt6-changes.qdoc | 39 |
1 files changed, 39 insertions, 0 deletions
diff --git a/src/corelib/doc/src/qt6-changes.qdoc b/src/corelib/doc/src/qt6-changes.qdoc index b92144a699..c34e952f20 100644 --- a/src/corelib/doc/src/qt6-changes.qdoc +++ b/src/corelib/doc/src/qt6-changes.qdoc @@ -795,4 +795,43 @@ PUBLIC_LIBRARIES Qt::Core5Compat \endcode + + \section2 QEvent and subclasses + + The QEvent class defined a copy constructor and an assignment operator, + in spite of being a polymorphic class. Copying classes with virtual methods + can result in slicing when assigning objects from different classes to each + other. Since copying and assigning often happens implicilty, this could + lead to hard-to-debug problems. + + In Qt 6, the copy constructor and assignment operator for QEvent subclasses + have been made protected to prevent implicit copying. If you need to copy + events, use the \l{QEvent::}{clone} method, which will return a heap-allocated + copy of the QEvent object. Make sure you delete the clone, perhaps using + std::unique_ptr, unless you post it (in which case Qt will delete it once it + has been delivered). + + In your QEvent subclasses, override clone(), and declare the protected and + default-implemented copy constructor and assignment operator like this: + + \code + class MyEvent : public QEvent + { + public: + // ... + + MyEvent *clone() const override { return new MyEvent(*this); } + + protected: + MyEvent(const MyEvent &other) = default; + MyEvent &operator=(const MyEvent &other) = default; + MyEvent(MyEvent &&) = delete; + MyEvent &operator=(MyEvent &&) = delete; + // member data + }; + \endcode + + Note that if your MyEvent class allocates memory (e.g. through a + pointer-to-implementation pattern), then you will have to implement + custom copy semantics. */ |