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Q_DECLARE_OPAQUE_POINTER: Document footgun

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Using Q_DECLARE_OPAQUE_POINTER means that the metatype system won't have
any knowledge about the type, beyond it being a pointer. That leads to
all kinds of unexpected errors in code that needs to introspect the
meta-type information to handle QObject's and gadgets, notably but not
limited to QML.

Warn about using Q_DECLARE_OPAQUE_POINTER for gadgets and QObject's, and
offer Q_MOC_INCLUDE as a potential alternative for some use cases.

Pick-to: 6.8 6.5
Task-number: QTBUG-127012
Task-number: QTBUG-134883
Change-Id: I4fe53f277d0aa51ef552d04f642364aa6613ef7c
Reviewed-by: Marc Mutz <marc.mutz@qt.io>
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>
(cherry picked from commit 0a567a7f926f433cbd1294131aac0e80dcfd7943)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>
This commit is contained in:
Fabian Kosmale 2025-03-20 11:42:27 +01:00 committed by Qt Cherry-pick Bot
parent bbceab7932
commit 4a79ed90f1
1 changed files with 10 additions and 0 deletions
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10
src/corelib/kernel/qmetatype.cpp
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@ -220,6 +220,16 @@ const char *QtMetaTypePrivate::typedefNameForType(const QtPrivate::QMetaTypeInte
to be registered with QMetaType using either Q_DECLARE_METATYPE()
or qRegisterMetaType().
Do not use this macro to avoid complaints or errors from \l moc about
incomplete property types when the pointee type is used as a complete type in
other contexts in the program. Use \l{Q_MOC_INCLUDE} instead when
the full definition of the type is available, but you prefer a forward
declaration in the header to reduce compilation times.
\warning Don't use Q_DECLARE_OPAQUE_POINTER with pointers to a Q_OBJECT or
a gadget class, as that risks introducing inconsistent information in the
meta-type system.
\sa Q_DECLARE_METATYPE(), qRegisterMetaType()
*/