Avoid UB in moc generated code

Introduce a Q_OFFSETOF() macro that uses the optional support of
offsetof() for non standard layout types and disables the
corresponding compiler warnings. All our supported compilers
support offsetof() on non standard layout types.

Use the macro to do the offset calculations required in moc
generated code to replace a manual offset calculation that
was dereferencing a null pointer.

Change-Id: I4aab3af3c8bbaa90372f2234aa1cf8399d023c22
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

1 files changed, 11 insertions, 0 deletions

diff --git a/src/corelib/global/qglobal.h b/src/corelib/global/qglobal.h
index 539e3e7cda..94de9fd79e 100644
--- a/src/corelib/global/qglobal.h
+++ b/src/corelib/global/qglobal.h
@@ -1316,6 +1316,17 @@ template <typename T> struct QEnableIf<true, T> { typedef T Type; };
 #define QT_DEFINE_PLATFORM_INTERFACE(...) QT_OVERLOADED_MACRO(QT_DEFINE_PLATFORM_INTERFACE, QPlatformInterface, __VA_ARGS__)
 #define QT_DEFINE_PRIVATE_PLATFORM_INTERFACE(...) QT_OVERLOADED_MACRO(QT_DEFINE_PLATFORM_INTERFACE, QPlatformInterface::Private, __VA_ARGS__)
 
+// This macro can be used to calculate member offsets for types with a non standard layout.
+// It uses the fact that offsetof() is allowed to support those types since C++17 as an optional
+// feature. All our compilers do support this, but some issue a warning, so we wrap the offsetof()
+// call in a macro that disables the compiler warning.
+#define Q_OFFSETOF(Class, member) \
+    []() -> size_t { \
+        QT_WARNING_PUSH QT_WARNING_DISABLE_INVALID_OFFSETOF \
+        return offsetof(Class, member); \
+        QT_WARNING_POP \
+    }()
+
 QT_END_NAMESPACE
 
 // We need to keep QTypeInfo, QSysInfo, QFlags, qDebug & family in qglobal.h for compatibility with Qt 4.