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, 1 insertions, 1 deletions

diff --git a/src/tools/moc/generator.cpp b/src/tools/moc/generator.cpp
index da3f22aa96..06b2ffc06f 100644
--- a/src/tools/moc/generator.cpp
+++ b/src/tools/moc/generator.cpp
@@ -1613,7 +1613,7 @@ void Generator::generateQPropertyApi()
     for (const PrivateQPropertyDef &property: cdef->privateQProperties) {
         auto printAccessor = [this, property](bool constAccessor = false) {
             const char *constOrNot = constAccessor ? "const " : " ";
-            fprintf(out, "    const size_t propertyMemberOffset = reinterpret_cast<size_t>(&(static_cast<%s *>(nullptr)->%s));\n", cdef->qualified.constData(), property.name.constData());
+            fprintf(out, "    const size_t propertyMemberOffset = Q_OFFSETOF(%s, %s);\n", cdef->qualified.constData(), property.name.constData());
             fprintf(out, "    %sauto *thisPtr = reinterpret_cast<%s%s *>(reinterpret_cast<%schar *>(this) - propertyMemberOffset);\n", constOrNot, constOrNot, cdef->qualified.constData(), constOrNot);
         };