Add QArrayData::sharedNullData()

Just to simplify a few operations, like detecting when a QChar* or char*
coming from a QString or QByteArray, respectively, were null data.

While you're not supposed to dereference the pointer returned by
QVector::data() unless you know that the array is non-empty, that is
permitted for QString and QByteArray. That is, QString().constData()
must return a valid pointer to a null QChar.

Change-Id: I80b4b62f203dc841e5c99c20c51d92ca576e4bfe
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>

1 files changed, 12 insertions, 0 deletions

diff --git a/src/corelib/tools/qarraydata.h b/src/corelib/tools/qarraydata.h
index a2d9901677..a91dd54262 100644
--- a/src/corelib/tools/qarraydata.h
+++ b/src/corelib/tools/qarraydata.h
@@ -122,6 +122,12 @@ struct Q_CORE_EXPORT QArrayData
 
     static const QArrayData shared_null[2];
     static QArrayData *sharedNull() noexcept { return const_cast<QArrayData*>(shared_null); }
+    static void *sharedNullData()
+    {
+        QArrayData *const null = const_cast<QArrayData *>(&shared_null[1]);
+        Q_ASSERT(sharedNull()->data() == null);
+        return null;
+    }
 };
 
 Q_DECLARE_OPERATORS_FOR_FLAGS(QArrayData::AllocationOptions)
@@ -273,6 +279,12 @@ struct QTypedArrayData
         Q_STATIC_ASSERT(sizeof(QTypedArrayData) == sizeof(QArrayData));
         return allocate(/* capacity */ 0);
     }
+
+    static T *sharedNullData()
+    {
+        Q_STATIC_ASSERT(sizeof(QTypedArrayData) == sizeof(QArrayData));
+        return static_cast<T *>(QArrayData::sharedNullData());
+    }
 };
 
 template <class T, size_t N>