QVarLengthArray: fix memory leak in (qsizetype) ctorv6.4.36.4.3

Instead of duplicating the logic of resize() to avoid three stores to
(ptr, a, s), and getting the memory management wrong (malloc()ed ptr
leaked when a following default-construction failed), simply call the
QVLA default ctor followed by std::uninitialized_default_construct_n().

After we called the QVLA default ctor, the lifetime of this object has
begun, so if the following code throws, ~QVLA will be called to clean
up. This was not the case when we didn't delegate the ctor: if the
body of this ctor threw, ~QVLA would _not_ have been called, since the
lifetime of the object had never started.

Since the dtor may now run, we need to maintain the class invariants,
so we can't just set the size() before we have constructed all
elements. This is where std::uninitialized_default_construct_n() comes
in: it's strongly exception-safe, so if a constructor throws, it will
destroy all previously-constructed elements, so that either we fail
and size() == 0 is true or it doesn't, then we can set size() to
asize.

Change-Id: I03408d8a473e8a31fa0086ef5ac551ce46a758f8
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
(cherry picked from commit 217f2ab3eb04b7489959e8e1134184e8071dbaf0)

0 files changed, 0 insertions, 0 deletions