xcb: Refactor align of xcb events

Use C++11 alignas instead of a union to pad the
xcb_*_event. It allows using the struct directly
without accessing a union member.

Change-Id: I221a6708ef7af844bd6b71a57dcbab75e1319c72
Reviewed-by: Jesus Fernandez <Jesus.Fernandez@qt.io>

src/plugins/platforms/xcb/qxcbclipboard.cpp

@@ -581,7 +581,7 @@ void QXcbClipboard::handleSelectionRequest(xcb_selection_request_event_t *req)
         return;
     }
 
-    Q_DECLARE_XCB_EVENT(event, xcb_selection_notify_event_t);
+    q_padded_xcb_event<xcb_selection_notify_event_t> event = {};
     event.response_type = XCB_SELECTION_NOTIFY;
     event.requestor = req->requestor;
     event.selection = req->selection;

src/plugins/platforms/xcb/qxcbconnection.h

@@ -772,18 +772,11 @@ struct QStdFreeDeleter {
         call##_reply(Q_XCB_REPLY_CONNECTION_ARG(__VA_ARGS__), call##_unchecked(__VA_ARGS__), nullptr) \
     )
 
-template <typename T>
-union q_padded_xcb_event {
-  T event;
-  char padding[32];
-};
-
 // The xcb_send_event() requires all events to have 32 bytes. It calls memcpy() on the
 // passed in event. If the passed in event is less than 32 bytes, memcpy() reaches into
 // unrelated memory.
-#define Q_DECLARE_XCB_EVENT(event_var, event_type) \
-    q_padded_xcb_event<event_type> store = {}; \
-    auto &event_var = store.event;
+template <typename T>
+struct alignas(32) q_padded_xcb_event : T { };
 
 QT_END_NAMESPACE
 

src/plugins/platforms/xcb/qxcbdrag.cpp

@@ -1112,8 +1112,7 @@ static xcb_window_t findXdndAwareParent(QXcbConnection *c, xcb_window_t window)
 void QXcbDrag::handleSelectionRequest(const xcb_selection_request_event_t *event)
 {
     qCDebug(lcQpaXDnd) << "handle selection request from target:" << event->requestor;
-
-    Q_DECLARE_XCB_EVENT(notify, xcb_selection_notify_event_t);
+    q_padded_xcb_event<xcb_selection_notify_event_t> notify = {};
     notify.response_type = XCB_SELECTION_NOTIFY;
     notify.requestor = event->requestor;
     notify.selection = event->selection;

src/plugins/platforms/xcb/qxcbwindow.cpp

@@ -800,7 +800,7 @@ void QXcbWindow::hide()
     xcb_unmap_window(xcb_connection(), m_window);
 
     // send synthetic UnmapNotify event according to icccm 4.1.4
-    Q_DECLARE_XCB_EVENT(event, xcb_unmap_notify_event_t);
+    q_padded_xcb_event<xcb_unmap_notify_event_t> event = {};
     event.response_type = XCB_UNMAP_NOTIFY;
     event.event = xcbScreen()->root();
     event.window = m_window;