QObject: optimize the common case of findChildren(QString())

Outside tests, all in-tree callers of QObject::findChildren() pass no
name to match, and in my experience that is also true for the vast
majority of out-of-tree users.

Avoid the temporary QString creation in the caller and the repeated
QString::isNull() checks in the implementation by overloading
findChildren() without a name argument and checking for name.isNull()
only once, forking off into separate helper functions.

Adjust in-tree callers that used an explicit `QString()` argument in
order to pass options, which goes to show that `name` should never
have been the first argument of findChilden() in the first place, even
though I appreciate the symmetry with findChild() (the use-cases of
which, however, are radically different).

Change a `findChildren().size() == 0` call found while scanning for
findChildren() calls to `!findChild()` as a drive-by.

Modernize loops in the various qt_qFindChild{,ren}_helper() overloads
to match how the new code looks.

[ChangeLog][QtCore][QObject] Added findChildren() overload taking no
name (thus optimizing this common case).

Change-Id: Ifc56e5438023d079b40c67f11ae274a3e128ad5e
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

2 files changed, 9 insertions, 9 deletions

diff --git a/src/widgets/widgets/qdockarealayout.cpp b/src/widgets/widgets/qdockarealayout.cpp
index 7776ae6613..ee200de57d 100644
--- a/src/widgets/widgets/qdockarealayout.cpp
+++ b/src/widgets/widgets/qdockarealayout.cpp
@@ -2607,7 +2607,7 @@ void QDockAreaLayout::removePlaceHolder(const QString &name)
     if (!index.isEmpty())
         remove(index);
     const auto groups =
-            mainWindow->findChildren<QDockWidgetGroupWindow *>(QString(), Qt::FindDirectChildrenOnly);
+            mainWindow->findChildren<QDockWidgetGroupWindow *>(Qt::FindDirectChildrenOnly);
     for (QDockWidgetGroupWindow *dwgw : groups) {
         index = dwgw->layoutInfo()->indexOfPlaceHolder(name);
         if (!index.isEmpty()) {
@@ -3045,7 +3045,7 @@ bool QDockAreaLayout::restoreDockWidget(QDockWidget *dockWidget)
 {
     QDockAreaLayoutItem *item = nullptr;
     const auto groups =
-            mainWindow->findChildren<QDockWidgetGroupWindow *>(QString(), Qt::FindDirectChildrenOnly);
+            mainWindow->findChildren<QDockWidgetGroupWindow *>(Qt::FindDirectChildrenOnly);
     for (QDockWidgetGroupWindow *dwgw : groups) {
         QList<int> index = dwgw->layoutInfo()->indexOfPlaceHolder(dockWidget->objectName());
         if (!index.isEmpty()) {
diff --git a/src/widgets/widgets/qmainwindowlayout.cpp b/src/widgets/widgets/qmainwindowlayout.cpp
index 59e2e4e57a..18dd05e63b 100644
--- a/src/widgets/widgets/qmainwindowlayout.cpp
+++ b/src/widgets/widgets/qmainwindowlayout.cpp
@@ -431,7 +431,7 @@ void QDockWidgetGroupWindow::destroyOrHideIfEmpty()
     }
 
     // Make sure to reparent the possibly floating or hidden QDockWidgets to the parent
-    const auto dockWidgets = findChildren<QDockWidget *>(QString(), Qt::FindDirectChildrenOnly);
+    const auto dockWidgets = findChildren<QDockWidget *>(Qt::FindDirectChildrenOnly);
     for (QDockWidget *dw : dockWidgets) {
         bool wasFloating = dw->isFloating();
         bool wasHidden = dw->isHidden();
@@ -451,7 +451,7 @@ void QDockWidgetGroupWindow::destroyOrHideIfEmpty()
             dw->show();
     }
 #if QT_CONFIG(tabbar)
-    const auto tabBars = findChildren<QTabBar *>(QString(), Qt::FindDirectChildrenOnly);
+    const auto tabBars = findChildren<QTabBar *>(Qt::FindDirectChildrenOnly);
     for (QTabBar *tb : tabBars)
         tb->setParent(parentWidget());
 #endif
@@ -1056,7 +1056,7 @@ void QMainWindowLayoutState::saveState(QDataStream &stream) const
     dockAreaLayout.saveState(stream);
 #if QT_CONFIG(tabbar)
     const QList<QDockWidgetGroupWindow *> floatingTabs =
-        mainWindow->findChildren<QDockWidgetGroupWindow *>(QString(), Qt::FindDirectChildrenOnly);
+        mainWindow->findChildren<QDockWidgetGroupWindow *>(Qt::FindDirectChildrenOnly);
 
     for (QDockWidgetGroupWindow *floating : floatingTabs) {
         if (floating->layoutInfo()->isEmpty())
@@ -1835,7 +1835,7 @@ QDockAreaLayoutInfo *QMainWindowLayout::dockInfo(QWidget *widget)
     if (info)
         return info;
     const auto groups =
-            parent()->findChildren<QDockWidgetGroupWindow*>(QString(), Qt::FindDirectChildrenOnly);
+            parent()->findChildren<QDockWidgetGroupWindow*>(Qt::FindDirectChildrenOnly);
     for (QDockWidgetGroupWindow *dwgw : groups) {
         info = dwgw->layoutInfo()->info(widget);
         if (info)
@@ -2104,7 +2104,7 @@ bool QMainWindowLayout::plug(QLayoutItem *widgetItem)
         previousPath = currentHoveredFloat->layoutInfo()->indexOf(widget);
         // Let's remove the widget from any possible group window
         const auto groups =
-                parent()->findChildren<QDockWidgetGroupWindow*>(QString(), Qt::FindDirectChildrenOnly);
+                parent()->findChildren<QDockWidgetGroupWindow*>(Qt::FindDirectChildrenOnly);
         for (QDockWidgetGroupWindow *dwgw : groups) {
             if (dwgw == currentHoveredFloat)
                 continue;
@@ -2134,7 +2134,7 @@ bool QMainWindowLayout::plug(QLayoutItem *widgetItem)
 #if QT_CONFIG(dockwidget)
     // Let's remove the widget from any possible group window
     const auto groups =
-            parent()->findChildren<QDockWidgetGroupWindow*>(QString(), Qt::FindDirectChildrenOnly);
+            parent()->findChildren<QDockWidgetGroupWindow*>(Qt::FindDirectChildrenOnly);
     for (QDockWidgetGroupWindow *dwgw : groups) {
         QList<int> path = dwgw->layoutInfo()->indexOf(widget);
         if (!path.isEmpty())
@@ -2730,7 +2730,7 @@ void QMainWindowLayout::applyState(QMainWindowLayoutState &newState, bool animat
 #if QT_CONFIG(dockwidget) && QT_CONFIG(tabwidget)
     QSet<QTabBar*> used = newState.dockAreaLayout.usedTabBars();
     const auto groups =
-            parent()->findChildren<QDockWidgetGroupWindow*>(QString(), Qt::FindDirectChildrenOnly);
+            parent()->findChildren<QDockWidgetGroupWindow*>(Qt::FindDirectChildrenOnly);
     for (QDockWidgetGroupWindow *dwgw : groups)
         used += dwgw->layoutInfo()->usedTabBars();