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>

1 files changed, 1 insertions, 1 deletions

diff --git a/src/corelib/doc/snippets/code/src_corelib_kernel_qobject.cpp b/src/corelib/doc/snippets/code/src_corelib_kernel_qobject.cpp
index fddda64b19..8a007899e3 100644
--- a/src/corelib/doc/snippets/code/src_corelib_kernel_qobject.cpp
+++ b/src/corelib/doc/snippets/code/src_corelib_kernel_qobject.cpp
@@ -449,7 +449,7 @@ QListWidget *list = parentWidget->findChild<QListWidget *>(QString(), Qt::FindDi
 
 
 //! [43]
-QList<QPushButton *> childButtons = parentWidget.findChildren<QPushButton *>(QString(), Qt::FindDirectChildrenOnly);
+QList<QPushButton *> childButtons = parentWidget.findChildren<QPushButton *>(Qt::FindDirectChildrenOnly);
 //! [43]
 
 //! [44]