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, 2 insertions, 0 deletions

diff --git a/tests/auto/corelib/kernel/qobject/tst_qobject.cpp b/tests/auto/corelib/kernel/qobject/tst_qobject.cpp
index f5c16faa56..3d52478299 100644
--- a/tests/auto/corelib/kernel/qobject/tst_qobject.cpp
+++ b/tests/auto/corelib/kernel/qobject/tst_qobject.cpp
@@ -755,6 +755,8 @@ void tst_QObject::findChildren()
     op = o.findChild<QObject*>("unnamed", Qt::FindDirectChildrenOnly);
     QCOMPARE(op, static_cast<QObject *>(0));
 
+    l = o.findChildren<QObject*>(Qt::FindDirectChildrenOnly);
+    QCOMPARE(l.size(), 5);
     l = o.findChildren<QObject*>(QString(), Qt::FindDirectChildrenOnly);
     QCOMPARE(l.size(), 5);
     l = o.findChildren<QObject*>("", Qt::FindDirectChildrenOnly);