QtWidgets: prevent detach attempts from first()/etc. use [dialogs, kernel, util, widgets]

The algorithm used was:
- If possible, just declare the container const
- Otherwise, for first()/last(), use constFirst()/constLast()
  and for front()/back(), to not destroy the use of the STL
  API subset, use qAsConst()

Did some caching of function returns here and there, and
converted one 0 to nullptr as a drive-by.

Also saves almost 4KiB in text size on optimized GCC 4.9
Linux AMD64 builds.

Change-Id: I04b7bfd68dc85c22de247cb65a310e1cbbca1e8c
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Lars Knoll <lars.knoll@theqtcompany.com>
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>

1 files changed, 2 insertions, 2 deletions

diff --git a/src/widgets/util/qcompleter.cpp b/src/widgets/util/qcompleter.cpp
index 9c61a3a263..9f167f5924 100644
--- a/src/widgets/util/qcompleter.cpp
+++ b/src/widgets/util/qcompleter.cpp
@@ -755,9 +755,9 @@ void QUnsortedModelEngine::filterOnDemand(int n)
     const QAbstractItemModel *model = c->proxy->sourceModel();
     int lastRow = model->rowCount(curParent) - 1;
     QIndexMapper im(curMatch.indices.last() + 1, lastRow);
-    int lastIndex = buildIndices(curParts.last(), curParent, n, im, &curMatch);
+    int lastIndex = buildIndices(curParts.constLast(), curParent, n, im, &curMatch);
     curMatch.partial = (lastRow != lastIndex);
-    saveInCache(curParts.last(), curParent, curMatch);
+    saveInCache(curParts.constLast(), curParent, curMatch);
 }
 
 QMatchData QUnsortedModelEngine::filter(const QString& part, const QModelIndex& parent, int n)