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, 1 insertions, 1 deletions

diff --git a/src/widgets/widgets/qtoolbararealayout.cpp b/src/widgets/widgets/qtoolbararealayout.cpp
index 2e79b502ea..05564bb6b1 100644
--- a/src/widgets/widgets/qtoolbararealayout.cpp
+++ b/src/widgets/widgets/qtoolbararealayout.cpp
@@ -347,7 +347,7 @@ void QToolBarAreaLayoutInfo::removeToolBar(QToolBar *toolBar)
 void QToolBarAreaLayoutInfo::insertToolBarBreak(QToolBar *before)
 {
     if (before == 0) {
-        if (!lines.isEmpty() && lines.last().toolBarItems.isEmpty())
+        if (!lines.isEmpty() && lines.constLast().toolBarItems.isEmpty())
             return;
         lines.append(QToolBarAreaLayoutLine(o));
         return;