QMessageBox: optimize textToCopy string construction

1. Keep 'separator' a QLatin1String.
   - saves at least two memory allocations
   - necessitates carrying the \n previously prepended to it
     around explicitly
2. Start adding to 'textToCopy' with op+=
   - saves one allocation, costs one -> ±0
   - preallocates more capacity than if we started with
     assignment
3. Collapse three unconditional op+= into one
   - more efficient usage of QStringBuilder
4. Don't collect button texts in a separate variable, but
   append to 'textToCopy' directly.
   - saves at least one memory allocation, probably more
     since the growth increments of 'textToCopy' should
     be larger (due to more content) than those of a new
     variable.

Also replace index-based iteration over the buttons with
C++11 range-for over a const QList. Avoids the detach that
happened previously, due to use of op[] instead of at(),
but frankly, I was just too lazy to separate this change.

Change-Id: I27a46a6a163c16d773124f140e085325b17ce5d1
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>

1 files changed, 10 insertions, 13 deletions

diff --git a/src/widgets/dialogs/qmessagebox.cpp b/src/widgets/dialogs/qmessagebox.cpp
index 4ad235051c..76a9de0f1c 100644
--- a/src/widgets/dialogs/qmessagebox.cpp
+++ b/src/widgets/dialogs/qmessagebox.cpp
@@ -1478,24 +1478,21 @@ void QMessageBox::keyPressEvent(QKeyEvent *e)
 
 #if defined(Q_OS_WIN)
         if (e == QKeySequence::Copy) {
-            QString separator = QString::fromLatin1("---------------------------\n");
-            QString textToCopy = separator;
-            separator.prepend(QLatin1Char('\n'));
-            textToCopy += windowTitle() + separator; // title
-            textToCopy += d->label->text() + separator; // text
+            const QLatin1String separator("---------------------------\n");
+            QString textToCopy;
+            textToCopy += separator + windowTitle() + QLatin1Char('\n') + separator // title
+                          + d->label->text() + QLatin1Char('\n') + separator;       // text
 
             if (d->informativeLabel)
-                textToCopy += d->informativeLabel->text() + separator;
+                textToCopy += d->informativeLabel->text() + QLatin1Char('\n') + separator;
 
-            QString buttonTexts;
-            QList<QAbstractButton *> buttons = d->buttonBox->buttons();
-            for (int i = 0; i < buttons.count(); i++) {
-                buttonTexts += buttons[i]->text() + QLatin1String("   ");
-            }
-            textToCopy += buttonTexts + separator;
+            const QList<QAbstractButton *> buttons = d->buttonBox->buttons();
+            for (const auto *button : buttons)
+                textToCopy += button->text() + QLatin1String("   ");
+            textToCopy += QLatin1Char('\n') + separator;
 #ifndef QT_NO_TEXTEDIT
             if (d->detailsText)
-                textToCopy += d->detailsText->text() + separator;
+                textToCopy += d->detailsText->text() + QLatin1Char('\n') + separator;
 #endif
             QApplication::clipboard()->setText(textToCopy);
             return;