Doc: use const& in foreach when applicable.

Const reference are slightly faster than doing a copy of the object
on each iteration.

Lead by example by having this pattern right in our documentation

1 files changed, 1 insertions, 1 deletions

diff --git a/doc/src/snippets/droparea.cpp b/doc/src/snippets/droparea.cpp
index bbcf82da02..bb49dd7e63 100644
--- a/doc/src/snippets/droparea.cpp
+++ b/doc/src/snippets/droparea.cpp
@@ -130,7 +130,7 @@ QPixmap DropArea::extractPixmap(const QByteArray &data, const QString &format)
     QList<QByteArray> imageFormats = QImageReader::supportedImageFormats();
     QPixmap pixmap;
 
-    foreach (QByteArray imageFormat, imageFormats) {
+    foreach (const QByteArray &imageFormat, imageFormats) {
         if (format.mid(6) == QString(imageFormat)) {
             pixmap.loadFromData(data, imageFormat);
             break;