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, 2 insertions, 2 deletions

diff --git a/doc/src/snippets/widgets-tutorial/nestedlayouts/main.cpp b/doc/src/snippets/widgets-tutorial/nestedlayouts/main.cpp
index 6a49be8..35d2fa7 100644
--- a/doc/src/snippets/widgets-tutorial/nestedlayouts/main.cpp
+++ b/doc/src/snippets/widgets-tutorial/nestedlayouts/main.cpp
@@ -72,9 +72,9 @@ int main(int argc, char *argv[])
         << (QStringList() << "David Bradley" << "42")
         << (QStringList() << "Knut Walters" << "25")
         << (QStringList() << "Andrea Jones" << "34");
-    foreach (QStringList row, rows) {
+    foreach (const QStringList &row, rows) {
         QList<QStandardItem *> items;
-        foreach (QString text, row)
+        foreach (const QString &text, row)
             items.append(new QStandardItem(text));
         model.appendRow(items);
     }