Optimise QStringList::join by pre-allocating the final size.

This avoids a number of reallocations due to appending.

This patch was contributed to us.

Task-number: QTBUG-3242
Reviewed-by: Thiago Macieira
(cherry picked from commit 7461ed5227e3002c4a6f74d458aa0255b7c1217d)

1 files changed, 10 insertions, 0 deletions

diff --git a/src/corelib/tools/qstringlist.cpp b/src/corelib/tools/qstringlist.cpp
index 4b0f488..e9bdf57 100644
--- a/src/corelib/tools/qstringlist.cpp
+++ b/src/corelib/tools/qstringlist.cpp
@@ -404,7 +404,17 @@ void QtPrivate::QStringList_replaceInStrings(QStringList *that, const QRegExp &r
 */
 QString QtPrivate::QStringList_join(const QStringList *that, const QString &sep)
 {
+    int totalLength = 0;
+    const int size = that->size();
+
+    for (int i = 0; i < size; ++i)
+        totalLength += that->at(i).size();
+
+    if(size > 0)
+        totalLength += sep.size() * (size - 1);
+
     QString res;
+    res.reserve(totalLength);
     for (int i = 0; i < that->size(); ++i) {
         if (i)
             res += sep;