Avoid spurious detaching in QDir::to/fromNativeSeparators

The new code avoids non-const detaching operations until needed and uses
a pointer into the "raw" QChar data from then on, thus skipping unneeded
checks on the reference count for further detaching.

These functions are used all the time by the file system classes so this
small optimization won't hurt. In particular, it will help users who
already use '/' when passing paths into Qt.

Reviewed-by: Peter Hartmann

1 files changed, 28 insertions, 10 deletions

diff --git a/src/corelib/io/qdir.cpp b/src/corelib/io/qdir.cpp
index b31cf69..f9196e0 100644
--- a/src/corelib/io/qdir.cpp
+++ b/src/corelib/io/qdir.cpp
@@ -802,14 +802,23 @@ QString QDir::convertSeparators(const QString &pathName)
 */
 QString QDir::toNativeSeparators(const QString &pathName)
 {
-    QString n(pathName);
 #if defined(Q_FS_FAT) || defined(Q_OS_OS2EMX) || defined(Q_OS_SYMBIAN)
-    for (int i = 0; i < (int)n.length(); ++i) {
-        if (n[i] == QLatin1Char('/'))
-            n[i] = QLatin1Char('\\');
+    int i = pathName.indexOf(QLatin1Char('/'));
+    if (i != -1) {
+        QString n(pathName);
+
+        QChar * const data = n.data();
+        data[i++] = QLatin1Char('\\');
+
+        for (; i < n.length(); ++i) {
+            if (data[i] == QLatin1Char('/'))
+                data[i] = QLatin1Char('\\');
+        }
+
+        return n;
     }
 #endif
-    return n;
+    return pathName;
 }
 
 /*!
@@ -826,14 +835,23 @@ QString QDir::toNativeSeparators(const QString &pathName)
 */
 QString QDir::fromNativeSeparators(const QString &pathName)
 {
-    QString n(pathName);
 #if defined(Q_FS_FAT) || defined(Q_OS_OS2EMX) || defined(Q_OS_SYMBIAN)
-    for (int i = 0; i < (int)n.length(); ++i) {
-        if (n[i] == QLatin1Char('\\'))
-            n[i] = QLatin1Char('/');
+    int i = pathName.indexOf(QLatin1Char('\\'));
+    if (i != -1) {
+        QString n(pathName);
+
+        QChar * const data = n.data();
+        data[i++] = QLatin1Char('/');
+
+        for (; i < n.length(); ++i) {
+            if (data[i] == QLatin1Char('\\'))
+                data[i] = QLatin1Char('/');
+        }
+
+        return n;
     }
 #endif
-    return n;
+    return pathName;
 }
 
 /*!