Use the fast operator+ (in the form of operator%)

QStringBuilder will precalculate the size of the string for us, which
avoids extra realloc() and moving data around.

Change-Id: I4e31964bb9bfffbe2083b3cb8c120e602160dfd8
Reviewed-by: David Faure <david.faure@kdab.com>
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@digia.com>
Reviewed-by: Kevin Ottens <kevin.ottens@kdab.com>

src/corelib/io/qlockfile_unix.cpp

@@ -142,13 +142,10 @@ QLockFile::LockError QLockFilePrivate::tryLock_sys()
 {
     // Assemble data, to write in a single call to write
     // (otherwise we'd have to check every write call)
-    QByteArray fileData;
-    fileData += QByteArray::number(QCoreApplication::applicationPid());
-    fileData += '\n';
-    fileData += qAppName().toUtf8();
-    fileData += '\n';
-    fileData += localHostName().toUtf8();
-    fileData += '\n';
+    // Use operator% from the fast builder to avoid multiple memory allocations.
+    QByteArray fileData = QByteArray::number(QCoreApplication::applicationPid()) % '\n'
+                          % qAppName().toUtf8() % '\n'
+                          % localHostName().toUtf8() % '\n';
 
     const QByteArray lockFileName = QFile::encodeName(fileName);
     const int fd = qt_safe_open(lockFileName.constData(), O_WRONLY | O_CREAT | O_EXCL, 0644);