diff --git a/src/plugins/diffeditor/diffeditorplugin.cpp b/src/plugins/diffeditor/diffeditorplugin.cpp
index 023a188a837..e27c3e892db 100644
--- a/src/plugins/diffeditor/diffeditorplugin.cpp
+++ b/src/plugins/diffeditor/diffeditorplugin.cpp
@@ -109,7 +109,7 @@ protected:
     void reload() override;
 
 private:
-    QString m_fileName;
+    const QString m_fileName;
 };
 
 DiffCurrentFileController::DiffCurrentFileController(IDocument *document, const QString &fileName) :
@@ -227,7 +227,7 @@ protected:
     void reload() override;
 
 private:
-    QStringList m_fileNames;
+    const QStringList m_fileNames;
 };
 
 DiffModifiedFilesController::DiffModifiedFilesController(IDocument *document, const QStringList &fileNames) :
@@ -289,8 +289,8 @@ protected:
     void reload() override;
 
 private:
-    QString m_leftFileName;
-    QString m_rightFileName;
+    const QString m_leftFileName;
+    const QString m_rightFileName;
 };
 
 DiffExternalFilesController::DiffExternalFilesController(IDocument *document, const QString &leftFileName,