diff --git a/src/base/bittorrent/peerinfo.cpp b/src/base/bittorrent/peerinfo.cpp
index 6d5deb46a..a5a3d8f60 100644
--- a/src/base/bittorrent/peerinfo.cpp
+++ b/src/base/bittorrent/peerinfo.cpp
@@ -43,7 +43,7 @@ PeerAddress::PeerAddress()
 {
 }
 
-PeerAddress::PeerAddress(QHostAddress ip, ushort port)
+PeerAddress::PeerAddress(const QHostAddress &ip, ushort port)
     : ip(ip)
     , port(port)
 {
diff --git a/src/base/bittorrent/peerinfo.h b/src/base/bittorrent/peerinfo.h
index bdeed4cad..9edac8552 100644
--- a/src/base/bittorrent/peerinfo.h
+++ b/src/base/bittorrent/peerinfo.h
@@ -45,7 +45,7 @@ namespace BitTorrent
         ushort port;
 
         PeerAddress();
-        PeerAddress(QHostAddress ip, ushort port);
+        PeerAddress(const QHostAddress &ip, ushort port);
     };
 
     class PeerInfo
diff --git a/src/base/bittorrent/session.cpp b/src/base/bittorrent/session.cpp
index 8d93d313c..14e10f1ab 100644
--- a/src/base/bittorrent/session.cpp
+++ b/src/base/bittorrent/session.cpp
@@ -3762,7 +3762,7 @@ void Session::readAlerts()
         handleAlert(a);
 }
 
-void Session::handleAlert(libt::alert *a)
+void Session::handleAlert(const libt::alert *a)
 {
     try {
         switch (a->type()) {
@@ -3784,53 +3784,53 @@ void Session::handleAlert(libt::alert *a)
             dispatchTorrentAlert(a);
             break;
         case libt::metadata_received_alert::alert_type:
-            handleMetadataReceivedAlert(static_cast<libt::metadata_received_alert*>(a));
+            handleMetadataReceivedAlert(static_cast<const libt::metadata_received_alert*>(a));
             dispatchTorrentAlert(a);
             break;
         case libt::state_update_alert::alert_type:
-            handleStateUpdateAlert(static_cast<libt::state_update_alert*>(a));
+            handleStateUpdateAlert(static_cast<const libt::state_update_alert*>(a));
             break;
         case libt::session_stats_alert::alert_type:
-            handleSessionStatsAlert(static_cast<libt::session_stats_alert*>(a));
+            handleSessionStatsAlert(static_cast<const libt::session_stats_alert*>(a));
             break;
         case libt::file_error_alert::alert_type:
-            handleFileErrorAlert(static_cast<libt::file_error_alert*>(a));
+            handleFileErrorAlert(static_cast<const libt::file_error_alert*>(a));
             break;
         case libt::add_torrent_alert::alert_type:
-            handleAddTorrentAlert(static_cast<libt::add_torrent_alert*>(a));
+            handleAddTorrentAlert(static_cast<const libt::add_torrent_alert*>(a));
             break;
         case libt::torrent_removed_alert::alert_type:
-            handleTorrentRemovedAlert(static_cast<libt::torrent_removed_alert*>(a));
+            handleTorrentRemovedAlert(static_cast<const libt::torrent_removed_alert*>(a));
             break;
         case libt::torrent_deleted_alert::alert_type:
-            handleTorrentDeletedAlert(static_cast<libt::torrent_deleted_alert*>(a));
+            handleTorrentDeletedAlert(static_cast<const libt::torrent_deleted_alert*>(a));
             break;
         case libt::torrent_delete_failed_alert::alert_type:
-            handleTorrentDeleteFailedAlert(static_cast<libt::torrent_delete_failed_alert*>(a));
+            handleTorrentDeleteFailedAlert(static_cast<const libt::torrent_delete_failed_alert*>(a));
             break;
         case libt::portmap_error_alert::alert_type:
-            handlePortmapWarningAlert(static_cast<libt::portmap_error_alert*>(a));
+            handlePortmapWarningAlert(static_cast<const libt::portmap_error_alert*>(a));
             break;
         case libt::portmap_alert::alert_type:
-            handlePortmapAlert(static_cast<libt::portmap_alert*>(a));
+            handlePortmapAlert(static_cast<const libt::portmap_alert*>(a));
             break;
         case libt::peer_blocked_alert::alert_type:
-            handlePeerBlockedAlert(static_cast<libt::peer_blocked_alert*>(a));
+            handlePeerBlockedAlert(static_cast<const libt::peer_blocked_alert*>(a));
             break;
         case libt::peer_ban_alert::alert_type:
-            handlePeerBanAlert(static_cast<libt::peer_ban_alert*>(a));
+            handlePeerBanAlert(static_cast<const libt::peer_ban_alert*>(a));
             break;
         case libt::url_seed_alert::alert_type:
-            handleUrlSeedAlert(static_cast<libt::url_seed_alert*>(a));
+            handleUrlSeedAlert(static_cast<const libt::url_seed_alert*>(a));
             break;
         case libt::listen_succeeded_alert::alert_type:
-            handleListenSucceededAlert(static_cast<libt::listen_succeeded_alert*>(a));
+            handleListenSucceededAlert(static_cast<const libt::listen_succeeded_alert*>(a));
             break;
         case libt::listen_failed_alert::alert_type:
-            handleListenFailedAlert(static_cast<libt::listen_failed_alert*>(a));
+            handleListenFailedAlert(static_cast<const libt::listen_failed_alert*>(a));
             break;
         case libt::external_ip_alert::alert_type:
-            handleExternalIPAlert(static_cast<libt::external_ip_alert*>(a));
+            handleExternalIPAlert(static_cast<const libt::external_ip_alert*>(a));
             break;
         }
     }
@@ -3839,7 +3839,7 @@ void Session::handleAlert(libt::alert *a)
     }
 }
 
-void Session::dispatchTorrentAlert(libt::alert *a)
+void Session::dispatchTorrentAlert(const libt::alert *a)
 {
     TorrentHandle *const torrent = m_torrents.value(static_cast<const libt::torrent_alert*>(a)->handle.info_hash());
     if (torrent)
@@ -3901,7 +3901,7 @@ void Session::createTorrentHandle(const libt::torrent_handle &nativeHandle)
         emit torrentNew(torrent);
 }
 
-void Session::handleAddTorrentAlert(libt::add_torrent_alert *p)
+void Session::handleAddTorrentAlert(const libt::add_torrent_alert *p)
 {
     if (p->error) {
         qDebug("/!\\ Error: Failed to add torrent!");
@@ -3914,7 +3914,7 @@ void Session::handleAddTorrentAlert(libt::add_torrent_alert *p)
     }
 }
 
-void Session::handleTorrentRemovedAlert(libt::torrent_removed_alert *p)
+void Session::handleTorrentRemovedAlert(const libt::torrent_removed_alert *p)
 {
     const InfoHash infoHash {p->info_hash};
 
@@ -3930,7 +3930,7 @@ void Session::handleTorrentRemovedAlert(libt::torrent_removed_alert *p)
     }
 }
 
-void Session::handleTorrentDeletedAlert(libt::torrent_deleted_alert *p)
+void Session::handleTorrentDeletedAlert(const libt::torrent_deleted_alert *p)
 {
     const InfoHash infoHash {p->info_hash};
 
@@ -3942,7 +3942,7 @@ void Session::handleTorrentDeletedAlert(libt::torrent_deleted_alert *p)
     LogMsg(tr("'%1' was removed from the transfer list and hard disk.", "'xxx.avi' was removed...").arg(tmpRemovingTorrentData.name));
 }
 
-void Session::handleTorrentDeleteFailedAlert(libt::torrent_delete_failed_alert *p)
+void Session::handleTorrentDeleteFailedAlert(const libt::torrent_delete_failed_alert *p)
 {
     const InfoHash infoHash {p->info_hash};
 
@@ -3958,7 +3958,7 @@ void Session::handleTorrentDeleteFailedAlert(libt::torrent_delete_failed_alert *
            , Log::CRITICAL);
 }
 
-void Session::handleMetadataReceivedAlert(libt::metadata_received_alert *p)
+void Session::handleMetadataReceivedAlert(const libt::metadata_received_alert *p)
 {
     const InfoHash hash {p->handle.info_hash()};
 
@@ -3970,7 +3970,7 @@ void Session::handleMetadataReceivedAlert(libt::metadata_received_alert *p)
     }
 }
 
-void Session::handleFileErrorAlert(libt::file_error_alert *p)
+void Session::handleFileErrorAlert(const libt::file_error_alert *p)
 {
     qDebug() << Q_FUNC_INFO;
     // NOTE: Check this function!
@@ -3988,18 +3988,18 @@ void Session::handleFileErrorAlert(libt::file_error_alert *p)
     }
 }
 
-void Session::handlePortmapWarningAlert(libt::portmap_error_alert *p)
+void Session::handlePortmapWarningAlert(const libt::portmap_error_alert *p)
 {
     Logger::instance()->addMessage(tr("UPnP/NAT-PMP: Port mapping failure, message: %1").arg(QString::fromStdString(p->message())), Log::CRITICAL);
 }
 
-void Session::handlePortmapAlert(libt::portmap_alert *p)
+void Session::handlePortmapAlert(const libt::portmap_alert *p)
 {
     qDebug("UPnP Success, msg: %s", p->message().c_str());
     Logger::instance()->addMessage(tr("UPnP/NAT-PMP: Port mapping successful, message: %1").arg(QString::fromStdString(p->message())), Log::INFO);
 }
 
-void Session::handlePeerBlockedAlert(libt::peer_blocked_alert *p)
+void Session::handlePeerBlockedAlert(const libt::peer_blocked_alert *p)
 {
     boost::system::error_code ec;
     const std::string ip = p->ip.to_string(ec);
@@ -4029,7 +4029,7 @@ void Session::handlePeerBlockedAlert(libt::peer_blocked_alert *p)
         Logger::instance()->addPeer(QString::fromLatin1(ip.c_str()), true, reason);
 }
 
-void Session::handlePeerBanAlert(libt::peer_ban_alert *p)
+void Session::handlePeerBanAlert(const libt::peer_ban_alert *p)
 {
     boost::system::error_code ec;
     const std::string ip = p->ip.address().to_string(ec);
@@ -4037,14 +4037,14 @@ void Session::handlePeerBanAlert(libt::peer_ban_alert *p)
         Logger::instance()->addPeer(QString::fromLatin1(ip.c_str()), false);
 }
 
-void Session::handleUrlSeedAlert(libt::url_seed_alert *p)
+void Session::handleUrlSeedAlert(const libt::url_seed_alert *p)
 {
     Logger::instance()->addMessage(tr("URL seed lookup failed for URL: '%1', message: %2")
                                    .arg(QString::fromStdString(p->server_url()))
                                    .arg(QString::fromStdString(p->message())), Log::CRITICAL);
 }
 
-void Session::handleListenSucceededAlert(libt::listen_succeeded_alert *p)
+void Session::handleListenSucceededAlert(const libt::listen_succeeded_alert *p)
 {
     boost::system::error_code ec;
     QString proto = "TCP";
@@ -4067,7 +4067,7 @@ void Session::handleListenSucceededAlert(libt::listen_succeeded_alert *p)
         it->force_reannounce();
 }
 
-void Session::handleListenFailedAlert(libt::listen_failed_alert *p)
+void Session::handleListenFailedAlert(const libt::listen_failed_alert *p)
 {
     boost::system::error_code ec;
     QString proto = "TCP";
@@ -4090,13 +4090,13 @@ void Session::handleListenFailedAlert(libt::listen_failed_alert *p)
         , Log::CRITICAL);
 }
 
-void Session::handleExternalIPAlert(libt::external_ip_alert *p)
+void Session::handleExternalIPAlert(const libt::external_ip_alert *p)
 {
     boost::system::error_code ec;
     Logger::instance()->addMessage(tr("External IP: %1", "e.g. External IP: 192.168.0.1").arg(p->external_address.to_string(ec).c_str()), Log::INFO);
 }
 
-void Session::handleSessionStatsAlert(libt::session_stats_alert *p)
+void Session::handleSessionStatsAlert(const libt::session_stats_alert *p)
 {
     const qreal interval = m_statsUpdateTimer.restart() / 1000.;
 
@@ -4161,7 +4161,7 @@ void Session::handleSessionStatsAlert(libt::session_stats_alert *p)
     emit statsUpdated();
 }
 
-void Session::handleStateUpdateAlert(libt::state_update_alert *p)
+void Session::handleStateUpdateAlert(const libt::state_update_alert *p)
 {
     for (const libt::torrent_status &status : p->status) {
         TorrentHandle *const torrent = m_torrents.value(status.info_hash);
diff --git a/src/base/bittorrent/session.h b/src/base/bittorrent/session.h
index 294155e7c..8f056204d 100644
--- a/src/base/bittorrent/session.h
+++ b/src/base/bittorrent/session.h
@@ -589,24 +589,24 @@ namespace BitTorrent
         void exportTorrentFile(TorrentHandle *const torrent, TorrentExportFolder folder = TorrentExportFolder::Regular);
         void saveTorrentResumeData(TorrentHandle *const torrent);
 
-        void handleAlert(libtorrent::alert *a);
-        void dispatchTorrentAlert(libtorrent::alert *a);
-        void handleAddTorrentAlert(libtorrent::add_torrent_alert *p);
-        void handleStateUpdateAlert(libtorrent::state_update_alert *p);
-        void handleMetadataReceivedAlert(libtorrent::metadata_received_alert *p);
-        void handleFileErrorAlert(libtorrent::file_error_alert *p);
-        void handleTorrentRemovedAlert(libtorrent::torrent_removed_alert *p);
-        void handleTorrentDeletedAlert(libtorrent::torrent_deleted_alert *p);
-        void handleTorrentDeleteFailedAlert(libtorrent::torrent_delete_failed_alert *p);
-        void handlePortmapWarningAlert(libtorrent::portmap_error_alert *p);
-        void handlePortmapAlert(libtorrent::portmap_alert *p);
-        void handlePeerBlockedAlert(libtorrent::peer_blocked_alert *p);
-        void handlePeerBanAlert(libtorrent::peer_ban_alert *p);
-        void handleUrlSeedAlert(libtorrent::url_seed_alert *p);
-        void handleListenSucceededAlert(libtorrent::listen_succeeded_alert *p);
-        void handleListenFailedAlert(libtorrent::listen_failed_alert *p);
-        void handleExternalIPAlert(libtorrent::external_ip_alert *p);
-        void handleSessionStatsAlert(libtorrent::session_stats_alert *p);
+        void handleAlert(const libtorrent::alert *a);
+        void dispatchTorrentAlert(const libtorrent::alert *a);
+        void handleAddTorrentAlert(const libtorrent::add_torrent_alert *p);
+        void handleStateUpdateAlert(const libtorrent::state_update_alert *p);
+        void handleMetadataReceivedAlert(const libtorrent::metadata_received_alert *p);
+        void handleFileErrorAlert(const libtorrent::file_error_alert *p);
+        void handleTorrentRemovedAlert(const libtorrent::torrent_removed_alert *p);
+        void handleTorrentDeletedAlert(const libtorrent::torrent_deleted_alert *p);
+        void handleTorrentDeleteFailedAlert(const libtorrent::torrent_delete_failed_alert *p);
+        void handlePortmapWarningAlert(const libtorrent::portmap_error_alert *p);
+        void handlePortmapAlert(const libtorrent::portmap_alert *p);
+        void handlePeerBlockedAlert(const libtorrent::peer_blocked_alert *p);
+        void handlePeerBanAlert(const libtorrent::peer_ban_alert *p);
+        void handleUrlSeedAlert(const libtorrent::url_seed_alert *p);
+        void handleListenSucceededAlert(const libtorrent::listen_succeeded_alert *p);
+        void handleListenFailedAlert(const libtorrent::listen_failed_alert *p);
+        void handleExternalIPAlert(const libtorrent::external_ip_alert *p);
+        void handleSessionStatsAlert(const libtorrent::session_stats_alert *p);
 
         void createTorrentHandle(const libtorrent::torrent_handle &nativeHandle);
 
diff --git a/src/base/bittorrent/torrenthandle.cpp b/src/base/bittorrent/torrenthandle.cpp
index 6cddf36ec..5261bc2cb 100644
--- a/src/base/bittorrent/torrenthandle.cpp
+++ b/src/base/bittorrent/torrenthandle.cpp
@@ -1717,7 +1717,7 @@ void TorrentHandle::handleAppendExtensionToggled()
     manageIncompleteFiles();
 }
 
-void TorrentHandle::handleAlert(libtorrent::alert *a)
+void TorrentHandle::handleAlert(const libtorrent::alert *a)
 {
     switch (a->type()) {
     case libt::stats_alert::alert_type:
diff --git a/src/base/bittorrent/torrenthandle.h b/src/base/bittorrent/torrenthandle.h
index 096759c20..1f30ad892 100644
--- a/src/base/bittorrent/torrenthandle.h
+++ b/src/base/bittorrent/torrenthandle.h
@@ -358,7 +358,7 @@ namespace BitTorrent
         // Session interface
         libtorrent::torrent_handle nativeHandle() const;
 
-        void handleAlert(libtorrent::alert *a);
+        void handleAlert(const libtorrent::alert *a);
         void handleStateUpdate(const libtorrent::torrent_status &nativeStatus);
         void handleTempPathChanged();
         void handleCategorySavePathChanged();
diff --git a/src/base/search/searchpluginmanager.cpp b/src/base/search/searchpluginmanager.cpp
index 767859326..098979163 100644
--- a/src/base/search/searchpluginmanager.cpp
+++ b/src/base/search/searchpluginmanager.cpp
@@ -533,7 +533,7 @@ void SearchPluginManager::parseVersionInfo(const QByteArray &info)
     }
 }
 
-bool SearchPluginManager::isUpdateNeeded(QString pluginName, PluginVersion newVersion) const
+bool SearchPluginManager::isUpdateNeeded(const QString &pluginName, PluginVersion newVersion) const
 {
     PluginInfo *plugin = pluginInfo(pluginName);
     if (!plugin) return true;
diff --git a/src/base/search/searchpluginmanager.h b/src/base/search/searchpluginmanager.h
index e5aa6ce88..ac485807d 100644
--- a/src/base/search/searchpluginmanager.h
+++ b/src/base/search/searchpluginmanager.h
@@ -102,7 +102,7 @@ private:
     void updateNova();
     void parseVersionInfo(const QByteArray &info);
     void installPlugin_impl(const QString &name, const QString &path);
-    bool isUpdateNeeded(QString pluginName, PluginVersion newVersion) const;
+    bool isUpdateNeeded(const QString &pluginName, PluginVersion newVersion) const;
 
     void versionInfoDownloaded(const QString &url, const QByteArray &data);
     void versionInfoDownloadFailed(const QString &url, const QString &reason);
diff --git a/src/gui/mainwindow.cpp b/src/gui/mainwindow.cpp
index 46ee3a16c..d2ecf2212 100644
--- a/src/gui/mainwindow.cpp
+++ b/src/gui/mainwindow.cpp
@@ -849,7 +849,7 @@ void MainWindow::finishedTorrent(BitTorrent::TorrentHandle *const torrent) const
 }
 
 // Notification when disk is full
-void MainWindow::fullDiskError(BitTorrent::TorrentHandle *const torrent, QString msg) const
+void MainWindow::fullDiskError(BitTorrent::TorrentHandle *const torrent, const QString &msg) const
 {
     showNotificationBaloon(tr("I/O Error", "i.e: Input/Output Error")
         , tr("An I/O error occurred for torrent '%1'.\n Reason: %2"
@@ -963,7 +963,7 @@ void MainWindow::askRecursiveTorrentDownloadConfirmation(BitTorrent::TorrentHand
     confirmBox->show();
 }
 
-void MainWindow::handleDownloadFromUrlFailure(QString url, QString reason) const
+void MainWindow::handleDownloadFromUrlFailure(const QString &url, const QString &reason) const
 {
     // Display a message box
     showNotificationBaloon(tr("URL download error")
@@ -1061,7 +1061,7 @@ bool MainWindow::unlockUI()
     return true;
 }
 
-void MainWindow::notifyOfUpdate(QString)
+void MainWindow::notifyOfUpdate(const QString &)
 {
     // Show restart message
     m_statusBar->showRestartRequired();
@@ -1565,7 +1565,7 @@ void MainWindow::updateGUI()
     }
 }
 
-void MainWindow::showNotificationBaloon(QString title, QString msg) const
+void MainWindow::showNotificationBaloon(const QString &title, const QString &msg) const
 {
     if (!isNotificationsEnabled()) return;
 #if (defined(Q_OS_UNIX) && !defined(Q_OS_MAC)) && defined(QT_DBUS_LIB)
diff --git a/src/gui/mainwindow.h b/src/gui/mainwindow.h
index 2f346ed59..52168ad5e 100644
--- a/src/gui/mainwindow.h
+++ b/src/gui/mainwindow.h
@@ -99,20 +99,20 @@ public:
     void activate();
     void cleanup();
 
-    void showNotificationBaloon(QString title, QString msg) const;
+    void showNotificationBaloon(const QString &title, const QString &msg) const;
 
 private slots:
     void showFilterContextMenu(const QPoint &);
     void balloonClicked();
     void writeSettings();
     void readSettings();
-    void fullDiskError(BitTorrent::TorrentHandle *const torrent, QString msg) const;
-    void handleDownloadFromUrlFailure(QString, QString) const;
+    void fullDiskError(BitTorrent::TorrentHandle *const torrent, const QString &msg) const;
+    void handleDownloadFromUrlFailure(const QString &, const QString &) const;
     void tabChanged(int newTab);
     bool defineUILockPassword();
     void clearUILockPassword();
     bool unlockUI();
-    void notifyOfUpdate(QString);
+    void notifyOfUpdate(const QString &);
     void showConnectionSettings();
     void minimizeWindow();
     // Keyboard shortcuts
diff --git a/src/gui/properties/trackerlistwidget.cpp b/src/gui/properties/trackerlistwidget.cpp
index 40b1263a4..1af932331 100644
--- a/src/gui/properties/trackerlistwidget.cpp
+++ b/src/gui/properties/trackerlistwidget.cpp
@@ -149,7 +149,7 @@ QList<QTreeWidgetItem*> TrackerListWidget::getSelectedTrackerItems() const
     return selectedTrackers;
 }
 
-void TrackerListWidget::setRowColor(const int row, QColor color)
+void TrackerListWidget::setRowColor(const int row, const QColor &color)
 {
     const int nbColumns = columnCount();
     QTreeWidgetItem *item = topLevelItem(row);
diff --git a/src/gui/properties/trackerlistwidget.h b/src/gui/properties/trackerlistwidget.h
index 378023e27..8fcde45eb 100644
--- a/src/gui/properties/trackerlistwidget.h
+++ b/src/gui/properties/trackerlistwidget.h
@@ -69,7 +69,7 @@ public:
     int visibleColumnsCount() const;
 
 public slots:
-    void setRowColor(int row, QColor color);
+    void setRowColor(int row, const QColor &color);
 
     void moveSelectionUp();
     void moveSelectionDown();
diff --git a/src/gui/search/pluginselectdialog.cpp b/src/gui/search/pluginselectdialog.cpp
index 432154d93..3543eaa63 100644
--- a/src/gui/search/pluginselectdialog.cpp
+++ b/src/gui/search/pluginselectdialog.cpp
@@ -228,7 +228,7 @@ void PluginSelectDialog::enableSelection(bool enable)
 }
 
 // Set the color of a row in data model
-void PluginSelectDialog::setRowColor(const int row, QString color)
+void PluginSelectDialog::setRowColor(const int row, const QString &color)
 {
     QTreeWidgetItem *item = m_ui->pluginsTree->topLevelItem(row);
     for (int i = 0; i < m_ui->pluginsTree->columnCount(); ++i) {
@@ -236,7 +236,7 @@ void PluginSelectDialog::setRowColor(const int row, QString color)
     }
 }
 
-QList<QTreeWidgetItem*> PluginSelectDialog::findItemsWithUrl(QString url)
+QList<QTreeWidgetItem*> PluginSelectDialog::findItemsWithUrl(const QString &url)
 {
     QList<QTreeWidgetItem*> res;
 
@@ -249,7 +249,7 @@ QList<QTreeWidgetItem*> PluginSelectDialog::findItemsWithUrl(QString url)
     return res;
 }
 
-QTreeWidgetItem *PluginSelectDialog::findItemWithID(QString id)
+QTreeWidgetItem *PluginSelectDialog::findItemWithID(const QString &id)
 {
     for (int i = 0; i < m_ui->pluginsTree->topLevelItemCount(); ++i) {
         QTreeWidgetItem *item = m_ui->pluginsTree->topLevelItem(i);
@@ -268,7 +268,7 @@ void PluginSelectDialog::loadSupportedSearchPlugins()
         addNewPlugin(name);
 }
 
-void PluginSelectDialog::addNewPlugin(QString pluginName)
+void PluginSelectDialog::addNewPlugin(const QString &pluginName)
 {
     QTreeWidgetItem *item = new QTreeWidgetItem(m_ui->pluginsTree);
     PluginInfo *plugin = m_pluginManager->pluginInfo(pluginName);
diff --git a/src/gui/search/pluginselectdialog.h b/src/gui/search/pluginselectdialog.h
index 7eadfda5e..c2754f8bf 100644
--- a/src/gui/search/pluginselectdialog.h
+++ b/src/gui/search/pluginselectdialog.h
@@ -51,8 +51,8 @@ public:
     explicit PluginSelectDialog(SearchPluginManager *pluginManager, QWidget *parent = nullptr);
     ~PluginSelectDialog();
 
-    QList<QTreeWidgetItem*> findItemsWithUrl(QString url);
-    QTreeWidgetItem *findItemWithID(QString id);
+    QList<QTreeWidgetItem*> findItemsWithUrl(const QString &url);
+    QTreeWidgetItem *findItemWithID(const QString &id);
 
 protected:
     void dropEvent(QDropEvent *event) override;
@@ -64,7 +64,7 @@ private slots:
     void on_installButton_clicked();
     void on_closeButton_clicked();
     void togglePluginState(QTreeWidgetItem*, int);
-    void setRowColor(int row, QString color);
+    void setRowColor(int row, const QString &color);
     void displayContextMenu(const QPoint &pos);
     void enableSelection(bool enable);
     void askForLocalPlugin();
@@ -81,7 +81,7 @@ private slots:
 
 private:
     void loadSupportedSearchPlugins();
-    void addNewPlugin(QString pluginName);
+    void addNewPlugin(const QString &pluginName);
     void startAsyncOp();
     void finishAsyncOp();
     void finishPluginUpdate();
diff --git a/src/gui/search/searchwidget.cpp b/src/gui/search/searchwidget.cpp
index 4b552b748..5ae242222 100644
--- a/src/gui/search/searchwidget.cpp
+++ b/src/gui/search/searchwidget.cpp
@@ -280,7 +280,7 @@ void SearchWidget::on_pluginsButton_clicked()
     new PluginSelectDialog(SearchPluginManager::instance(), this);
 }
 
-void SearchWidget::searchTextEdited(QString)
+void SearchWidget::searchTextEdited(const QString &)
 {
     // Enable search button
     m_ui->searchButton->setText(tr("Search"));
diff --git a/src/gui/search/searchwidget.h b/src/gui/search/searchwidget.h
index 153beea15..e303b569d 100644
--- a/src/gui/search/searchwidget.h
+++ b/src/gui/search/searchwidget.h
@@ -74,7 +74,7 @@ private:
     void fillCatCombobox();
     void fillPluginComboBox();
     void selectActivePage();
-    void searchTextEdited(QString);
+    void searchTextEdited(const QString &);
     void updateButtons();
 
     QString selectedCategory() const;
diff --git a/src/gui/transferlistwidget.cpp b/src/gui/transferlistwidget.cpp
index c46bb64e2..4fabfd69b 100644
--- a/src/gui/transferlistwidget.cpp
+++ b/src/gui/transferlistwidget.cpp
@@ -338,7 +338,7 @@ TransferListModel *TransferListWidget::getSourceModel() const
     return m_listModel;
 }
 
-void TransferListWidget::previewFile(QString filePath)
+void TransferListWidget::previewFile(const QString &filePath)
 {
     Utils::Misc::openPath(filePath);
 }
@@ -854,7 +854,7 @@ void TransferListWidget::renameSelectedTorrent()
     }
 }
 
-void TransferListWidget::setSelectionCategory(QString category)
+void TransferListWidget::setSelectionCategory(const QString &category)
 {
     for (const QModelIndex &index : asConst(selectionModel()->selectedRows()))
         m_listModel->setData(m_listModel->index(mapToSource(index).row(), TransferListModel::TR_CATEGORY), category, Qt::DisplayRole);
@@ -1173,7 +1173,7 @@ void TransferListWidget::currentChanged(const QModelIndex &current, const QModel
     emit currentTorrentChanged(torrent);
 }
 
-void TransferListWidget::applyCategoryFilter(QString category)
+void TransferListWidget::applyCategoryFilter(const QString &category)
 {
     if (category.isNull())
         m_sortFilterModel->disableCategoryFilter();
diff --git a/src/gui/transferlistwidget.h b/src/gui/transferlistwidget.h
index 2807d0325..ff29b97a3 100644
--- a/src/gui/transferlistwidget.h
+++ b/src/gui/transferlistwidget.h
@@ -56,7 +56,7 @@ public:
     TransferListModel *getSourceModel() const;
 
 public slots:
-    void setSelectionCategory(QString category);
+    void setSelectionCategory(const QString &category);
     void addSelectionTag(const QString &tag);
     void removeSelectionTag(const QString &tag);
     void clearSelectionTags();
@@ -90,11 +90,11 @@ public slots:
     void displayDLHoSMenu(const QPoint&);
     void applyNameFilter(const QString &name);
     void applyStatusFilter(int f);
-    void applyCategoryFilter(QString category);
+    void applyCategoryFilter(const QString &category);
     void applyTagFilter(const QString &tag);
     void applyTrackerFilterAll();
     void applyTrackerFilter(const QStringList &hashes);
-    void previewFile(QString filePath);
+    void previewFile(const QString &filePath);
     void renameSelectedTorrent();
 
 protected: