diff --git a/kcmkwin/kwindecoration/buttons.h b/kcmkwin/kwindecoration/buttons.h
index 965b6cb42a..1c62b2545d 100644
--- a/kcmkwin/kwindecoration/buttons.h
+++ b/kcmkwin/kwindecoration/buttons.h
@@ -112,7 +112,7 @@ class ButtonSource : public QListWidget
     Q_OBJECT
 
 public:
-    explicit ButtonSource(QWidget *parent = 0);
+    explicit ButtonSource(QWidget *parent = nullptr);
     virtual ~ButtonSource();
 
     QSize sizeHint() const;
@@ -143,7 +143,7 @@ class ButtonDropSite: public QFrame
     Q_OBJECT
 
 public:
-    explicit ButtonDropSite(QWidget* parent = 0);
+    explicit ButtonDropSite(QWidget* parent = nullptr);
     ~ButtonDropSite();
 
     // Allow external classes access our buttons - ensure buttons are
@@ -196,7 +196,7 @@ class ButtonPositionWidget : public QWidget
     Q_OBJECT
 
 public:
-    explicit ButtonPositionWidget(QWidget *parent = 0);
+    explicit ButtonPositionWidget(QWidget *parent = nullptr);
     ~ButtonPositionWidget();
 
     QList<KDecorationDefines::DecorationButton> buttonsLeft() const;
diff --git a/kcmkwin/kwindecoration/buttonsconfigdialog.h b/kcmkwin/kwindecoration/buttonsconfigdialog.h
index 740a32ba87..5ebbf55714 100644
--- a/kcmkwin/kwindecoration/buttonsconfigdialog.h
+++ b/kcmkwin/kwindecoration/buttonsconfigdialog.h
@@ -43,7 +43,7 @@ class KWinDecorationButtonsConfigDialog : public QDialog
 {
     Q_OBJECT
 public:
-    KWinDecorationButtonsConfigDialog(DecorationButtons const *buttons, bool showTooltips, QWidget* parent = 0, Qt::WFlags flags = 0);
+    KWinDecorationButtonsConfigDialog(DecorationButtons const *buttons, bool showTooltips, QWidget* parent = nullptr, Qt::WFlags flags = 0);
     ~KWinDecorationButtonsConfigDialog();
 
     bool customPositions() const;
diff --git a/kcmkwin/kwindecoration/configdialog.cpp b/kcmkwin/kwindecoration/configdialog.cpp
index b06eb1ce72..5083d604c6 100644
--- a/kcmkwin/kwindecoration/configdialog.cpp
+++ b/kcmkwin/kwindecoration/configdialog.cpp
@@ -74,8 +74,8 @@ KWinDecorationConfigDialog::KWinDecorationConfigDialog(QString deco, const QList
     : QDialog(parent, flags)
     , m_borderSizes(borderSizes)
     , m_kwinConfig(KSharedConfig::openConfig("kwinrc"))
-    , m_pluginObject(0)
-    , m_pluginConfigWidget(0)
+    , m_pluginObject(nullptr)
+    , m_pluginConfigWidget(nullptr)
 {
     m_ui = new KWinDecorationConfigForm(this);
     setWindowTitle(i18n("Decoration Options"));
@@ -92,7 +92,7 @@ KWinDecorationConfigDialog::KWinDecorationConfigDialog(QString deco, const QList
     KLibrary library(styleToConfigLib(deco));
     if (library.load()) {
         KLibrary::void_function_ptr alloc_ptr = library.resolveFunction("allocate_config");
-        if (alloc_ptr != NULL) {
+        if (alloc_ptr != nullptr) {
             allocatePlugin = (QObject * (*)(KConfigGroup & conf, QWidget * parent))alloc_ptr;
             KConfigGroup config(m_kwinConfig, "Style");
             m_pluginConfigWidget = new QWidget(this);
diff --git a/kcmkwin/kwindecoration/configdialog.h b/kcmkwin/kwindecoration/configdialog.h
index 2f53b4a601..1b99f39d17 100644
--- a/kcmkwin/kwindecoration/configdialog.h
+++ b/kcmkwin/kwindecoration/configdialog.h
@@ -54,7 +54,7 @@ class KWinDecorationConfigDialog : public QDialog, public KDecorationDefines
     Q_OBJECT
 public:
     KWinDecorationConfigDialog(QString decoLib, const QList<QVariant>& borderSizes,
-                               KDecorationDefines::BorderSize size, QWidget* parent = 0,
+                               KDecorationDefines::BorderSize size, QWidget* parent = nullptr,
                                Qt::WFlags flags = 0);
     ~KWinDecorationConfigDialog();
 
diff --git a/kcmkwin/kwindecoration/decorationmodel.h b/kcmkwin/kwindecoration/decorationmodel.h
index 58da01acc7..f55e6ddd5e 100644
--- a/kcmkwin/kwindecoration/decorationmodel.h
+++ b/kcmkwin/kwindecoration/decorationmodel.h
@@ -89,7 +89,7 @@ public:
         QmlMainScriptRole = Qt::UserRole + 14,
         CloseOnDblClickRole = Qt::UserRole + 15
     };
-    explicit DecorationModel(KSharedConfigPtr config, QObject* parent = 0);
+    explicit DecorationModel(KSharedConfigPtr config, QObject* parent = nullptr);
     ~DecorationModel();
 
     virtual QVariant data(const QModelIndex& index, int role = Qt::DisplayRole) const;
diff --git a/kcmkwin/kwindecoration/kwindecoration.cpp b/kcmkwin/kwindecoration/kwindecoration.cpp
index 3172d77fae..b1c79097bf 100644
--- a/kcmkwin/kwindecoration/kwindecoration.cpp
+++ b/kcmkwin/kwindecoration/kwindecoration.cpp
@@ -73,8 +73,8 @@ KWinDecorationModule::KWinDecorationModule(QWidget* parent, const QVariantList &
     : KCModule(parent)
     , kwinConfig(KSharedConfig::openConfig("kwinrc"))
     , m_showTooltips(false)
-    , m_model(NULL)
-    , m_proxyModel(NULL)
+    , m_model(nullptr)
+    , m_proxyModel(nullptr)
     , m_configLoaded(false)
     , m_decorationButtons(new DecorationButtons(this))
     , m_listView(new QQuickView())
@@ -381,7 +381,7 @@ void KWinDecorationModule::slotConfigureDecoration()
         form->buttonSizesCombo->setCurrentIndex(index.data(DecorationModel::ButtonSizeRole).toInt());
         form->closeWindowsDoubleClick->setChecked(index.data(DecorationModel::CloseOnDblClickRole).toBool());
         // in case of QmlDecoration look for a config.ui in the package structure
-        KConfigDialogManager *configManager = NULL;
+        KConfigDialogManager *configManager = nullptr;
         if (index.data(DecorationModel::TypeRole).toInt() == DecorationModelData::QmlDecoration) {
             const QString packageName = index.data(DecorationModel::AuroraeNameRole).toString();
             const QString uiPath = QStandardPaths::locate(QStandardPaths::GenericDataLocation, "kwin/decorations/" + packageName + "/contents/ui/config.ui");
diff --git a/kcmkwin/kwindecoration/kwindecoration.h b/kcmkwin/kwindecoration/kwindecoration.h
index fb70ba0891..fb9ac7cf12 100644
--- a/kcmkwin/kwindecoration/kwindecoration.h
+++ b/kcmkwin/kwindecoration/kwindecoration.h
@@ -58,7 +58,7 @@ class DecorationButtons : public QObject, public KDecorationDefines
     Q_OBJECT
     Q_PROPERTY(bool customButtonPositions READ customPositions WRITE setCustomPositions NOTIFY customPositionsChanged)
 public:
-    explicit DecorationButtons(QObject *parent = 0);
+    explicit DecorationButtons(QObject *parent = nullptr);
     virtual ~DecorationButtons();
 
     bool customPositions() const;
diff --git a/kcmkwin/kwindecoration/preview.h b/kcmkwin/kwindecoration/preview.h
index df3ef3bdab..05404fb354 100644
--- a/kcmkwin/kwindecoration/preview.h
+++ b/kcmkwin/kwindecoration/preview.h
@@ -84,7 +84,7 @@ public:
     //       and an inactive window.
     enum Windows { Inactive = 0, Active, NumWindows };
 
-    explicit KDecorationPreview(QWidget* parent = NULL);
+    explicit KDecorationPreview(QWidget* parent = nullptr);
     virtual ~KDecorationPreview();
     void disablePreview();
     KDecorationFactory *factory() const;
diff --git a/kcmkwin/kwindesktop/main.cpp b/kcmkwin/kwindesktop/main.cpp
index 3d835a1d23..28902b2943 100644
--- a/kcmkwin/kwindesktop/main.cpp
+++ b/kcmkwin/kwindesktop/main.cpp
@@ -59,8 +59,8 @@ KWinDesktopConfigForm::KWinDesktopConfigForm(QWidget* parent)
 KWinDesktopConfig::KWinDesktopConfig(QWidget* parent, const QVariantList& args)
     : KCModule(KAboutData::pluginData(QStringLiteral("kcm_kwindesktop")), parent, args)
     , m_config(KSharedConfig::openConfig("kwinrc"))
-    , m_actionCollection(NULL)
-    , m_switchDesktopCollection(NULL)
+    , m_actionCollection(nullptr)
+    , m_switchDesktopCollection(nullptr)
 {
     init();
 }
diff --git a/kcmkwin/kwinrules/detectwidget.cpp b/kcmkwin/kwinrules/detectwidget.cpp
index 723bdee7bb..035acc1b45 100644
--- a/kcmkwin/kwinrules/detectwidget.cpp
+++ b/kcmkwin/kwinrules/detectwidget.cpp
@@ -176,7 +176,7 @@ void DetectDialog::selectWindow()
     // use a dialog, so that all user input is blocked
     // use WX11BypassWM and moving away so that it's not actually visible
     // grab only mouse, so that keyboard can be used e.g. for switching windows
-    grabber.reset(new QDialog(0, Qt::X11BypassWindowManagerHint));
+    grabber.reset(new QDialog(nullptr, Qt::X11BypassWindowManagerHint));
     grabber->move(-1000, -1000);
     grabber->setModal(true);
     grabber->show();
@@ -233,7 +233,7 @@ WId DetectDialog::findWindow()
         unsigned char* prop;
         if (XGetWindowProperty(QX11Info::display(), child, wm_state, 0, 0, False, AnyPropertyType,
                               &type, &format, &nitems, &after, &prop) == Success) {
-            if (prop != NULL)
+            if (prop != nullptr)
                 XFree(prop);
             if (type != None)
                 return child;
diff --git a/kcmkwin/kwinrules/detectwidget.h b/kcmkwin/kwinrules/detectwidget.h
index 4fdd3614f2..6c7e4efac3 100644
--- a/kcmkwin/kwinrules/detectwidget.h
+++ b/kcmkwin/kwinrules/detectwidget.h
@@ -39,7 +39,7 @@ class DetectWidget
 {
     Q_OBJECT
 public:
-    explicit DetectWidget(QWidget* parent = NULL);
+    explicit DetectWidget(QWidget* parent = nullptr);
 };
 
 class DetectDialog
@@ -47,7 +47,7 @@ class DetectDialog
 {
     Q_OBJECT
 public:
-    explicit DetectDialog(QWidget* parent = NULL, const char* name = NULL);
+    explicit DetectDialog(QWidget* parent = nullptr, const char* name = nullptr);
     void detect(WId window, int secs = 0);
     QByteArray selectedClass() const;
     bool selectedWholeClass() const;
diff --git a/kcmkwin/kwinrules/main.cpp b/kcmkwin/kwinrules/main.cpp
index 678e25a76e..98885948f3 100644
--- a/kcmkwin/kwinrules/main.cpp
+++ b/kcmkwin/kwinrules/main.cpp
@@ -72,7 +72,7 @@ static Rules* findRule(const QList< Rules* >& rules, Window wid, bool whole_app)
                        NET::WMName | NET::WMWindowType,
                        NET::WM2WindowClass | NET::WM2WindowRole | NET::WM2ClientMachine);
     if (!info.valid())  // shouldn't really happen
-        return NULL;
+        return nullptr;
     ClientMachine clientMachine;
     clientMachine.resolve(info.win(), info.groupLeader());
     QByteArray wmclass_class = info.windowClassClass().toLower();
@@ -83,7 +83,7 @@ static Rules* findRule(const QList< Rules* >& rules, Window wid, bool whole_app)
                                            | NET::UtilityMask | NET::SplashMask);
     QString title = info.name();
     QByteArray machine = clientMachine.hostName();
-    Rules* best_match = NULL;
+    Rules* best_match = nullptr;
     int match_quality = 0;
     for (QList< Rules* >::ConstIterator it = rules.constBegin();
             it != rules.constEnd();
@@ -136,7 +136,7 @@ static Rules* findRule(const QList< Rules* >& rules, Window wid, bool whole_app)
             match_quality = quality;
         }
     }
-    if (best_match != NULL)
+    if (best_match != nullptr)
         return best_match;
     Rules* ret = new Rules;
     if (whole_app) {
@@ -215,7 +215,7 @@ static int edit(Window wid, bool whole_app)
         dlg.setWindowTitle(i18nc("Window caption for the application wide rules dialog", "Edit Application-Specific Settings"));
     // dlg.edit() creates new Rules instance if edited
     Rules* edited_rule = dlg.edit(orig_rule, wid, true);
-    if (edited_rule == NULL || edited_rule->isEmpty()) {
+    if (edited_rule == nullptr || edited_rule->isEmpty()) {
         rules.removeAll(orig_rule);
         delete orig_rule;
         if (orig_rule != edited_rule)
diff --git a/kcmkwin/kwinrules/ruleslist.cpp b/kcmkwin/kwinrules/ruleslist.cpp
index 842a22ab51..0353c66da1 100644
--- a/kcmkwin/kwinrules/ruleslist.cpp
+++ b/kcmkwin/kwinrules/ruleslist.cpp
@@ -72,20 +72,20 @@ void KCMRulesList::activeChanged()
     QListWidgetItem *item = rules_listbox->currentItem();
     int itemRow = rules_listbox->row(item);
 
-    if (item != NULL)   // make current==selected
+    if (item != nullptr)   // make current==selected
         rules_listbox->setCurrentItem(item, QItemSelectionModel::ClearAndSelect);
-    modify_button->setEnabled(item != NULL);
-    delete_button->setEnabled(item != NULL);
-    export_button->setEnabled(item != NULL);
-    moveup_button->setEnabled(item != NULL && itemRow > 0);
-    movedown_button->setEnabled(item != NULL && itemRow < (rules_listbox->count() - 1));
+    modify_button->setEnabled(item != nullptr);
+    delete_button->setEnabled(item != nullptr);
+    export_button->setEnabled(item != nullptr);
+    moveup_button->setEnabled(item != nullptr && itemRow > 0);
+    movedown_button->setEnabled(item != nullptr && itemRow < (rules_listbox->count() - 1));
 }
 
 void KCMRulesList::newClicked()
 {
     RulesDialog dlg(this);
-    Rules* rule = dlg.edit(NULL, 0, false);
-    if (rule == NULL)
+    Rules* rule = dlg.edit(nullptr, 0, false);
+    if (rule == nullptr)
         return;
     int pos = rules_listbox->currentRow() + 1;
     rules_listbox->insertItem(pos , rule->description);
@@ -191,7 +191,7 @@ void KCMRulesList::importClicked()
                 }
                 else
                     rules[i] = new_rule;
-                new_rule = 0;
+                new_rule = nullptr;
                 break;
             }
         }
@@ -199,7 +199,7 @@ void KCMRulesList::importClicked()
         // don't add "to be deleted" if not present
         if (remove) {
             delete new_rule;
-            new_rule = 0;
+            new_rule = nullptr;
         }
 
         // plain insertion
@@ -234,7 +234,7 @@ void KCMRulesList::load()
     if (rules.count() > 0)
         rules_listbox->setCurrentItem(rules_listbox->item(0));
     else
-        rules_listbox->setCurrentItem(NULL);
+        rules_listbox->setCurrentItem(nullptr);
     activeChanged();
 }
 
diff --git a/kcmkwin/kwinrules/ruleslist.h b/kcmkwin/kwinrules/ruleslist.h
index 19b8768f42..eb9a1ad1a0 100644
--- a/kcmkwin/kwinrules/ruleslist.h
+++ b/kcmkwin/kwinrules/ruleslist.h
@@ -32,7 +32,7 @@ class KCMRulesList
 {
     Q_OBJECT
 public:
-    explicit KCMRulesList(QWidget* parent = NULL);
+    explicit KCMRulesList(QWidget* parent = nullptr);
     virtual ~KCMRulesList();
     void load();
     void save();
diff --git a/kcmkwin/kwinrules/ruleswidget.cpp b/kcmkwin/kwinrules/ruleswidget.cpp
index ea322abbf3..370ae1a8d9 100644
--- a/kcmkwin/kwinrules/ruleswidget.cpp
+++ b/kcmkwin/kwinrules/ruleswidget.cpp
@@ -54,7 +54,7 @@ namespace KWin
     rule_##var->setWhatsThis( type##RuleDesc );
 
 RulesWidget::RulesWidget(QWidget* parent)
-    : detect_dlg(NULL)
+    : detect_dlg(nullptr)
 {
     Q_UNUSED(parent);
     QRegularExpressionValidator* validator = new QRegularExpressionValidator(QRegularExpression("[0-9\-+,xX:]*"), this);
@@ -426,7 +426,7 @@ static NET::WindowType comboToType(int val)
 void RulesWidget::setRules(Rules* rules)
 {
     Rules tmp;
-    if (rules == NULL)
+    if (rules == nullptr)
         rules = &tmp; // empty
     description->setText(rules->description);
     wmclass->setText(rules->wmclass);
@@ -634,7 +634,7 @@ STRING_MATCH_COMBO(machine)
 
 void RulesWidget::detectClicked()
 {
-    assert(detect_dlg == NULL);
+    assert(detect_dlg == nullptr);
     detect_dlg = new DetectDialog;
     connect(detect_dlg, SIGNAL(detectionDone(bool)), this, SLOT(detected(bool)));
     detect_dlg->detect(0, Ui::RulesWidgetBase::detection_delay->value());
@@ -675,7 +675,7 @@ void RulesWidget::detected(bool ok)
         prefillUnusedValues(info);
     }
     delete detect_dlg;
-    detect_dlg = NULL;
+    detect_dlg = nullptr;
     detect_dlg_ok = ok;
 }
 
diff --git a/kcmkwin/kwinrules/ruleswidget.h b/kcmkwin/kwinrules/ruleswidget.h
index 7e0e7b4cad..1420986618 100644
--- a/kcmkwin/kwinrules/ruleswidget.h
+++ b/kcmkwin/kwinrules/ruleswidget.h
@@ -40,7 +40,7 @@ class RulesWidget
 {
     Q_OBJECT
 public:
-    explicit RulesWidget(QWidget* parent = NULL);
+    explicit RulesWidget(QWidget* parent = nullptr);
     void setRules(Rules* r);
     Rules* rules() const;
     bool finalCheck();
@@ -116,7 +116,7 @@ class RulesDialog
 {
     Q_OBJECT
 public:
-    explicit RulesDialog(QWidget* parent = NULL, const char* name = NULL);
+    explicit RulesDialog(QWidget* parent = nullptr, const char* name = nullptr);
     Rules* edit(Rules* r, WId window, bool show_hints);
 protected:
     virtual void accept();
@@ -132,7 +132,7 @@ class EditShortcut
 {
     Q_OBJECT
 public:
-    explicit EditShortcut(QWidget* parent = NULL);
+    explicit EditShortcut(QWidget* parent = nullptr);
 protected Q_SLOTS:
     void editShortcut();
     void clearShortcut();
@@ -143,7 +143,7 @@ class EditShortcutDialog
 {
     Q_OBJECT
 public:
-    explicit EditShortcutDialog(QWidget* parent = NULL, const char* name = NULL);
+    explicit EditShortcutDialog(QWidget* parent = nullptr, const char* name = nullptr);
     void setShortcut(const QString& cut);
     QString shortcut() const;
 private:
@@ -156,7 +156,7 @@ class ShortcutDialog
 {
     Q_OBJECT
 public:
-    explicit ShortcutDialog(const QKeySequence& cut, QWidget* parent = NULL);
+    explicit ShortcutDialog(const QKeySequence& cut, QWidget* parent = nullptr);
     virtual void accept();
     QKeySequence shortcut() const;
 private:
diff --git a/kcmkwin/kwintabbox/layoutpreview.h b/kcmkwin/kwintabbox/layoutpreview.h
index 0203909eda..8180d34589 100644
--- a/kcmkwin/kwintabbox/layoutpreview.h
+++ b/kcmkwin/kwintabbox/layoutpreview.h
@@ -48,7 +48,7 @@ class ExampleClientModel : public QAbstractListModel
 {
     Q_OBJECT
 public:
-    explicit ExampleClientModel(QObject *parent = NULL);
+    explicit ExampleClientModel(QObject *parent = nullptr);
     virtual ~ExampleClientModel();
 
     virtual QVariant data(const QModelIndex &index, int role = Qt::DisplayRole) const;
diff --git a/kcmkwin/kwintabbox/main.cpp b/kcmkwin/kwintabbox/main.cpp
index 5bda6887e0..70b18bfbfd 100644
--- a/kcmkwin/kwintabbox/main.cpp
+++ b/kcmkwin/kwintabbox/main.cpp
@@ -461,7 +461,7 @@ void KWinTabBoxConfig::updateConfigFromUi(const KWin::KWinTabBoxConfigForm* ui,
 
 #define CHECK_CURRENT_TABBOX_UI \
     Q_ASSERT(sender());\
-    KWinTabBoxConfigForm *ui = 0;\
+    KWinTabBoxConfigForm *ui = nullptr;\
     QObject *dad = sender();\
     while (!ui && (dad = dad->parent()))\
         ui = qobject_cast<KWinTabBoxConfigForm*>(dad);\