Mark explicit.

playground.h

@@ -32,7 +32,7 @@ class PlayGround : public QGraphicsView
   Q_OBJECT
 
 public:
-  PlayGround(TopLevel *parent);
+  explicit PlayGround(TopLevel *parent);
   ~PlayGround();
 
   enum LoadError { NoError, OldFileVersionError, OtherError };

playgrounddelegate.h

@@ -25,7 +25,7 @@
 class PlaygroundDelegate : public QAbstractItemDelegate
 {
   public:
-    PlaygroundDelegate(QObject *parent = 0);
+    explicit PlaygroundDelegate(QObject *parent = 0);
   private:
     virtual QSize sizeHint(const QStyleOptionViewItem& option, const QModelIndex& index) const;
     virtual void paint(QPainter* painter, const QStyleOptionViewItem& option, const QModelIndex& index) const;

soundfactory.h

@@ -26,7 +26,7 @@ class SoundFactory
 {
 public:
 
-  SoundFactory(TopLevel *parent);
+  explicit SoundFactory(TopLevel *parent);
   ~SoundFactory();
 
   bool loadLanguage(const QString &selectedLanguageFile);