--- a/src/document/io/MusicXMLLoader.h
+++ b/src/document/io/MusicXMLLoader.h
@@ -18,8 +18,7 @@
 #ifndef RG_MUSICXMLLOADER_H
 #define RG_MUSICXMLLOADER_H
 
-
-class QString;
+#include <QString>
 
 
 namespace Rosegarden
--- a/src/gui/studio/AudioPluginLV2GUIWindow.h
+++ b/src/gui/studio/AudioPluginLV2GUIWindow.h
@@ -32,6 +32,8 @@
 #include <string>
 #include <vector>
 
+#include <QWidget>
+
 
 namespace Rosegarden
 {
--- a/src/sound/LV2PluginDatabase.h
+++ b/src/sound/LV2PluginDatabase.h
@@ -19,6 +19,7 @@
 #include <QString>
 
 #include <map>
+#include <vector>
 
 
 namespace Rosegarden