Headers should also include what they use. All of the types should defined inorder to allow the header to be included stand-alone. For example, if aheader uses a GtkWidget*, it should #include . Or, if aheaders uses a GPtrArray*, it should #include to ensure thatall of the types are declared, whether by pointers/opaquely or fully, asrequired. Since all headers will use a G_BEGIN_DECLS and G_END_DECLSguard for C++, the bare minimum for a header is to include glib.h or or gtkcompat.h or some other header that makes thosemacros available.