(glib:define-gboxed-opaque icon-source "GtkIconSource"
  :export t
  :type-initializer "gtk_icon_source_get_type"
  :alloc (%icon-source-new))  

The gtk:icon-source structure is opaque, and has no user visible fields. A gtk:icon-source instance contains a gdk-pixbuf:pixbuf object, or image filename, that serves as the base image for one or more of the icons in a gtk:icon-set instance, along with a specification for which icons in the icon set will be based on that pixbuf or image file.