put 'VCALL' into its own lib header

There will be some use for it at other places too (config system)

src/lib/vcall.h

@@ -0,0 +1,63 @@
+/*
+  vcall.h  -  helper macros for virtual (method) calls in C
+
+  Copyright (C)                 Lumiera.org
+    2010,                       Christian Thaeter <ct@pipapo.org>
+
+  This program is free software; you can redistribute it and/or
+  modify it under the terms of the GNU General Public License as
+  published by the Free Software Foundation; either version 2 of the
+  License, or (at your option) any later version.
+
+  This program is distributed in the hope that it will be useful,
+  but WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+  GNU General Public License for more details.
+
+  You should have received a copy of the GNU General Public License
+  along with this program; if not, write to the Free Software
+  Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
+*/
+
+#ifndef LUMIERA_VCALL_H
+#define LUMIERA_VCALL_H
+
+#include <nobug.h>
+
+/**
+ * @file
+ * This allows one to do polymorphic programming in C by referencing a vtable
+ * member which contains function pointers to a structure and then calling
+ * this 'virtual' functions through the VCALL macro.
+ */
+
+/*
+  TODO usage example, see btree.h
+*/
+
+/**
+ * helper macro for calling vtable functions
+ * just adds syntacic sugar VCALL(obj, func, params...)
+ * translates to obj->vtable->func(obj, params...) plus some saftey checks
+ */
+#define LUMIERA_VCALL(self, function, ...)              \
+  ({                                                    \
+    REQUIRE (self);                                     \
+    REQUIRE (self->vtable->function);                   \
+    self->vtable->function (self, ## __VA_ARGS__);      \
+  })
+
+#ifndef VCALL
+#define VCALL LUMIERA_VCALL
+#else
+#error "WTF! someone defined VCALL!"
+#endif
+
+#endif
+/*
+// Local Variables:
+// mode: C
+// c-file-style: "gnu"
+// indent-tabs-mode: nil
+// End:
+*/