qobject: Add a special null QObject

I'm going to fix the JSON parser to recognize null.  The obvious
representation of JSON null as (QObject *)NULL doesn't work, because
the parser already uses it as an error value.  Perhaps we should
change it to free NULL for null, but that's more than I can do right
now.  Create a special null QObject instead.

The existing QDict, QList, and QString all represent something that
is a pointer in C and could therefore be associated with NULL.  But
right now, all three of these sub-types are always non-null once
created, so the new null sentinel object is intentionally unrelated
to them.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>

1 files changed, 10 insertions, 1 deletions

diff --git a/include/qapi/qmp/qobject.h b/include/qapi/qmp/qobject.h
index 099129657a..84b2d9fef5 100644
--- a/include/qapi/qmp/qobject.h
+++ b/include/qapi/qmp/qobject.h
@@ -3,7 +3,7 @@
  *
  * Based on ideas by Avi Kivity <avi@redhat.com>
  *
- * Copyright (C) 2009 Red Hat Inc.
+ * Copyright (C) 2009, 2015 Red Hat Inc.
  *
  * Authors:
  *  Luiz Capitulino <lcapitulino@redhat.com>
@@ -37,6 +37,7 @@
 
 typedef enum {
     QTYPE_NONE,    /* sentinel value, no QObject has this type code */
+    QTYPE_QNULL,
     QTYPE_QINT,
     QTYPE_QSTRING,
     QTYPE_QDICT,
@@ -110,4 +111,12 @@ static inline qtype_code qobject_type(const QObject *obj)
     return obj->type->code;
 }
 
+extern QObject qnull_;
+
+static inline QObject *qnull(void)
+{
+    qobject_incref(&qnull_);
+    return &qnull_;
+}
+
 #endif /* QOBJECT_H */