Introduce QObject

This commit introduces the qobject.h header file, it contains
basic QObject definitions and helper macros.
Signed-off-by: NLuiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

qobject.h

+/*
+ * QEMU Object Model.
+ *
+ * Based on ideas by Avi Kivity <avi@redhat.com>
+ *
+ * Copyright (C) 2009 Red Hat Inc.
+ *
+ * Authors:
+ *  Luiz Capitulino <lcapitulino@redhat.com>
+ *
+ * This work is licensed under the terms of the GNU GPL, version 2.  See
+ * the COPYING file in the top-level directory.
+ *
+ * QObject Reference Counts Terminology
+ * ------------------------------------
+ *
+ *  - Returning references: A function that returns an object may
+ *  return it as either a weak or a strong reference.  If the reference
+ *  is strong, you are responsible for calling QDECREF() on the reference
+ *  when you are done.
+ *
+ *  If the reference is weak, the owner of the reference may free it at
+ *  any time in the future.  Before storing the reference anywhere, you
+ *  should call QINCREF() to make the reference strong.
+ *
+ *  - Transferring ownership: when you transfer ownership of a reference
+ *  by calling a function, you are no longer responsible for calling
+ *  QDECREF() when the reference is no longer needed.  In other words,
+ *  when the function returns you must behave as if the reference to the
+ *  passed object was weak.
+ */
+#ifndef QOBJECT_H
+#define QOBJECT_H
+
+#include <stddef.h>
+#include <assert.h>
+
+typedef enum {
+    QTYPE_NONE,
+} qtype_code;
+
+struct QObject;
+
+typedef struct QType {
+    qtype_code code;
+    void (*destroy)(struct QObject *);
+} QType;
+
+typedef struct QObject {
+    const QType *type;
+    size_t refcnt;
+} QObject;
+
+/* Objects definitions must include this */
+#define QObject_HEAD  \
+    QObject base
+
+/* Get the 'base' part of an object */
+#define QOBJECT(obj) (&obj->base)
+
+/* High-level interface for qobject_incref() */
+#define QINCREF(obj)      \
+    assert(obj != NULL);  \
+    qobject_incref(QOBJECT(obj))
+
+/* High-level interface for qobject_decref() */
+#define QDECREF(obj)              \
+    assert(obj != NULL);          \
+    qobject_decref(QOBJECT(obj))
+
+/* Initialize an object to default values */
+#define QOBJECT_INIT(obj, qtype_type)   \
+    obj->base.refcnt = 1;               \
+    obj->base.type   = qtype_type
+
+/**
+ * qobject_incref(): Increment QObject's reference count
+ */
+static inline void qobject_incref(QObject *obj)
+{
+    obj->refcnt++;
+}
+
+/**
+ * qobject_decref(): Decrement QObject's reference count, deallocate
+ * when it reaches zero
+ */
+static inline void qobject_decref(QObject *obj)
+{
+    if (--obj->refcnt == 0) {
+        assert(obj->type != NULL);
+        assert(obj->type->destroy != NULL);
+        obj->type->destroy(obj);
+    }
+}
+
+/**
+ * qobject_type(): Return the QObject's type
+ */
+static inline qtype_code qobject_type(const QObject *obj)
+{
+    assert(obj->type != NULL);
+    return obj->type->code;
+}
+
+#endif /* QOBJECT_H */