qobject.h

   1 /*
   2  * QEMU Object Model.
   3  *
   4  * Based on ideas by Avi Kivity <avi@redhat.com>
   5  *
   6  * Copyright (C) 2009 Red Hat Inc.
   7  *
   8  * Authors:
   9  *  Luiz Capitulino <lcapitulino@redhat.com>
  10  *
  11  * This work is licensed under the terms of the GNU GPL, version 2.  See
  12  * the COPYING file in the top-level directory.
  13  *
  14  * QObject Reference Counts Terminology
  15  * ------------------------------------
  16  *
  17  *  - Returning references: A function that returns an object may
  18  *  return it as either a weak or a strong reference.  If the reference
  19  *  is strong, you are responsible for calling QDECREF() on the reference
  20  *  when you are done.
  21  *
  22  *  If the reference is weak, the owner of the reference may free it at
  23  *  any time in the future.  Before storing the reference anywhere, you
  24  *  should call QINCREF() to make the reference strong.
  25  *
  26  *  - Transferring ownership: when you transfer ownership of a reference
  27  *  by calling a function, you are no longer responsible for calling
  28  *  QDECREF() when the reference is no longer needed.  In other words,
  29  *  when the function returns you must behave as if the reference to the
  30  *  passed object was weak.
  31  */
  32 #ifndef QOBJECT_H
  33 #define QOBJECT_H
  34
  35 #include <stddef.h>
  36 #include <assert.h>
  37
  38 typedef enum {
  39     QTYPE_NONE,
  40     QTYPE_QINT,
  41     QTYPE_QSTRING,
  42     QTYPE_QDICT,
  43 } qtype_code;
  44
  45 struct QObject;
  46
  47 typedef struct QType {
  48     qtype_code code;
  49     void (*destroy)(struct QObject *);
  50 } QType;
  51
  52 typedef struct QObject {
  53     const QType *type;
  54     size_t refcnt;
  55 } QObject;
  56
  57 /* Objects definitions must include this */
  58 #define QObject_HEAD  \
  59     QObject base
  60
  61 /* Get the 'base' part of an object */
  62 #define QOBJECT(obj) (&obj->base)
  63
  64 /* High-level interface for qobject_incref() */
  65 #define QINCREF(obj)      \
  66     assert(obj != NULL);  \
  67     qobject_incref(QOBJECT(obj))
  68
  69 /* High-level interface for qobject_decref() */
  70 #define QDECREF(obj)              \
  71     assert(obj != NULL);          \
  72     qobject_decref(QOBJECT(obj))
  73
  74 /* Initialize an object to default values */
  75 #define QOBJECT_INIT(obj, qtype_type)   \
  76     obj->base.refcnt = 1;               \
  77     obj->base.type   = qtype_type
  78
  79 /**
  80  * qobject_incref(): Increment QObject's reference count
  81  */
  82 static inline void qobject_incref(QObject *obj)
  83 {
  84     obj->refcnt++;
  85 }
  86
  87 /**
  88  * qobject_decref(): Decrement QObject's reference count, deallocate
  89  * when it reaches zero
  90  */
  91 static inline void qobject_decref(QObject *obj)
  92 {
  93     if (--obj->refcnt == 0) {
  94         assert(obj->type != NULL);
  95         assert(obj->type->destroy != NULL);
  96         obj->type->destroy(obj);
  97     }
  98 }
  99
 100 /**
 101  * qobject_type(): Return the QObject's type
 102  */
 103 static inline qtype_code qobject_type(const QObject *obj)
 104 {
 105     assert(obj->type != NULL);
 106     return obj->type->code;
 107 }
 108
 109 #endif /* QOBJECT_H */