1 ===========================
2 The QEMU Object Model (QOM)
3 ===========================
7 The QEMU Object Model provides a framework for registering user creatable
8 types and instantiating objects from those types. QOM provides the following
11 - System for dynamically registering types
12 - Support for single-inheritance of types
13 - Multiple inheritance of stateless interfaces
16 :caption: Creating a minimal type
20 #define TYPE_MY_DEVICE "my-device"
22 // No new virtual functions: we can reuse the typedef for the
24 typedef DeviceClass MyDeviceClass;
25 typedef struct MyDevice
32 static const TypeInfo my_device_info = {
33 .name = TYPE_MY_DEVICE,
34 .parent = TYPE_DEVICE,
35 .instance_size = sizeof(MyDevice),
38 static void my_device_register_types(void)
40 type_register_static(&my_device_info);
43 type_init(my_device_register_types)
45 In the above example, we create a simple type that is described by #TypeInfo.
46 #TypeInfo describes information about the type including what it inherits
47 from, the instance and class size, and constructor/destructor hooks.
49 Alternatively several static types could be registered using helper macro
54 static const TypeInfo device_types_info[] = {
56 .name = TYPE_MY_DEVICE_A,
57 .parent = TYPE_DEVICE,
58 .instance_size = sizeof(MyDeviceA),
61 .name = TYPE_MY_DEVICE_B,
62 .parent = TYPE_DEVICE,
63 .instance_size = sizeof(MyDeviceB),
67 DEFINE_TYPES(device_types_info)
69 Every type has an #ObjectClass associated with it. #ObjectClass derivatives
70 are instantiated dynamically but there is only ever one instance for any
71 given type. The #ObjectClass typically holds a table of function pointers
72 for the virtual methods implemented by this type.
74 Using object_new(), a new #Object derivative will be instantiated. You can
75 cast an #Object to a subclass (or base-class) type using
76 object_dynamic_cast(). You typically want to define macro wrappers around
77 OBJECT_CHECK() and OBJECT_CLASS_CHECK() to make it easier to convert to a
81 :caption: Typecasting macros
83 #define MY_DEVICE_GET_CLASS(obj) \
84 OBJECT_GET_CLASS(MyDeviceClass, obj, TYPE_MY_DEVICE)
85 #define MY_DEVICE_CLASS(klass) \
86 OBJECT_CLASS_CHECK(MyDeviceClass, klass, TYPE_MY_DEVICE)
87 #define MY_DEVICE(obj) \
88 OBJECT_CHECK(MyDevice, obj, TYPE_MY_DEVICE)
93 Before an object is initialized, the class for the object must be
94 initialized. There is only one class object for all instance objects
95 that is created lazily.
97 Classes are initialized by first initializing any parent classes (if
98 necessary). After the parent class object has initialized, it will be
99 copied into the current class object and any additional storage in the
100 class object is zero filled.
102 The effect of this is that classes automatically inherit any virtual
103 function pointers that the parent class has already initialized. All
104 other fields will be zero filled.
106 Once all of the parent classes have been initialized, #TypeInfo::class_init
107 is called to let the class being instantiated provide default initialize for
108 its virtual functions. Here is how the above example might be modified
109 to introduce an overridden virtual function:
112 :caption: Overriding a virtual function
116 void my_device_class_init(ObjectClass *klass, void *class_data)
118 DeviceClass *dc = DEVICE_CLASS(klass);
119 dc->reset = my_device_reset;
122 static const TypeInfo my_device_info = {
123 .name = TYPE_MY_DEVICE,
124 .parent = TYPE_DEVICE,
125 .instance_size = sizeof(MyDevice),
126 .class_init = my_device_class_init,
129 Introducing new virtual methods requires a class to define its own
130 struct and to add a .class_size member to the #TypeInfo. Each method
131 will also have a wrapper function to call it easily:
134 :caption: Defining an abstract class
138 typedef struct MyDeviceClass
142 void (*frobnicate) (MyDevice *obj);
145 static const TypeInfo my_device_info = {
146 .name = TYPE_MY_DEVICE,
147 .parent = TYPE_DEVICE,
148 .instance_size = sizeof(MyDevice),
149 .abstract = true, // or set a default in my_device_class_init
150 .class_size = sizeof(MyDeviceClass),
153 void my_device_frobnicate(MyDevice *obj)
155 MyDeviceClass *klass = MY_DEVICE_GET_CLASS(obj);
157 klass->frobnicate(obj);
163 Interfaces allow a limited form of multiple inheritance. Instances are
164 similar to normal types except for the fact that are only defined by
165 their classes and never carry any state. As a consequence, a pointer to
166 an interface instance should always be of incomplete type in order to be
167 sure it cannot be dereferenced. That is, you should define the
168 'typedef struct SomethingIf SomethingIf' so that you can pass around
169 ``SomethingIf *si`` arguments, but not define a ``struct SomethingIf { ... }``.
170 The only things you can validly do with a ``SomethingIf *`` are to pass it as
171 an argument to a method on its corresponding SomethingIfClass, or to
172 dynamically cast it to an object that implements the interface.
177 A *method* is a function within the namespace scope of
178 a class. It usually operates on the object instance by passing it as a
179 strongly-typed first argument.
180 If it does not operate on an object instance, it is dubbed
183 Methods cannot be overloaded. That is, the #ObjectClass and method name
184 uniquely identity the function to be called; the signature does not vary
185 except for trailing varargs.
187 Methods are always *virtual*. Overriding a method in
188 #TypeInfo.class_init of a subclass leads to any user of the class obtained
189 via OBJECT_GET_CLASS() accessing the overridden function.
190 The original function is not automatically invoked. It is the responsibility
191 of the overriding class to determine whether and when to invoke the method
194 To invoke the method being overridden, the preferred solution is to store
195 the original value in the overriding class before overriding the method.
196 This corresponds to ``{super,base}.method(...)`` in Java and C#
197 respectively; this frees the overriding class from hardcoding its parent
198 class, which someone might choose to change at some point.
201 :caption: Overriding a virtual method
203 typedef struct MyState MyState;
205 typedef void (*MyDoSomething)(MyState *obj);
207 typedef struct MyClass {
208 ObjectClass parent_class;
210 MyDoSomething do_something;
213 static void my_do_something(MyState *obj)
218 static void my_class_init(ObjectClass *oc, void *data)
220 MyClass *mc = MY_CLASS(oc);
222 mc->do_something = my_do_something;
225 static const TypeInfo my_type_info = {
227 .parent = TYPE_OBJECT,
228 .instance_size = sizeof(MyState),
229 .class_size = sizeof(MyClass),
230 .class_init = my_class_init,
233 typedef struct DerivedClass {
234 MyClass parent_class;
236 MyDoSomething parent_do_something;
239 static void derived_do_something(MyState *obj)
241 DerivedClass *dc = DERIVED_GET_CLASS(obj);
244 dc->parent_do_something(obj);
245 // do something else here
248 static void derived_class_init(ObjectClass *oc, void *data)
250 MyClass *mc = MY_CLASS(oc);
251 DerivedClass *dc = DERIVED_CLASS(oc);
253 dc->parent_do_something = mc->do_something;
254 mc->do_something = derived_do_something;
257 static const TypeInfo derived_type_info = {
258 .name = TYPE_DERIVED,
260 .class_size = sizeof(DerivedClass),
261 .class_init = derived_class_init,
264 Alternatively, object_class_by_name() can be used to obtain the class and
265 its non-overridden methods for a specific type. This would correspond to
266 ``MyClass::method(...)`` in C++.
268 The first example of such a QOM method was #CPUClass.reset,
269 another example is #DeviceClass.realize.
271 Standard type declaration and definition macros
272 ===============================================
274 A lot of the code outlined above follows a standard pattern and naming
275 convention. To reduce the amount of boilerplate code that needs to be
276 written for a new type there are two sets of macros to generate the
277 common parts in a standard format.
279 A type is declared using the OBJECT_DECLARE macro family. In types
280 which do not require any virtual functions in the class, the
281 OBJECT_DECLARE_SIMPLE_TYPE macro is suitable, and is commonly placed
285 :caption: Declaring a simple type
287 OBJECT_DECLARE_SIMPLE_TYPE(MyDevice, my_device,
290 This is equivalent to the following:
293 :caption: Expansion from declaring a simple type
295 typedef struct MyDevice MyDevice;
296 typedef struct MyDeviceClass MyDeviceClass;
298 G_DEFINE_AUTOPTR_CLEANUP_FUNC(MyDeviceClass, object_unref)
300 #define MY_DEVICE_GET_CLASS(void *obj) \
301 OBJECT_GET_CLASS(MyDeviceClass, obj, TYPE_MY_DEVICE)
302 #define MY_DEVICE_CLASS(void *klass) \
303 OBJECT_CLASS_CHECK(MyDeviceClass, klass, TYPE_MY_DEVICE)
304 #define MY_DEVICE(void *obj)
305 OBJECT_CHECK(MyDevice, obj, TYPE_MY_DEVICE)
307 struct MyDeviceClass {
308 DeviceClass parent_class;
311 The 'struct MyDevice' needs to be declared separately.
312 If the type requires virtual functions to be declared in the class
313 struct, then the alternative OBJECT_DECLARE_TYPE() macro can be
314 used. This does the same as OBJECT_DECLARE_SIMPLE_TYPE(), but without
315 the 'struct MyDeviceClass' definition.
317 To implement the type, the OBJECT_DEFINE macro family is available.
318 In the simple case the OBJECT_DEFINE_TYPE macro is suitable:
321 :caption: Defining a simple type
323 OBJECT_DEFINE_TYPE(MyDevice, my_device, MY_DEVICE, DEVICE)
325 This is equivalent to the following:
328 :caption: Expansion from defining a simple type
330 static void my_device_finalize(Object *obj);
331 static void my_device_class_init(ObjectClass *oc, void *data);
332 static void my_device_init(Object *obj);
334 static const TypeInfo my_device_info = {
335 .parent = TYPE_DEVICE,
336 .name = TYPE_MY_DEVICE,
337 .instance_size = sizeof(MyDevice),
338 .instance_init = my_device_init,
339 .instance_finalize = my_device_finalize,
340 .class_size = sizeof(MyDeviceClass),
341 .class_init = my_device_class_init,
345 my_device_register_types(void)
347 type_register_static(&my_device_info);
349 type_init(my_device_register_types);
351 This is sufficient to get the type registered with the type
352 system, and the three standard methods now need to be implemented
353 along with any other logic required for the type.
355 If the type needs to implement one or more interfaces, then the
356 OBJECT_DEFINE_TYPE_WITH_INTERFACES() macro can be used instead.
357 This accepts an array of interface type names.
360 :caption: Defining a simple type implementing interfaces
362 OBJECT_DEFINE_TYPE_WITH_INTERFACES(MyDevice, my_device,
364 { TYPE_USER_CREATABLE },
367 If the type is not intended to be instantiated, then then
368 the OBJECT_DEFINE_ABSTRACT_TYPE() macro can be used instead:
371 :caption: Defining a simple abstract type
373 OBJECT_DEFINE_ABSTRACT_TYPE(MyDevice, my_device,
381 .. kernel-doc:: include/qom/object.h