docs/devel/qom.rst

.. _qom:

===========================
The QEMU Object Model (QOM)
===========================

.. highlight:: c

The QEMU Object Model provides a framework for registering user creatable
types and instantiating objects from those types.  QOM provides the following
features:

- System for dynamically registering types
- Support for single-inheritance of types
- Multiple inheritance of stateless interfaces

.. code-block:: c
   :caption: Creating a minimal type

   #include "qdev.h"

   #define TYPE_MY_DEVICE "my-device"

   // No new virtual functions: we can reuse the typedef for the
   // superclass.
   typedef DeviceClass MyDeviceClass;
   typedef struct MyDevice
   {
       DeviceState parent;

       int reg0, reg1, reg2;
   } MyDevice;

   static const TypeInfo my_device_info = {
       .name = TYPE_MY_DEVICE,
       .parent = TYPE_DEVICE,
       .instance_size = sizeof(MyDevice),
   };

   static void my_device_register_types(void)
   {
       type_register_static(&my_device_info);
   }

   type_init(my_device_register_types)

In the above example, we create a simple type that is described by #TypeInfo.
#TypeInfo describes information about the type including what it inherits
from, the instance and class size, and constructor/destructor hooks.

Alternatively several static types could be registered using helper macro
DEFINE_TYPES()

.. code-block:: c

   static const TypeInfo device_types_info[] = {
       {
           .name = TYPE_MY_DEVICE_A,
           .parent = TYPE_DEVICE,
           .instance_size = sizeof(MyDeviceA),
       },
       {
           .name = TYPE_MY_DEVICE_B,
           .parent = TYPE_DEVICE,
           .instance_size = sizeof(MyDeviceB),
       },
   };

   DEFINE_TYPES(device_types_info)

Every type has an #ObjectClass associated with it.  #ObjectClass derivatives
are instantiated dynamically but there is only ever one instance for any
given type.  The #ObjectClass typically holds a table of function pointers
for the virtual methods implemented by this type.

Using object_new(), a new #Object derivative will be instantiated.  You can
cast an #Object to a subclass (or base-class) type using
object_dynamic_cast().  You typically want to define macro wrappers around
OBJECT_CHECK() and OBJECT_CLASS_CHECK() to make it easier to convert to a
specific type:

.. code-block:: c
   :caption: Typecasting macros

   #define MY_DEVICE_GET_CLASS(obj) \
      OBJECT_GET_CLASS(MyDeviceClass, obj, TYPE_MY_DEVICE)
   #define MY_DEVICE_CLASS(klass) \
      OBJECT_CLASS_CHECK(MyDeviceClass, klass, TYPE_MY_DEVICE)
   #define MY_DEVICE(obj) \
      OBJECT_CHECK(MyDevice, obj, TYPE_MY_DEVICE)

In case the ObjectClass implementation can be built as module a
module_obj() line must be added to make sure qemu loads the module
when the object is needed.

.. code-block:: c

   module_obj(TYPE_MY_DEVICE);

Class Initialization
====================

Before an object is initialized, the class for the object must be
initialized.  There is only one class object for all instance objects
that is created lazily.

Classes are initialized by first initializing any parent classes (if
necessary).  After the parent class object has initialized, it will be
copied into the current class object and any additional storage in the
class object is zero filled.

The effect of this is that classes automatically inherit any virtual
function pointers that the parent class has already initialized.  All
other fields will be zero filled.

Once all of the parent classes have been initialized, #TypeInfo::class_init
is called to let the class being instantiated provide default initialize for
its virtual functions.  Here is how the above example might be modified
to introduce an overridden virtual function:

.. code-block:: c
   :caption: Overriding a virtual function

   #include "qdev.h"

   void my_device_class_init(ObjectClass *klass, void *class_data)
   {
       DeviceClass *dc = DEVICE_CLASS(klass);
       dc->reset = my_device_reset;
   }

   static const TypeInfo my_device_info = {
       .name = TYPE_MY_DEVICE,
       .parent = TYPE_DEVICE,
       .instance_size = sizeof(MyDevice),
       .class_init = my_device_class_init,
   };

Introducing new virtual methods requires a class to define its own
struct and to add a .class_size member to the #TypeInfo.  Each method
will also have a wrapper function to call it easily:

.. code-block:: c
   :caption: Defining an abstract class

   #include "qdev.h"

   typedef struct MyDeviceClass
   {
       DeviceClass parent;

       void (*frobnicate) (MyDevice *obj);
   } MyDeviceClass;

   static const TypeInfo my_device_info = {
       .name = TYPE_MY_DEVICE,
       .parent = TYPE_DEVICE,
       .instance_size = sizeof(MyDevice),
       .abstract = true, // or set a default in my_device_class_init
       .class_size = sizeof(MyDeviceClass),
   };

   void my_device_frobnicate(MyDevice *obj)
   {
       MyDeviceClass *klass = MY_DEVICE_GET_CLASS(obj);

       klass->frobnicate(obj);
   }

Interfaces
==========

Interfaces allow a limited form of multiple inheritance.  Instances are
similar to normal types except for the fact that are only defined by
their classes and never carry any state.  As a consequence, a pointer to
an interface instance should always be of incomplete type in order to be
sure it cannot be dereferenced.  That is, you should define the
'typedef struct SomethingIf SomethingIf' so that you can pass around
``SomethingIf *si`` arguments, but not define a ``struct SomethingIf { ... }``.
The only things you can validly do with a ``SomethingIf *`` are to pass it as
an argument to a method on its corresponding SomethingIfClass, or to
dynamically cast it to an object that implements the interface.

Methods
=======

A *method* is a function within the namespace scope of
a class. It usually operates on the object instance by passing it as a
strongly-typed first argument.
If it does not operate on an object instance, it is dubbed
*class method*.

Methods cannot be overloaded. That is, the #ObjectClass and method name
uniquely identity the function to be called; the signature does not vary
except for trailing varargs.

Methods are always *virtual*. Overriding a method in
#TypeInfo.class_init of a subclass leads to any user of the class obtained
via OBJECT_GET_CLASS() accessing the overridden function.
The original function is not automatically invoked. It is the responsibility
of the overriding class to determine whether and when to invoke the method
being overridden.

To invoke the method being overridden, the preferred solution is to store
the original value in the overriding class before overriding the method.
This corresponds to ``{super,base}.method(...)`` in Java and C#
respectively; this frees the overriding class from hardcoding its parent
class, which someone might choose to change at some point.

.. code-block:: c
   :caption: Overriding a virtual method

   typedef struct MyState MyState;

   typedef void (*MyDoSomething)(MyState *obj);

   typedef struct MyClass {
       ObjectClass parent_class;

       MyDoSomething do_something;
   } MyClass;

   static void my_do_something(MyState *obj)
   {
       // do something
   }

   static void my_class_init(ObjectClass *oc, void *data)
   {
       MyClass *mc = MY_CLASS(oc);

       mc->do_something = my_do_something;
   }

   static const TypeInfo my_type_info = {
       .name = TYPE_MY,
       .parent = TYPE_OBJECT,
       .instance_size = sizeof(MyState),
       .class_size = sizeof(MyClass),
       .class_init = my_class_init,
   };

   typedef struct DerivedClass {
       MyClass parent_class;

       MyDoSomething parent_do_something;
   } DerivedClass;

   static void derived_do_something(MyState *obj)
   {
       DerivedClass *dc = DERIVED_GET_CLASS(obj);

       // do something here
       dc->parent_do_something(obj);
       // do something else here
   }

   static void derived_class_init(ObjectClass *oc, void *data)
   {
       MyClass *mc = MY_CLASS(oc);
       DerivedClass *dc = DERIVED_CLASS(oc);

       dc->parent_do_something = mc->do_something;
       mc->do_something = derived_do_something;
   }

   static const TypeInfo derived_type_info = {
       .name = TYPE_DERIVED,
       .parent = TYPE_MY,
       .class_size = sizeof(DerivedClass),
       .class_init = derived_class_init,
   };

Alternatively, object_class_by_name() can be used to obtain the class and
its non-overridden methods for a specific type. This would correspond to
``MyClass::method(...)`` in C++.

The first example of such a QOM method was #CPUClass.reset,
another example is #DeviceClass.realize.

Standard type declaration and definition macros
===============================================

A lot of the code outlined above follows a standard pattern and naming
convention. To reduce the amount of boilerplate code that needs to be
written for a new type there are two sets of macros to generate the
common parts in a standard format.

A type is declared using the OBJECT_DECLARE macro family. In types
which do not require any virtual functions in the class, the
OBJECT_DECLARE_SIMPLE_TYPE macro is suitable, and is commonly placed
in the header file:

.. code-block:: c
   :caption: Declaring a simple type

   OBJECT_DECLARE_SIMPLE_TYPE(MyDevice, MY_DEVICE)

This is equivalent to the following:

.. code-block:: c
   :caption: Expansion from declaring a simple type

   typedef struct MyDevice MyDevice;
   typedef struct MyDeviceClass MyDeviceClass;

   G_DEFINE_AUTOPTR_CLEANUP_FUNC(MyDeviceClass, object_unref)

   #define MY_DEVICE_GET_CLASS(void *obj) \
           OBJECT_GET_CLASS(MyDeviceClass, obj, TYPE_MY_DEVICE)
   #define MY_DEVICE_CLASS(void *klass) \
           OBJECT_CLASS_CHECK(MyDeviceClass, klass, TYPE_MY_DEVICE)
   #define MY_DEVICE(void *obj)
           OBJECT_CHECK(MyDevice, obj, TYPE_MY_DEVICE)

   struct MyDeviceClass {
       DeviceClass parent_class;
   };

The 'struct MyDevice' needs to be declared separately.
If the type requires virtual functions to be declared in the class
struct, then the alternative OBJECT_DECLARE_TYPE() macro can be
used. This does the same as OBJECT_DECLARE_SIMPLE_TYPE(), but without
the 'struct MyDeviceClass' definition.

To implement the type, the OBJECT_DEFINE macro family is available.
In the simple case the OBJECT_DEFINE_TYPE macro is suitable:

.. code-block:: c
   :caption: Defining a simple type

   OBJECT_DEFINE_TYPE(MyDevice, my_device, MY_DEVICE, DEVICE)

This is equivalent to the following:

.. code-block:: c
   :caption: Expansion from defining a simple type

   static void my_device_finalize(Object *obj);
   static void my_device_class_init(ObjectClass *oc, void *data);
   static void my_device_init(Object *obj);

   static const TypeInfo my_device_info = {
       .parent = TYPE_DEVICE,
       .name = TYPE_MY_DEVICE,
       .instance_size = sizeof(MyDevice),
       .instance_init = my_device_init,
       .instance_finalize = my_device_finalize,
       .class_size = sizeof(MyDeviceClass),
       .class_init = my_device_class_init,
   };

   static void
   my_device_register_types(void)
   {
       type_register_static(&my_device_info);
   }
   type_init(my_device_register_types);

This is sufficient to get the type registered with the type
system, and the three standard methods now need to be implemented
along with any other logic required for the type.

If the type needs to implement one or more interfaces, then the
OBJECT_DEFINE_TYPE_WITH_INTERFACES() macro can be used instead.
This accepts an array of interface type names.

.. code-block:: c
   :caption: Defining a simple type implementing interfaces

   OBJECT_DEFINE_TYPE_WITH_INTERFACES(MyDevice, my_device,
                                      MY_DEVICE, DEVICE,
                                      { TYPE_USER_CREATABLE },
                                      { NULL })

If the type is not intended to be instantiated, then the
OBJECT_DEFINE_ABSTRACT_TYPE() macro can be used instead:

.. code-block:: c
   :caption: Defining a simple abstract type

   OBJECT_DEFINE_ABSTRACT_TYPE(MyDevice, my_device,
                               MY_DEVICE, DEVICE)


API Reference
-------------

.. kernel-doc:: include/qom/object.h