qom: Extend documentation on QOM method concepts

Add a documentation section "Methods" and discuss among others how to
handle overriding virtual methods.

Clarify DeviceClass::realize documentation and refer to the above.
Signed-off-by: NAndreas Färber <afaerber@suse.de>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

hw/qdev-core.h

@@ -60,14 +60,20 @@ struct VMStateDescription;
  * The @init callback is considered private to a particular bus implementation
  * (immediate abstract child types of TYPE_DEVICE). Derived leaf types set an
  * "init" callback on their parent class instead.
+ *
  * Any type may override the @realize and/or @unrealize callbacks but needs
- * to call (and thus save) the parent type's implementation if so desired.
- * Usually this means storing the previous value of, e.g., @realized inside
- * the type's class structure and overwriting it with a function that first
- * invokes the stored callback, then performs any additional steps.
+ * to call the parent type's implementation if keeping their functionality
+ * is desired. Refer to QOM documentation for further discussion and examples.
+ *
+ * <note>
+ *   <para>
  * If a type derived directly from TYPE_DEVICE implements @realize, it does
  * not need to implement @init and therefore does not need to store and call
  * #DeviceClass' default @realize callback.
+ * For other types consult the documentation and implementation of the
+ * respective parent types.
+ *   </para>
+ * </note>
  */
 typedef struct DeviceClass {
     /*< private >*/

include/qom/object.h

@@ -147,9 +147,9 @@ typedef struct InterfaceInfo InterfaceInfo;
  *   </programlisting>
  * </example>
  *
- * Introducing new virtual functions requires a class to define its own
- * struct and to add a .class_size member to the TypeInfo.  Each function
- * will also have a wrapper to call it easily:
+ * 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:
  *
  * <example>
  *   <title>Defining an abstract class</title>
@@ -186,6 +186,104 @@ typedef struct InterfaceInfo InterfaceInfo;
  * similar to normal types except for the fact that are only defined by
  * their classes and never carry any state.  You can dynamically cast an object
  * to one of its #Interface types and vice versa.
+ *
+ * # Methods #
+ *
+ * A <emphasis>method</emphasis> 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
+ * <emphasis>class method</emphasis>.
+ *
+ * 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 <emphasis>virtual</emphasis>. 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 responsability
+ * 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.
+ *
+ * <example>
+ *   <title>Overriding a virtual method</title>
+ *   <programlisting>
+ * 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;
+ * } MyClass;
+ *
+ * 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 = my_class_init,
+ * };
+ *   </programlisting>
+ * </example>
+ *
+ * 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.
  */