[RFC] include/qom/object.h: New OBJECT_DEFINE_SIMPLE_TYPE{, _WITH_INTEFACES} macros

We have an OBJECT_DEFINE_TYPE_EXTENDED macro, plus several variations
on it, which emits the boilerplate for the TypeInfo and ensures it is
registered with the type system.  However, all the existing macros
insist that the type being defined has its own FooClass struct, so
they aren't useful for the common case of a simple leaf class which
doesn't have any new methods or any other need for its own class
struct (that is, for the kind of type that OBJECT_DECLARE_SIMPLE_TYPE
declares).

Pull the actual implementation of OBJECT_DEFINE_TYPE_EXTENDED out
into a new DO_OBJECT_DEFINE_TYPE_EXTENDED which parameterizes the
value we use for the class_size field.  This lets us add a new
OBJECT_DEFINE_SIMPLE_TYPE which does the same job as the various
existing OBJECT_DEFINE_*_TYPE_* family macros for this kind of simple
type, and the variant OBJECT_DEFINE_SIMPLE_TYPE_WITH_INTERFACES for
when the type will implement some interfaces.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
---
I've marked this RFC largely because this patch doesn't include
any uses of the new macros. I wanted them for a series I'm currently
working on, but I wanted to send this out early so it could have an
extended review period and a bit more visibility than if I stuck
it inside an 8 patch series about some other topic. (In fact, this
is the second time I looked at the OBJECT_DEFINE_* macros and found
they weren't suitable for the common case kind of type. The first
time around I went back to writing the type out the old fashioned
way, but this time I figured I'd try improving the macros.)

 docs/devel/qom.rst   |  34 +++++++++++--
 include/qom/object.h | 114 +++++++++++++++++++++++++++++++++----------
 2 files changed, 117 insertions(+), 31 deletions(-)

On Mon, Feb 12, 2024 at 05:49:25PM +0000, Peter Maydell wrote:
> We have an OBJECT_DEFINE_TYPE_EXTENDED macro, plus several variations
> on it, which emits the boilerplate for the TypeInfo and ensures it is
> registered with the type system.  However, all the existing macros
> insist that the type being defined has its own FooClass struct, so
> they aren't useful for the common case of a simple leaf class which
> doesn't have any new methods or any other need for its own class
> struct (that is, for the kind of type that OBJECT_DECLARE_SIMPLE_TYPE
> declares).
> 
> Pull the actual implementation of OBJECT_DEFINE_TYPE_EXTENDED out
> into a new DO_OBJECT_DEFINE_TYPE_EXTENDED which parameterizes the
> value we use for the class_size field.  This lets us add a new
> OBJECT_DEFINE_SIMPLE_TYPE which does the same job as the various
> existing OBJECT_DEFINE_*_TYPE_* family macros for this kind of simple
> type, and the variant OBJECT_DEFINE_SIMPLE_TYPE_WITH_INTERFACES for
> when the type will implement some interfaces.
> 
> Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
> ---
> I've marked this RFC largely because this patch doesn't include
> any uses of the new macros. I wanted them for a series I'm currently
> working on, but I wanted to send this out early so it could have an
> extended review period and a bit more visibility than if I stuck
> it inside an 8 patch series about some other topic. (In fact, this
> is the second time I looked at the OBJECT_DEFINE_* macros and found
> they weren't suitable for the common case kind of type. The first
> time around I went back to writing the type out the old fashioned
> way, but this time I figured I'd try improving the macros.)

The macros were intended to simplify QOM boilerplate. So given that
they're not currently addressing a common use case of no-Class objects,
it makes sense to extend the macros as you suggest here.

>  docs/devel/qom.rst   |  34 +++++++++++--
>  include/qom/object.h | 114 +++++++++++++++++++++++++++++++++----------
>  2 files changed, 117 insertions(+), 31 deletions(-)

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>

> 
> diff --git a/docs/devel/qom.rst b/docs/devel/qom.rst
> index 9918fac7f21..0889ca949c1 100644
> --- a/docs/devel/qom.rst
> +++ b/docs/devel/qom.rst
> @@ -348,12 +348,14 @@ 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:
> +For the simplest case of a leaf class which doesn't need any of its
> +own virtual functions (i.e. which was declared with OBJECT_DECLARE_SIMPLE_TYPE)
> +the OBJECT_DEFINE_SIMPLE_TYPE macro is suitable:
>  
>  .. code-block:: c
>     :caption: Defining a simple type
>  
> -   OBJECT_DEFINE_TYPE(MyDevice, my_device, MY_DEVICE, DEVICE)
> +   OBJECT_DEFINE_SIMPLE_TYPE(MyDevice, my_device, MY_DEVICE, DEVICE)
>  
>  This is equivalent to the following:
>  
> @@ -370,7 +372,6 @@ This is equivalent to the following:
>         .instance_size = sizeof(MyDevice),
>         .instance_init = my_device_init,
>         .instance_finalize = my_device_finalize,
> -       .class_size = sizeof(MyDeviceClass),
>         .class_init = my_device_class_init,
>     };
>  
> @@ -385,13 +386,36 @@ 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 class needs its own virtual methods, or has some other
> +per-class state it needs to store in its own class struct,
> +then you can use the OBJECT_DEFINE_TYPE macro. This does the
> +same thing as OBJECT_DEFINE_SIMPLE_TYPE, but it also sets the
> +class_size of the type to the size of the class struct.
> +
> +.. code-block:: c
> +   :caption: Defining a type which needs a class struct
> +
> +   OBJECT_DEFINE_TYPE(MyDevice, my_device, MY_DEVICE, DEVICE)
> +
>  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.
> +OBJECT_DEFINE_SIMPLE_TYPE_WITH_INTERFACES() and
> +OBJECT_DEFINE_TYPE_WITH_INTERFACES() macros can be used instead.
> +These accept an array of interface type names. The difference between
> +them is that the former is for simple leaf classes that don't need
> +a class struct, and the latter is for when you will be defining
> +a class struct.
>  
>  .. code-block:: c
>     :caption: Defining a simple type implementing interfaces
>  
> +   OBJECT_DEFINE_SIMPLE_TYPE_WITH_INTERFACES(MyDevice, my_device,
> +                                             MY_DEVICE, DEVICE,
> +                                             { TYPE_USER_CREATABLE },
> +                                             { NULL })
> +
> +.. code-block:: c
> +   :caption: Defining a type implementing interfaces
> +
>     OBJECT_DEFINE_TYPE_WITH_INTERFACES(MyDevice, my_device,
>                                        MY_DEVICE, DEVICE,
>                                        { TYPE_USER_CREATABLE },
> diff --git a/include/qom/object.h b/include/qom/object.h
> index afccd24ca7a..f52ab216cdd 100644
> --- a/include/qom/object.h
> +++ b/include/qom/object.h
> @@ -258,6 +258,51 @@ struct Object
>      DECLARE_INSTANCE_CHECKER(InstanceType, MODULE_OBJ_NAME, TYPE_##MODULE_OBJ_NAME)
>  
>  
> +/**
> + * DO_OBJECT_DEFINE_TYPE_EXTENDED:
> + * @ModuleObjName: the object name with initial caps
> + * @module_obj_name: the object name in lowercase with underscore separators
> + * @MODULE_OBJ_NAME: the object name in uppercase with underscore separators
> + * @PARENT_MODULE_OBJ_NAME: the parent object name in uppercase with underscore
> + *                          separators
> + * @ABSTRACT: boolean flag to indicate whether the object can be instantiated
> + * @CLASS_SIZE: size of the type's class
> + * @...: list of initializers for "InterfaceInfo" to declare implemented interfaces
> + *
> + * This is the base macro used to implement all the OBJECT_DEFINE_*
> + * macros. It should never be used directly in a source file.
> + */
> +#define DO_OBJECT_DEFINE_TYPE_EXTENDED(ModuleObjName, module_obj_name, \
> +                                       MODULE_OBJ_NAME, \
> +                                       PARENT_MODULE_OBJ_NAME, \
> +                                       ABSTRACT, CLASS_SIZE, ...) \
> +    static void \
> +    module_obj_name##_finalize(Object *obj); \
> +    static void \
> +    module_obj_name##_class_init(ObjectClass *oc, void *data); \
> +    static void \
> +    module_obj_name##_init(Object *obj); \
> +    \
> +    static const TypeInfo module_obj_name##_info = { \
> +        .parent = TYPE_##PARENT_MODULE_OBJ_NAME, \
> +        .name = TYPE_##MODULE_OBJ_NAME, \
> +        .instance_size = sizeof(ModuleObjName), \
> +        .instance_align = __alignof__(ModuleObjName), \
> +        .instance_init = module_obj_name##_init, \
> +        .instance_finalize = module_obj_name##_finalize, \
> +        .class_size = CLASS_SIZE, \
> +        .class_init = module_obj_name##_class_init, \
> +        .abstract = ABSTRACT, \
> +        .interfaces = (InterfaceInfo[]) { __VA_ARGS__ } , \
> +    }; \
> +    \
> +    static void \
> +    module_obj_name##_register_types(void) \
> +    { \
> +        type_register_static(&module_obj_name##_info); \
> +    } \
> +    type_init(module_obj_name##_register_types);
> +
>  /**
>   * OBJECT_DEFINE_TYPE_EXTENDED:
>   * @ModuleObjName: the object name with initial caps
> @@ -284,32 +329,10 @@ struct Object
>  #define OBJECT_DEFINE_TYPE_EXTENDED(ModuleObjName, module_obj_name, \
>                                      MODULE_OBJ_NAME, PARENT_MODULE_OBJ_NAME, \
>                                      ABSTRACT, ...) \
> -    static void \
> -    module_obj_name##_finalize(Object *obj); \
> -    static void \
> -    module_obj_name##_class_init(ObjectClass *oc, void *data); \
> -    static void \
> -    module_obj_name##_init(Object *obj); \
> -    \
> -    static const TypeInfo module_obj_name##_info = { \
> -        .parent = TYPE_##PARENT_MODULE_OBJ_NAME, \
> -        .name = TYPE_##MODULE_OBJ_NAME, \
> -        .instance_size = sizeof(ModuleObjName), \
> -        .instance_align = __alignof__(ModuleObjName), \
> -        .instance_init = module_obj_name##_init, \
> -        .instance_finalize = module_obj_name##_finalize, \
> -        .class_size = sizeof(ModuleObjName##Class), \
> -        .class_init = module_obj_name##_class_init, \
> -        .abstract = ABSTRACT, \
> -        .interfaces = (InterfaceInfo[]) { __VA_ARGS__ } , \
> -    }; \
> -    \
> -    static void \
> -    module_obj_name##_register_types(void) \
> -    { \
> -        type_register_static(&module_obj_name##_info); \
> -    } \
> -    type_init(module_obj_name##_register_types);
> +    DO_OBJECT_DEFINE_TYPE_EXTENDED(ModuleObjName, module_obj_name, \
> +                                   MODULE_OBJ_NAME, PARENT_MODULE_OBJ_NAME, \
> +                                   ABSTRACT, sizeof(ModuleObjName##Class), \
> +                                   __VA_ARGS__)
>  
>  /**
>   * OBJECT_DEFINE_TYPE:
> @@ -368,6 +391,45 @@ struct Object
>                                  MODULE_OBJ_NAME, PARENT_MODULE_OBJ_NAME, \
>                                  true, { NULL })
>  
> +/**
> + * OBJECT_DEFINE_SIMPLE_TYPE_WITH_INTERFACES:
> + * @ModuleObjName: the object name with initial caps
> + * @module_obj_name: the object name in lowercase with underscore separators
> + * @MODULE_OBJ_NAME: the object name in uppercase with underscore separators
> + * @PARENT_MODULE_OBJ_NAME: the parent object name in uppercase with underscore
> + *                          separators
> + *
> + * This is a variant of OBJECT_DEFINE_TYPE_EXTENDED, which is suitable for
> + * the case of a non-abstract type, with interfaces, and with no requirement
> + * for a class struct.
> + */
> +#define OBJECT_DEFINE_SIMPLE_TYPE_WITH_INTERFACES(ModuleObjName, \
> +                                                  module_obj_name, \
> +                                                  MODULE_OBJ_NAME, \
> +                                                  PARENT_MODULE_OBJ_NAME, ...) \
> +    DO_OBJECT_DEFINE_TYPE_EXTENDED(ModuleObjName, module_obj_name, \
> +                                   MODULE_OBJ_NAME, PARENT_MODULE_OBJ_NAME, \
> +                                   false, 0, __VA_ARGS__)
> +
> +/**
> + * OBJECT_DEFINE_SIMPLE_TYPE:
> + * @ModuleObjName: the object name with initial caps
> + * @module_obj_name: the object name in lowercase with underscore separators
> + * @MODULE_OBJ_NAME: the object name in uppercase with underscore separators
> + * @PARENT_MODULE_OBJ_NAME: the parent object name in uppercase with underscore
> + *                          separators
> + *
> + * This is a variant of OBJECT_DEFINE_TYPE_EXTENDED, which is suitable for
> + * the common case of a non-abstract type, without any interfaces, and with
> + * no requirement for a class struct. If you declared your type with
> + * OBJECT_DECLARE_SIMPLE_TYPE then this is probably the right choice for
> + * defining it.
> + */
> +#define OBJECT_DEFINE_SIMPLE_TYPE(ModuleObjName, module_obj_name, \
> +                                  MODULE_OBJ_NAME, PARENT_MODULE_OBJ_NAME) \
> +    OBJECT_DEFINE_SIMPLE_TYPE_WITH_INTERFACES(ModuleObjName, module_obj_name, \
> +        MODULE_OBJ_NAME, PARENT_MODULE_OBJ_NAME, { NULL })
> +
>  /**
>   * struct TypeInfo:
>   * @name: The name of the type.
> -- 
> 2.34.1
> 

With regards,
Daniel