Understand macro MP_DEFINE_CONST_OBJ_TYPE in-depth

[bhcuong2008]

I'm trying to understand some inner of MicroPython. One of it is

MP_DEFINE_CONST_OBJ_TYPE(
    mp_type_type,
    MP_QSTR_type,
    MP_TYPE_FLAG_NONE,
    make_new, type_make_new,
    print, type_print,
    call, type_call,
    attr, type_attr
    );

The macro definition MP_DEFINE_CONST_OBJ_TYPE defined as:
#define MP_DEFINE_CONST_OBJ_TYPE(...) MP_DEFINE_CONST_OBJ_TYPE_EXPAND(MP_DEFINE_CONST_OBJ_TYPE_NARGS(__VA_ARGS__, _INV, 12, _INV, 11, _INV, 10, _INV, 9, _INV, 8, _INV, 7, _INV, 6, _INV, 5, _INV, 4, _INV, 3, _INV, 2, _INV, 1, _INV, 0)(mp_obj_type_t, __VA_ARGS__))

With macro: #define MP_DEFINE_CONST_OBJ_TYPE_EXPAND(x) x, it will be:
MP_DEFINE_CONST_OBJ_TYPE_NARGS(__VA_ARGS__, _INV, 12, _INV, 11, _INV, 10, _INV, 9, _INV, 8, _INV, 7, _INV, 6, _INV, 5, _INV, 4, _INV, 3, _INV, 2, _INV, 1, _INV, 0)(mp_obj_type_t, __VA_ARGS__)

I could understand __VA_ARGS__ but not _INV, 12, _INV, 11,...and this macro
MP_DEFINE_CONST_OBJ_TYPE_NARGS(_1, _2, _3, _4, _5, _6, _7, _8, _9, _10, _11, _12, _13, _14, _15, _16, _17, _18, _19, _20, _21, _22, _23, _24, _25, _26, _27, _28, N, ...) MP_DEFINE_CONST_OBJ_TYPE_NARGS_##N(mp_obj_type_t, __VA_ARGS__)

This is so complex to my understanding of C, so could someone help me to explain it?
Thank you so much.

[Josverl]

I do not know for sure, and the below is an explanation by GH Copilot an AI grounded on this repo. So to be sure it also needs human validation.

Let me know if it is incorrect, then I can delete it

The macro MP_DEFINE_CONST_OBJ_TYPE is part of MicroPython's internal implementation for defining constant object types. Here's a detailed breakdown of its expansion and function:

Initial Macro

MP_DEFINE_CONST_OBJ_TYPE(
    mp_type_type,
    MP_QSTR_type,
    MP_TYPE_FLAG_NONE,
    make_new, type_make_new,
    print, type_print,
    call, type_call,
    attr, type_attr
);

Macro Definition

The macro MP_DEFINE_CONST_OBJ_TYPE is defined as:

#define MP_DEFINE_CONST_OBJ_TYPE(...) \
    MP_DEFINE_CONST_OBJ_TYPE_EXPAND( \
        MP_DEFINE_CONST_OBJ_TYPE_NARGS(__VA_ARGS__, _INV, 12, _INV, 11, _INV, 10, _INV, 9, _INV, 8, _INV, 7, _INV, 6, _INV, 5, _INV, 4, _INV, 3, _INV, 2, _INV, 1, _INV, 0)(mp_obj_type_t, __VA_ARGS__))

Key Components

1. __VA_ARGS__:
   This is a standard C preprocessor feature that represents the variable arguments passed to the macro. For example, in the given usage:

   mp_type_type, MP_QSTR_type, MP_TYPE_FLAG_NONE, make_new, type_make_new, print, type_print, call, type_call, attr, type_attr

   These are the arguments passed to __VA_ARGS__.

2. MP_DEFINE_CONST_OBJ_TYPE_EXPAND(x):
   This is defined as:

   #define MP_DEFINE_CONST_OBJ_TYPE_EXPAND(x) x

   It simply expands its argument x into its full form. This is useful for forcing macro expansion in some complex cases.

3. MP_DEFINE_CONST_OBJ_TYPE_NARGS:
   This macro is the most complex part. It determines the number of arguments passed to __VA_ARGS__ and selects the correct macro to call.

   • _INV, 12, _INV, 11, ...:
     These are markers and indices used to count the number of arguments. The _INV values are placeholders for positions that are not valid indices, while the numbers represent valid positions.

   • The macro MP_DEFINE_CONST_OBJ_TYPE_NARGS itself is defined as:

     #define MP_DEFINE_CONST_OBJ_TYPE_NARGS(_1, _2, _3, _4, ..., N, ...) \
         MP_DEFINE_CONST_OBJ_TYPE_NARGS_##N(mp_obj_type_t, __VA_ARGS__)

     It takes a large number of arguments, extracts the Nth argument (the count), and uses it to determine which specific macro to call.

4. Macro Selection:
   The result of MP_DEFINE_CONST_OBJ_TYPE_NARGS is a specialized macro, e.g., MP_DEFINE_CONST_OBJ_TYPE_NARGS_12, which is then invoked with the arguments.

Purpose

This macro system achieves two goals:

1. Flexible Argument Handling:
   It allows MP_DEFINE_CONST_OBJ_TYPE to handle a variable number of arguments without explicitly defining multiple versions of the macro.

2. Type Definition:
   It generates the necessary code to define a constant type object in MicroPython, including its attributes, methods, and flags.

Simplified View

Given the complexity, the macro essentially:

• Counts the number of arguments passed.
• Selects the appropriate version of the macro.
• Expands into a definition of a constant object type (mp_obj_type_t).

[bhcuong2008]

@Josverl , it's great explanation. But I'm still stuck here,
MP_DEFINE_CONST_OBJ_TYPE_NARGS(__VA_ARGS__, _INV, 12, _INV, 11, _INV, 10, _INV, 9, _INV, 8, _INV, 7, _INV, 6, _INV, 5, _INV, 4, _INV, 3, _INV, 2, _INV, 1, _INV, 0)(mp_obj_type_t, __VA_ARGS__)

It's like the function call with parameter (mp_obj_type_t, __VA_ARGS__). And _INV is a keyword like __VA_ARGS__ or just an optional with any name. I google but not found about it.

And macro, #define MP_DEFINE_CONST_OBJ_TYPE_NARGS(_1, _2, _3, _4, ..., N, ...), how to extract the 29th argument (N)?
(_1, _2, _3, _4, _5, _6, _7, _8, _9, _10, _11, _12, _13, _14, _15, _16, _17, _18, _19, _20, _21, _22, _23, _24, _25, _26, _27, _28 what will it be?

At the end, is it called with the macro as you said, is it right?

MP_DEFINE_CONST_OBJ_TYPE_NARGS_12(
    mp_obj_type_t, mp_type_type,
    MP_QSTR_type,
    MP_TYPE_FLAG_NONE,
    make_new, type_make_new,
    print, type_print,
    call, type_call,
    attr, type_attr)

[Josverl]

Sorry that's beyond my skills

[bhcuong2008]

According to my understanding, it will define like this. But I'm still confused about mp_type_type.

const mp_obj_type_t mp_type_type = {
{ &mp_type_type },
.name = MP_QSTR_type,
...
...
}

[jonnor]

What is confusing about mp_type_type ?

[bhcuong2008]

@jonnor Sorry for late reply.
That line defines struct mp_type_type , but inside struct, it uses &mp_type_type. I mean that &mp_type_type does not exist at that time, how could it be?

Thank you.