explaining some C macro magic

macro_explanation.c

// so a cool trick with macros in C (and C++) is that since macros inside of
// macros are stille evaluated by the preprocessor, you can use macro names as
// parameters to other macros (and even construct macro names out of out of
// parameters!) - so using this trick if we have some macro like
// this:

#include <stddef.h>

#define MY_TYPES_ITER(_F, ...)   \
    _F(FOO, foo, 0, __VA_ARGS__) \
    _F(BAR, bar, 1, __VA_ARGS__) \
    _F(BAZ, baz, 2, __VA_ARGS__) \

// then we can first use this trick to create an enum with the members
// MY_TYPE_FOO, MY_TYPE_BAR, MY_TYPE_BAZ

#define DECL_TYPES_ENUM_MEMBER(uc, lc, i, ...) \
    MY_TYPE_##uc = i,

typedef enum my_type {
    MY_TYPES_ITER(DECL_TYPES_ENUM_MEMBER)
} my_type_e;

// and this expands to
//
// enum my_types {
//     MY_TYPE_FOO = 0,
//     MY_TYPE_BAR = 1,
//     MY_TYPE_BAZ = 2,
// };
//
// which can be pretty useful - better yet though, if we have all of our types
// defined...

typedef struct foo { int x; } foo_t;
typedef struct bar { float y; } bar_t;
typedef struct baz { const char *z; } baz_t;

// then we can *also* use this trick to automatically make a big ol' union of
// that type *using* the enum as the type descriminator

typedef struct {
    union {
#define DECL_TYPES_UNION_MEMBER(uc, lc, ...) lc##_t lc;
        MY_TYPES_ITER(DECL_TYPES_UNION_MEMBER)
    };

    my_type_e type;
} my_types_t;

// and now my_types_t has a union of a foo_t, bar_t, and baz_t
// pretty cool, right?

// so *even better* than this, if we also list out the fields on each type
// (you should probably do this on each struct definition but we'll do it here
// for the sake of chronology)...

#define FOO_FIELDS(_F, ...) \
    _F(x, __VA_ARGS__)

#define BAR_FIELDS(_F, ...) \
    _F(y, __VA_ARGS__)

#define BAZ_FIELDS(_F, ...) \
    _F(z, __VA_ARGS__)

// and then we use _Generic to map types to another enum...

typedef enum field_type {
    FIELD_TYPE_INT,
    FIELD_TYPE_FLOAT,
    FIELD_TYPE_STR,
    // ... (fill out as you need)
} field_type_e;

#define TYPE_TO_FIELD_TYPE(x) _Generic(*((x*) NULL), \
    int: FIELD_TYPE_INT,                             \
    float: FIELD_TYPE_FLOAT,                         \
    const char*: FIELD_TYPE_STR                      \
    )

// and then we define structs to contain information about fields...

typedef struct field_desc {
    field_type_e type;
    const char *name;
    int offset, size;
} field_desc_t;

typedef struct type_desc {
    field_desc_t fields[16];
} type_desc_t;

// then we can use the above macros to autogenerate type info (!) which can be
// used for all sorts of stuff, including serialization!

#define TYPE_OF_FIELD(parent, field) __typeof__(((parent*) NULL)->field)

#define DO_FIELD_DESC(fname, pname)                              \
    {                                                            \
        .type = TYPE_TO_FIELD_TYPE(TYPE_OF_FIELD(pname, fname)), \
        .name = #fname,                                          \
        .size = sizeof(TYPE_OF_FIELD(pname, fname)),              \
        .offset = offsetof(pname, fname),                        \
    },

#define DO_TYPE_DESC(uc, lc, i, ...) \
    [i] = { .fields = { uc##_FIELDS(DO_FIELD_DESC, lc##_t) } },

// this array contains all type info for each field on each of our structs!
// automatically !!!!

const type_desc_t types[] = {
    MY_TYPES_ITER(DO_TYPE_DESC)
}

h

h

this is what C does to a man

I C

@jdah you should write a C compiler someday. bonus points if it doesn't choke on this C preprocessor incantation

🥴🥴🥴

h

h

Seems like you've invented a new programming paradigm: "Macro-oriented programming"

Seems like you've invented a new programming paradigm: "Macro-oriented programming"

This stuff is like half a century old.

This is very similar to X macros