C23_Variant_Implementation.c

#include <stdlib.h>
#include <string.h>
#include <stdio.h>

typedef enum {
    thing_flag,
    thing_number,
    thing_nil
} Thing;

typedef struct nil_t {} Nil;

typedef struct variant_t {
    Thing tag;
    union {
        int num;
        bool flag;
        Nil foo; // placeholder
    } data;
} BootlegVariant;

/// "generic" constructor

#define GEN_BOOTLEG_VARIANT_INIT(type)\
void bootleg_variant_init_##type ( BootlegVariant* self, type arg ) {\
    _Generic((arg),\
    int: self->data.num = arg,\
    bool: self->data.flag = arg,\
    default:\
        self->data.foo = (Nil){}\
    );\
    _Generic((arg),\
    int:  self->tag = thing_number,\
    bool: self->tag = thing_flag,\
    default:\
        self->tag = thing_nil\
    );\
}\

#define GEN_BOOTLEG_VARIANT_GET(type)\
bool bootleg_variant_get_##type(const BootlegVariant* self, type* out_result) {\
    bool access_ok = true;\
    _Generic((*out_result),\
    int: access_ok = self->tag == thing_number,\
    bool: access_ok = self->tag == thing_flag,\
    default: access_ok = false\
    );\
    \
    if (access_ok) {\
    _Generic((*out_result),\
    int: *out_result = self->data.num,\
    bool: *out_result = self->data.flag,\
    default: 0\
    );\
    }\
    return access_ok;\
}\

/// fancy macro for calling "generic" functions

#define BOOTLEG_VARIANT_INIT_CALL(type, self_ptr, init_arg) bootleg_variant_init_##type(self_ptr, init_arg);\

#define BOOTLEG_VARIANT_GET_CALL(type, self_ptr, out_arg) bootleg_variant_get_##type(self_ptr, out_arg)\

/// generate "generic" functions for types

GEN_BOOTLEG_VARIANT_INIT(int)
GEN_BOOTLEG_VARIANT_INIT(bool)
GEN_BOOTLEG_VARIANT_GET(int)
GEN_BOOTLEG_VARIANT_GET(bool)

int main() {
    BootlegVariant variant;
    int answer = 0;
    bool something = false;
    BOOTLEG_VARIANT_INIT_CALL(int, &variant, 42)

    if (BOOTLEG_VARIANT_GET_CALL(bool, &variant, &something)) {
        puts("Yay!");
    } else {
        puts("Nay!");
    }

    if (BOOTLEG_VARIANT_GET_CALL(int, &variant, &answer)) {
        puts("Yay!");
    } else {
        puts("Nay!");
    }

    printf("answer = %i\n", answer);
}