Stupidly simple set of macros to mimic GO-like INTERFACE in C

go_like_interface.c

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

/*
 * PROs:
 * * can have variables too as interface members
 * 
 * CONs:
 * * INTERFACE type can be directly declared and used as a normal struct
 * * INTERFACE_EMBED() macros injects a field in the struct
 */

#define INTERFACE(interface, ...)            typedef struct { __VA_ARGS__ } interface
#define INTERFACE_EMBED(interface)           interface _interface
#define INTERFACE_OF(ptr, interface)         ((interface *)ptr + offsetof(typeof(*ptr), _interface))
#define INTERFACE_INIT(ptr, interface, ...)  memcpy(INTERFACE_OF(ptr, interface), &(interface){ __VA_ARGS__ }, sizeof(interface));

static void foo(void);

// Declare the interface, in this case named "test" with 2 fields, an int and a function pointer
INTERFACE(test, 
    int val;
    void (*cb)(void);
);

// Declare a struct that implements "test" interface.
// Note, test interface member must still be initialized.
typedef struct {
    int bar;
    double x;
    INTERFACE_EMBED(test);
} myS;

static void foo(void) {
    printf("World\n");
}

static void my_test(test *t) {
    printf("Hello %d!\n", t->val);
    t->cb();
}

int main() {
    myS t = {0};
    
    // Init interface members
    INTERFACE_INIT((&t), test, 5, foo);
    // Call a function on interface
    my_test(INTERFACE_OF((&t), test));
    return 0;
}