Function overrides in C

#import <Foundation/Foundation.h>
#import <pthread.h>

typedef void (^MyBlock)(void);
typedef void (*MyFunc)(void *);

static void Block(MyBlock block)
{
    NSLog(@"Block called with %p", block);
}

static void Object(id obj, SEL sel)
{
    NSLog(@"Object called with %p", obj);
}

static void Function(MyFunc func, void *userData, MyFunc destructor)
{
    NSLog(@"Function called with %p", func);
}

#define TYPE_IS_TYPE(x, y) __builtin_types_compatible_p(x, y)
#define EXPR_IS_TYPE(expr, type) TYPE_IS_TYPE(__typeof__(expr), type)

#define	CHOOSE(p, a, b) __builtin_choose_expr((p), (a), (b))

#define DEFAULT_TYPE(expr, type) CHOOSE(EXPR_IS_TYPE(expr, type), (expr), (type)NULL)

#define FUNCTYPE(x) __typeof__((__typeof__(x) *){ NULL })

#define THING_IMPL(x, y, z, ...) \
    CHOOSE(EXPR_IS_TYPE(x, MyBlock), Block(DEFAULT_TYPE((x), MyBlock)), \
    CHOOSE(EXPR_IS_TYPE(x, id),      Object(DEFAULT_TYPE((x), id), (y)), \
    CHOOSE(EXPR_IS_TYPE(x, MyFunc) || TYPE_IS_TYPE(FUNCTYPE(x), MyFunc), \
                                     Function((MyFunc)(x), (y), (z)), \
                                     assert(0))))

#define THING(x, ...) THING_IMPL(x, ## __VA_ARGS__, 0, 0)

static void f(void *x)
{
}

int main(int argc, char **argv)
{
    NSString *obj = @"hello";
    
    THING(^{});
    THING(obj, @selector(abc));
    THING(f, NULL, f);
    THING(&f, NULL, &f);
    
    return 0;
}