#include <stdio.h>

typedef struct _foo {

    union {
        char c;
        int i;
    } data;

    void (*print)(struct _foo *);

} foo;

static void print_char(foo *bar)
{
    printf( "data.char: %cn", bar->data.c );
}

static void print_int(foo *bar)
{
    printf( "data.int : %dn", bar->data.i );
}

int main(void)
{
    foo int_foo  = { .data.i = 1,   .print = print_int  };
    foo char_foo = { .data.c = 'a', .print = print_char };
    foo *foo_ptr;

    foo_ptr = &char_foo;
    foo_ptr->print(foo_ptr);

    foo_ptr = &int_foo;
    foo_ptr->print(foo_ptr);

    return 0;
}