Struct-Komposition und Zeiger-Typkonvertierung für polymorphe Objekt-Untertypisierung

#include <stdio.h>
 
typedef struct BaseObject {
    int id;
    const char *type_name;
} BaseObject;
 
typedef struct Widget {
    BaseObject base; /* Base struct placed as the first member */
    int width;
    int height;
} Widget;
 
void render_object(const BaseObject *obj) {
    printf("Rendering [%s] ID: %d\n", obj->type_name, obj->id);
}
 
int main(void) {
    Widget w = {
        .base = { .id = 101, .type_name = "UI_Widget" },
        .width = 1920,
        .height = 1080
    };
    /* Standard C guarantees offsetof(Widget, base) == 0, enabling safe base pointer casts */
    render_object((const BaseObject *)&w);
    return 0;
}