Thread: Downcasting to access a variable?

Here's a slimmed down version of the code:

Code:

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

typedef struct Entity {
    int x, y;
} Entity;

typedef struct Player {
    Entity e;
    int health;
} Player;

typedef struct Bullet {
    Entity e;
    int lifeTime;
} Bullet;

int main() {

    Entity *e = malloc(sizeof *e);

    ((Bullet *)e)->lifeTime = 5;

    e->x = 32;
    e->y = 55;

    printf("%d , %d , %d", e->x, e->y, ((Bullet *)e)->lifeTime);

    free(e);

    return 0;
}

On this line, you allocate space for an Entity object, not a Bullet object:

Code:

Entity *e = malloc(sizeof *e);

Therefore, this line effectively attempts to access memory out of bounds:

Code:

((Bullet *)e)->lifeTime = 5;

You could try this instead:

Code:

Entity *e = malloc(sizeof(Bullet));