Đa hình thủ công bằng vtable

#ifndef HINH_H
#define HINH_H

typedef struct Hinh Hinh;

typedef struct {
    const char *ten;
    double (*dien_tich)(const Hinh *);
    double (*chu_vi)(const Hinh *);
    void   (*huy)(Hinh *);
} HinhVTable;

struct Hinh {
    const HinhVTable *vt;
};

/* Ham tao cua tung loai */
Hinh *tao_tron(double r);
Hinh *tao_chu_nhat(double a, double b);

/* Giao dien chung, khong biet loai gi */
double hinh_dien_tich(const Hinh *h);
double hinh_chu_vi(const Hinh *h);
void   hinh_huy(Hinh *h);

#endif /* HINH_H */

#include <stdlib.h>

#include "hinh.h"

#define PI 3.141592653589793

typedef struct {
    Hinh   base;          /* PHAI la truong DAU TIEN */
    double r;
} HinhTron;

static double tron_dt(const Hinh *h) {
    const HinhTron *t = (const HinhTron *)h;

    return PI * t->r * t->r;
}

static double tron_cv(const Hinh *h) {
    const HinhTron *t = (const HinhTron *)h;

    return 2 * PI * t->r;
}

static void tron_huy(Hinh *h) { free(h); }

static const HinhVTable TRON_VT = {
    .ten       = "Tron",
    .dien_tich = tron_dt,
    .chu_vi    = tron_cv,
    .huy       = tron_huy,
};

Hinh *tao_tron(double r) {
    HinhTron *t = malloc(sizeof *t);

    if (!t) return NULL;

    t->base.vt = &TRON_VT;
    t->r       = r;

    return &t->base;
}

Hinh *ds[] = {
    tao_tron(2.0),
    tao_chu_nhat(3.0, 4.0),
    tao_tron(1.0),
};

double tong = 0;

for (size_t i = 0; i < 3; ++i) {
    printf("%-8s dien tich %7.4f\n",
           ds[i]->vt->ten, hinh_dien_tich(ds[i]));

    tong += hinh_dien_tich(ds[i]);
}

for (size_t i = 0; i < 3; ++i) hinh_huy(ds[i]);

/* Vong lap nay KHONG BIET co bao nhieu loai hinh ton tai.
   Them HinhTamGiac vao du an: vong lap nay khong doi mot ky tu. */

/* Chuan C, muc 6.7.2.1:

   "A pointer to a structure object, suitably converted, points to
    its initial member, and vice versa."

   Dich: con tro toi mot doi tuong struct, sau khi chuyen doi
   thich hop, tro toi thanh vien DAU TIEN cua no, va nguoc lai.

   Nen:
     (Hinh *)con_tro_tron          hop le, cho ra &tron->base
     (HinhTron *)con_tro_hinh      hop le, VOI DIEU KIEN
                                   doi tuong that su la mot HinhTron */

struct file_operations {
    struct module *owner;
    loff_t  (*llseek) (struct file *, loff_t, int);
    ssize_t (*read)   (struct file *, char __user *, size_t, loff_t *);
    ssize_t (*write)  (struct file *, const char __user *, size_t, loff_t *);
    int     (*open)   (struct inode *, struct file *);
    int     (*release)(struct inode *, struct file *);
    long    (*unlocked_ioctl)(struct file *, unsigned int, unsigned long);
    /* ... vai chuc con tro ham nua ... */
};

static ssize_t den_read(struct file *f, char __user *bo_dem,
                        size_t n, loff_t *vt) { /* ... */ }

static ssize_t den_write(struct file *f, const char __user *bo_dem,
                         size_t n, loff_t *vt) { /* ... */ }

static const struct file_operations DEN_FOPS = {
    .owner   = THIS_MODULE,
    .read    = den_read,
    .write   = den_write,
    .open    = den_open,
    .release = den_release,
};

/* Dang ky voi nhan */
cdev_init(&den_cdev, &DEN_FOPS);

/* Tu day tro di, khi tien trinh nguoi dung goi read() tren
   /dev/den, nhan goi DEN_FOPS.read, ma khong he biet gi ve
   trinh dieu khien nay.

   Day chinh xac la co che o tren, chi khac ten. */