Vector tổng quát: hai cách

#ifndef VEC_H
#define VEC_H

#include <stddef.h>

typedef struct {
    void  *d;         /* vung du lieu tho */
    size_t n;         /* so phan tu dang dung */
    size_t cap;       /* so phan tu da cap phat */
    size_t esz;       /* kich thuoc MOT phan tu */
} Vec;

void  vec_khoi_tao(Vec *v, size_t esz);
void  vec_giai_phong(Vec *v);

int   vec_day(Vec *v, const void *phan_tu);
void *vec_tai(const Vec *v, size_t i);       /* NULL neu ngoai vung */

#endif /* VEC_H */

#include <stdlib.h>
#include <string.h>

#include "vec.h"

void vec_khoi_tao(Vec *v, size_t esz) {
    v->d = NULL;
    v->n = v->cap = 0;
    v->esz = esz;
}

void vec_giai_phong(Vec *v) {
    free(v->d);
    vec_khoi_tao(v, v->esz);
}

int vec_day(Vec *v, const void *phan_tu) {
    if (v->n == v->cap) {
        size_t c = v->cap ? v->cap * 2 : 4;

        if (c > SIZE_MAX / v->esz) return -1;      /* chong tran */

        void *t = realloc(v->d, c * v->esz);

        if (!t) return -1;

        v->d   = t;
        v->cap = c;
    }

    memcpy((char *)v->d + v->n * v->esz, phan_tu, v->esz);
    ++v->n;

    return 0;
}

void *vec_tai(const Vec *v, size_t i) {
    return i < v->n ? (char *)v->d + i * v->esz : NULL;
}

Vec a;
vec_khoi_tao(&a, sizeof(int));

for (int i = 0; i < 100; ++i)
    if (vec_day(&a, &i) != 0) return 1;        /* PHAI truyen dia chi */

int *p = vec_tai(&a, 50);
printf("%d\n", p ? *p : -1);                   /* PHAI ep kieu ngam */

vec_giai_phong(&a);

#ifndef VEC_MACRO_H
#define VEC_MACRO_H

#include <stdlib.h>

#define DINH_NGHIA_VECTOR(T, ten)                                     \
    typedef struct { T *d; size_t n, cap; } ten;                      \
                                                                      \
    static inline void ten##_khoi_tao(ten *v) {                       \
        v->d = NULL;                                                  \
        v->n = v->cap = 0;                                            \
    }                                                                 \
                                                                      \
    static inline void ten##_giai_phong(ten *v) {                     \
        free(v->d);                                                   \
        ten##_khoi_tao(v);                                            \
    }                                                                 \
                                                                      \
    static inline int ten##_day(ten *v, T x) {                        \
        if (v->n == v->cap) {                                         \
            size_t c = v->cap ? v->cap * 2 : 4;                       \
            T *t = realloc(v->d, c * sizeof *t);                      \
                                                                      \
            if (!t) return -1;                                        \
                                                                      \
            v->d   = t;                                               \
            v->cap = c;                                               \
        }                                                             \
                                                                      \
        v->d[v->n++] = x;                                             \
                                                                      \
        return 0;                                                     \
    }

#endif /* VEC_MACRO_H */

#include "vec-macro.h"

DINH_NGHIA_VECTOR(int,    VecInt)
DINH_NGHIA_VECTOR(double, VecDouble)

int main(void) {
    VecInt a;
    VecInt_khoi_tao(&a);

    for (int i = 0; i < 100; ++i)
        if (VecInt_day(&a, i) != 0) return 1;     /* truyen GIA TRI, khong phai dia chi */

    printf("%d\n", a.d[50]);                       /* truy cap TRUC TIEP, co kieu */

    VecInt_giai_phong(&a);

    return 0;
}

/* Tep nay duoc include NHIEU LAN, moi lan voi mot T khac.
   KHONG co header guard, va do la co y. */

#ifndef T
#error "Phai dinh nghia T truoc khi include vec-mau.h"
#endif

#ifndef TEN
#error "Phai dinh nghia TEN truoc khi include vec-mau.h"
#endif

#define NOI_(a, b) a##b
#define NOI(a, b)  NOI_(a, b)

typedef struct { T *d; size_t n, cap; } TEN;

static inline void NOI(TEN, _khoi_tao)(TEN *v) {
    v->d = NULL;
    v->n = v->cap = 0;
}

static inline int NOI(TEN, _day)(TEN *v, T x) {
    if (v->n == v->cap) {
        size_t c = v->cap ? v->cap * 2 : 4;
        T *t = realloc(v->d, c * sizeof *t);

        if (!t) return -1;

        v->d   = t;
        v->cap = c;
    }

    v->d[v->n++] = x;

    return 0;
}

#undef NOI
#undef NOI_
#undef T
#undef TEN

#define T   int
#define TEN VecInt
#include "vec-mau.h"

#define T   double
#define TEN VecDouble
#include "vec-mau.h"

/* Uu diem so voi macro mot dong:
     - ma nam trong tep .h binh thuong, khong co dau gach cheo nguoc
     - trinh soan thao to mau va thut le dung
     - thong bao loi tro vao dong that trong vec-mau.h

   Nhuoc diem:
     - cu phap la, nguoi doc lan dau khong hieu
     - phai nho #undef moi macro o cuoi, neu khong thi lan include
       thu hai bao "T redefined"
     - la cach ma thu vien C++ dung truoc khi co template */