Closure trong C

/* Trong mot ngon ngu co closure: */

int he_so = 10;
auto nhan = [he_so](int x) { return x * he_so; };   /* C++ */

/* Ham nhan "bat" duoc bien he_so. Doi tuong nhan mang theo
   ban sao cua he_so, va no song lau hon pham vi noi he_so
   duoc khai bao.

   Trong C, ban phai tu lam ca hai viec do:
     1. goi gia tri bat duoc vao dau do
     2. truyen cho do cho ham */

/* Mot "closure" la mot cap: ham, va con tro toi moi truong */
typedef struct {
    int (*goi)(void *ctx, int x);
    void *ctx;
} HamInt;

static int ap_dung(HamInt f, int x) { return f.goi(f.ctx, x); }

void map_int(int *a, size_t n, HamInt f) {
    for (size_t i = 0; i < n; ++i)
        a[i] = f.goi(f.ctx, a[i]);
}

/* Moi truong bat duoc: mot struct do ban dinh nghia */
typedef struct { int n; } CtxNhan;

static int than_nhan(void *ctx, int x) {
    return x * ((CtxNhan *)ctx)->n;
}

int main(void) {
    int     a[] = { 1, 2, 3, 4, 5 };
    CtxNhan c   = { .n = 10 };
    HamInt  nhan10 = { than_nhan, &c };      /* "lambda bat bien n" */

    map_int(a, 5, nhan10);

    for (int i = 0; i < 5; ++i) printf("%d ", a[i]);

    return 0;
}

/* Nhan f(a, b) va gia tri a, tra ve g(b) = f(a, b) */

typedef struct {
    int (*goi)(void *ctx, int x);
    void *ctx;
    void (*huy)(void *ctx);          /* de giai phong ngu canh */
} HamInt;

typedef struct { int a; int (*f)(int, int); } CtxApDung;

static int than_ap_dung(void *ctx, int b) {
    CtxApDung *c = ctx;

    return c->f(c->a, b);
}

static void huy_ap_dung(void *ctx) { free(ctx); }

HamInt ap_dung_tung_phan(int (*f)(int, int), int a) {
    CtxApDung *c = malloc(sizeof *c);

    if (!c) return (HamInt){ NULL, NULL, NULL };

    c->a = a;
    c->f = f;

    return (HamInt){ than_ap_dung, c, huy_ap_dung };
}

/* Dung: */
static int cong(int a, int b) { return a + b; }

HamInt cong5 = ap_dung_tung_phan(cong, 5);

printf("%d\n", cong5.goi(cong5.ctx, 10));   /* 15 */

cong5.huy(cong5.ctx);                        /* dung quen */

#define CLOSURE(fn, ...) \
    ((HamInt){ (fn), &(CtxNhan){ __VA_ARGS__ } })

int a[] = { 1, 2, 3, 4, 5 };

map_int(a, 5, CLOSURE(than_nhan, .n = 10));

/* Bung ra thanh:
     map_int(a, 5, (HamInt){ than_nhan, &(CtxNhan){ .n = 10 } });

   Doi tuong CtxNhan vo danh duoc tao ngay tai cho, va no song
   toi het KHOI LENH chua bieu thuc nay. Trong suot loi goi
   map_int thi no con song, nen an toan. */

/* MO RONG GCC, khong co trong chuan, Clang va MSVC khong ho tro */

int main(void) {
    int a[] = { 1, 2, 3 };
    int he_so = 10;

    int nhan(int x) { return x * he_so; }     /* bat duoc he_so */

    ap_dung(a, 3, nhan);

    return 0;
}

/* MO RONG CLANG: clang -fblocks, tren Linux can them -lBlocksRuntime */

#include <Block.h>

typedef int (^BlockInt)(int);

BlockInt tao_cong(int n) {
    return Block_copy(^(int x) { return x + n; });   /* chep tu stack sang heap */
}

int main(void) {
    __block int dem = 0;                  /* __block: cho phep block GHI vao */

    void (^tang)(void) = ^{ dem++; };

    tang(); tang();
    printf("%d\n", dem);                  /* 2 */

    BlockInt f = tao_cong(5);
    printf("%d\n", f(10));                 /* 15, closure song lau hon ham tao */

    Block_release(f);

    return 0;
}

/* MO RONG GNU, chay tren GCC va Clang, KHONG chay tren MSVC */

#define LAMBDA(kieu, than) ({ kieu __fn__ than __fn__; })

int (*bp)(int) = LAMBDA(int, (int x) { return x * x; });

printf("%d\n", bp(7));      /* 49 */

qsort(a, n, sizeof a[0],
      LAMBDA(int, (const void *p, const void *q) {
          int x = *(const int *)p, y = *(const int *)q;

          return (x > y) - (x < y);
      }));