在Objective-C中有两个星号**是什么意思？

Question

我明白有一个星号*是指针,有两个**是什么意思？

我从文档中偶然发现了这个:

- (NSAppleEventDescriptor *)executeAndReturnError:(NSDictionary **)errorInfo

Answer 1

它是一个指向指针的指针,就像在C中一样(尽管它采用奇怪的方括号语法,Objective-C基于):

char c;
char *pc = &c;
char **ppc = &pc;
char ***pppc = &ppc;

等等,无限期(或直到你用完可变空间).

它通常用于将指针传递给必须能够更改指针本身的函数(例如为可变大小的对象重新分配内存).

=====

根据您的示例请求显示如何使用它,这里是我为另一篇文章编写的一些代码,用于说明它.它是一个appendStr()管理自己分配的功能(你仍然需要释放最终版本).最初将string(char *)设置为NULL,函数本身将根据需要分配空间.

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

void appendToStr (int *sz, char **str, char *app) {
    char *newstr;
    int reqsz;

    /* If no string yet, create it with a bit of space. */

    if (*str == NULL) {
        *sz = strlen (app) + 10;
        if ((*str = malloc (*sz)) == NULL) {
            *sz = 0;
            return;
        }
        strcpy (*str, app);
        return;
    }

 

    /* If not enough room in string, expand it. We could use realloc
       but I've kept it as malloc/cpy/free to ensure the address
       changes (for the program output). */

    reqsz = strlen (*str) + strlen (app) + 1;
    if (reqsz > *sz) {
        *sz = reqsz + 10;
        if ((newstr = malloc (*sz)) == NULL) {
            free (*str);
            *str = NULL;
            *sz = 0;
            return;
        }
        strcpy (newstr, *str);
        free (*str);
        *str = newstr;
    }

    /* Append the desired string to the (now) long-enough buffer. */

    strcat (*str, app);
}

 

static void dump(int sz, char *x) {
    if (x == NULL)
        printf ("%8p   [%2d]   %3d   [%s]\n", x, sz, 0, "");
    else
        printf ("%8p   [%2d]   %3d   [%s]\n", x, sz, strlen (x), x);
}

static char *arr[] = {"Hello.", " My", " name", " is", " Pax",
                      " and"," I", " am", " old."};

int main (void) {
    int i;
    char *x = NULL;
    int sz = 0;

    printf (" Pointer   Size   Len   Value\n");
    printf (" -------   ----   ---   -----\n");
    dump (sz, x);
    for (i = 0; i < sizeof (arr) / sizeof (arr[0]); i++) {
        appendToStr (&sz, &x, arr[i]);
        dump (sz, x);
    }
}

代码输出以下内容.当前分配的内存用尽扩展字符串的空间时(注释处),您可以看到指针如何变化:

 Pointer   Size   Len   Value
 -------   ----   ---   -----
# NULL pointer here since we've not yet put anything in.
     0x0   [ 0]     0   []

# The first time we put in something, we allocate space (+10 chars).
0x6701b8   [16]     6   [Hello.]
0x6701b8   [16]     9   [Hello. My]
0x6701b8   [16]    14   [Hello. My name]

# Adding " is" takes length to 17 so we need more space.
0x6701d0   [28]    17   [Hello. My name is]
0x6701d0   [28]    21   [Hello. My name is Pax]
0x6701d0   [28]    25   [Hello. My name is Pax and]
0x6701d0   [28]    27   [Hello. My name is Pax and I]

# Ditto for adding " am".
0x6701f0   [41]    30   [Hello. My name is Pax and I am]
0x6701f0   [41]    35   [Hello. My name is Pax and I am old.]

在这种情况下,您传入,**str因为您需要能够更改*str值.

=====

或者以下,对不在数组中的字符串执行展开的冒泡排序(哦,耻辱!).它通过直接交换字符串的地址来实现.

#include <stdio.h>

static void sort (char **s1, char **s2, char **s3, char **s4, char **s5) {
    char *t;

    if (strcmp (*s1, *s2) > 0) { t = *s1; *s1 = *s2; *s2 = t; }
    if (strcmp (*s2, *s3) > 0) { t = *s2; *s2 = *s3; *s3 = t; }
    if (strcmp (*s3, *s4) > 0) { t = *s3; *s3 = *s4; *s4 = t; }
    if (strcmp (*s4, *s5) > 0) { t = *s4; *s4 = *s5; *s5 = t; }

    if (strcmp (*s1, *s2) > 0) { t = *s1; *s1 = *s2; *s2 = t; }
    if (strcmp (*s2, *s3) > 0) { t = *s2; *s2 = *s3; *s3 = t; }
    if (strcmp (*s3, *s4) > 0) { t = *s3; *s3 = *s4; *s4 = t; }

    if (strcmp (*s1, *s2) > 0) { t = *s1; *s1 = *s2; *s2 = t; }
    if (strcmp (*s2, *s3) > 0) { t = *s2; *s2 = *s3; *s3 = t; }

    if (strcmp (*s1, *s2) > 0) { t = *s1; *s1 = *s2; *s2 = t; }
}

int main (int argCount, char *argVar[]) {
    char *a = "77";
    char *b = "55";
    char *c = "99";
    char *d = "88";
    char *e = "66";

    printf ("Unsorted: [%s] [%s] [%s] [%s] [%s]\n", a, b, c, d, e);
    sort (&a,&b,&c,&d,&e);
    printf ("  Sorted: [%s] [%s] [%s] [%s] [%s]\n", a, b, c, d, e);
    return 0;
}

产生:

Unsorted: [77] [55] [99] [88] [66]
  Sorted: [55] [66] [77] [88] [99]

不要介意排序的实现,只需注意变量的传递方式,char **以便它们可以轻松交换.任何真正的排序都可能是作用于真正的数据数组而不是单个变量,但这不是示例的重点.