在Objective-C中有两个星号**是什么意思? [英] What does having two asterisk ** in Objective-C mean?
问题描述
我知道有一个星号*是一个指针，有两个**是什么意思?
I understand having one asterisk * is a pointer, what does having two ** mean?
我从文档中偶然发现了这一点:
I stumble upon this from the documentation:
- (NSAppleEventDescriptor *)executeAndReturnError:(NSDictionary **)errorInfo
推荐答案
它是指向指针的指针，就像在C语言中一样(尽管它的方括号语法很奇怪，但Objective-C却是基于它的):
It's a pointer to a pointer, just like in C (which, despite its strange square-bracket syntax, Objective-C is based on):
char c;
char *pc = &c;
char **ppc = &pc;
char ***pppc = &ppc;
以此类推，然后是无限的(或者直到您用完可变空间为止).
and so on, ad infinitum (or until you run out of variable space).
通常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).
It's often used to pass a pointer to a function that must be able to change the pointer itself (such as re-allocating memory for a variable-sized object).
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在您请求一个示例来说明如何使用它之后，这是我为另一篇文章编写的一些代码来说明它.这是一个appendStr()函数，它管理自己的分配(您仍然必须释放最终版本).最初，您将字符串(char *)设置为NULL，并且函数本身将根据需要分配空间.
Following your request for a sample that shows how to use it, here's some code I wrote for another post which illustrates it. It's an appendStr() function which manages its own allocations (you still have to free the final version). Initially you set the string (char *) to NULL and the function itself will allocate space as needed.
#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.您可以看到当前分配的内存用完扩展字符串的空间时指针如何变化(在注释处):
The code outputs the following. You can see how the pointer changes when the currently allocated memory runs out of space for the expanded string (at the comments):
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，您需要输入**str，因为您需要能够更改*str值.
In that case, you pass in **str since you need to be able to change the *str value.
=====
或以下命令，它对不在数组中的字符串进行展开的气泡排序(哦，太可惜了！).它是通过直接交换字符串的地址来实现的.
Or the following, which does an unrolled bubble sort (oh, the shame!) on strings that aren't in an array. It does this by directly exchanging the addresses of the strings.
#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]
不用管排序的实现，只需要注意变量作为char **传递即可轻松进行交换.任何真正的排序都可能会作用于真正的数据数组上，而不是单个变量上，但这不是示例的重点.
Never mind the implementation of sort, just notice that the variables are passed as char ** so that they can be swapped easily. Any real sort would probably be acting on a true array of data rather than individual variables but that's not the point of the example.
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