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copied!<p>It's a pointer to a pointer, just like in C (which, despite its strange square-bracket syntax, Objective-C is based on):</p> <pre><code>char c; char *pc = &c; char **ppc = &pc; char ***pppc = &ppc; </code></pre> <p>and so on, ad infinitum (or until you run out of variable space).</p> <p>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).</p> <p>=====</p> <p>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 <code>appendStr()</code> function which manages its own allocations (you still have to free the final version). Initially you set the string (<code>char *</code>) to NULL and the function itself will allocate space as needed.</p> <pre><code>#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; } </code></pre> <p> </p> <pre><code> /* 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); } </code></pre> <p> </p> <pre><code>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); } } </code></pre> <p>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):</p> <pre><code> 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.] </code></pre> <p>In that case, you pass in <code>**str</code> since you need to be able to change the <code>*str</code> value.</p> <p>=====</p> <p>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.</p> <pre><code>#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; } </code></pre> <p>which produces:</p> <pre><code>Unsorted: [77] [55] [99] [88] [66] Sorted: [55] [66] [77] [88] [99] </code></pre> <p>Never mind the implementation of sort, just notice that the variables are passed as <code>char **</code> 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.</p>

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