c - 指针所需的基本帮助(双重间接)

Question

我前段时间问过一个我不记得如何操作基本指针的帐户，有人给了我一个非常好的演示

例如

char *ptr = "hello" (hello = a char array)

所以现在 *ptr 指向 'h'

ptr++ 意味着移动 ptr 指向下一个元素，为了获得它的值，我执行 *ptr 并且这给了我 e

---

到目前为止一切正常，我希望 :D，但现在我需要操作一个 char **ptr 并且想知道我如何以模仿二维数组效果的方式做到这一点？

一些基本技巧将不胜感激，因为我需要做一个具有 **ptr 的作业来模仿二维数组，并且不知道它是如何做到的，这意味着我什至无法解决它在纸上(例如，如何取消引用 **ptr，如何获取 [x][y] 值等)

谢谢

Answer 1

如果所有地址都已正确设置，您可以像为数组下标一样为指针下标。

假设如下声明:

char **ptr;

这里是各种表达式的类型:

       Expression        Type      Equivalent expressions (yield same value)     
       ----------        ----      -----------------------------------------
              ptr        char **   &ptr[0]
             *ptr        char *    ptr[0] 
         *(ptr+i)        char *    ptr[i]; &ptr[i][0]
            **ptr        char      ptr[0][0]
      *(*(ptr+i))        char      ptr[i][0]; *ptr[i]
    *(*(ptr+i)+j)        char      ptr[i][j]

thus:

• ptr can be treated as though it was an array of strings (2-d array of char)
• ptr[i] points to the beginning of the i'th string in the list
• ptr[i][j] is the value of the j'th character of the i'th string in the list
• The expressions ptr++ and ++ptr will advance ptr to point to the next string
• The expressions (*ptr)++ and ++(*ptr) will advance *ptr to point to the next character

As for setting up your pointers, this arrangement assumes everything has already been allocated as static arrays or dynamically through malloc. You cannot just write

char **ptr = {"foo", "bar", "bletch"}; // using aggregate initializer on 
                                       // non-aggregate type; bad juju,
                                       // a.k.a undefined behavior

或

char **ptr;          // ptr has not been initialized to point anywhere 
ptr[0] = "foo";      // dereferencing ptr via subscript invokes undefined
ptr[1] = "bar";      // behavior
ptr[2] = "bletch";

通常，当您像使用数组一样使用指针时，您将使用 malloc 或类似的东西来分配缓冲区:

char **ptr = malloc(sizeof *ptr * N);
if (ptr)
{
  ptr[0] = "foo";    // ptr[i] gets address of
  ptr[1] = "bar";    // string literal
  ptr[2] = "bletch";
  ...
}

或

char **ptr = malloc(sizeof *ptr * N);
if (ptr)
{
  size_t i;
  for (i = 0; i < N; i++)
  {
    ptr[i] = malloc(sizeof *ptr[i] * M); // strictly speaking, the sizeof
    if (ptr[i])                          // is not necessary here
    {
      //initialize ptr[i]
    }
  }
}