在C语言中什么时候应该使用双重间接?谁能举个例子解释一下吗?
我所知道的是双间接是指向指针的指针。为什么我需要指向指针的指针?
在C语言中什么时候应该使用双重间接?谁能举个例子解释一下吗?
我所知道的是双间接是指向指针的指针。为什么我需要指向指针的指针?
当前回答
1. 基本概念-
当你申报如下:-
1. Char *ch -(称为字符指针) - ch为单个字符的地址。 - (*ch)将解引用字符的值。
2. Char **ch - 'ch'包含字符指针数组的地址。(如1) '*ch'包含单个字符的地址。(注意它与1不同,因为声明不同)。 (**ch)将解引用到字符的确切值..
添加更多指针将扩展数据类型的维度,从字符扩展到字符串,再扩展到字符串数组,等等……你可以把它和一维,二维,三维矩阵联系起来。
指针的用法取决于你如何声明它。
这是一个简单的代码..
int main()
{
char **p;
p = (char **)malloc(100);
p[0] = (char *)"Apple"; // or write *p, points to location of 'A'
p[1] = (char *)"Banana"; // or write *(p+1), points to location of 'B'
cout << *p << endl; //Prints the first pointer location until it finds '\0'
cout << **p << endl; //Prints the exact character which is being pointed
*p++; //Increments for the next string
cout << *p;
}
2. 双指针的另一个应用 (这也包括引用传递)
假设您想从函数中更新一个字符。如果你尝试以下方法:-
void func(char ch)
{
ch = 'B';
}
int main()
{
char ptr;
ptr = 'A';
printf("%c", ptr);
func(ptr);
printf("%c\n", ptr);
}
输出为AA。这是行不通的,因为您已经将“按值传递”传递给了函数。
正确的做法是-
void func( char *ptr) //Passed by Reference
{
*ptr = 'B';
}
int main()
{
char *ptr;
ptr = (char *)malloc(sizeof(char) * 1);
*ptr = 'A';
printf("%c\n", *ptr);
func(ptr);
printf("%c\n", *ptr);
}
现在扩展这个要求,更新字符串而不是字符。 为此,需要将函数中的形参作为双指针接收。
void func(char **str)
{
strcpy(str, "Second");
}
int main()
{
char **str;
// printf("%d\n", sizeof(char));
*str = (char **)malloc(sizeof(char) * 10); //Can hold 10 character pointers
int i = 0;
for(i=0;i<10;i++)
{
str = (char *)malloc(sizeof(char) * 1); //Each pointer can point to a memory of 1 character.
}
strcpy(str, "First");
printf("%s\n", str);
func(str);
printf("%s\n", str);
}
在本例中,method使用双指针作为参数来更新字符串的值。
其他回答
有点晚了,但希望这能帮助到一些人。
在C语言中,数组总是在堆栈上分配内存,因此函数不能返回 一个(非静态)数组,因为内存分配在堆栈上 当执行到达当前块的末尾时自动释放。 当你想处理二维数组时,这真的很烦人 (即矩阵),并实现一些可以改变和返回矩阵的函数。 要实现这一点,可以使用指针对指针来实现矩阵 动态分配内存:
/* Initializes a matrix */
double** init_matrix(int num_rows, int num_cols){
// Allocate memory for num_rows float-pointers
double** A = calloc(num_rows, sizeof(double*));
// return NULL if the memory couldn't allocated
if(A == NULL) return NULL;
// For each double-pointer (row) allocate memory for num_cols floats
for(int i = 0; i < num_rows; i++){
A[i] = calloc(num_cols, sizeof(double));
// return NULL if the memory couldn't allocated
// and free the already allocated memory
if(A[i] == NULL){
for(int j = 0; j < i; j++){
free(A[j]);
}
free(A);
return NULL;
}
}
return A;
}
这里有一个例子:
double** double* double
------------- ---------------------------------------------------------
A ------> | A[0] | ----> | A[0][0] | A[0][1] | A[0][2] | ........ | A[0][cols-1] |
| --------- | ---------------------------------------------------------
| A[1] | ----> | A[1][0] | A[1][1] | A[1][2] | ........ | A[1][cols-1] |
| --------- | ---------------------------------------------------------
| . | .
| . | .
| . | .
| --------- | ---------------------------------------------------------
| A[i] | ----> | A[i][0] | A[i][1] | A[i][2] | ........ | A[i][cols-1] |
| --------- | ---------------------------------------------------------
| . | .
| . | .
| . | .
| --------- | ---------------------------------------------------------
| A[rows-1] | ----> | A[rows-1][0] | A[rows-1][1] | ... | A[rows-1][cols-1] |
------------- ---------------------------------------------------------
The double-pointer-to-double-pointer A points to the first element A[0] of a memory block whose elements are double-pointers itself. You can imagine these double-pointers as the rows of the matrix. That's the reason why every double-pointer allocates memory for num_cols elements of type double. Furthermore A[i] points to the i-th row, i.e. A[i] points to A[i][0] and that's just the first double-element of the memory block for the i-th row. Finally, you can access the element in the i-th row and j-th column easily with A[i][j].
下面是一个完整的例子来演示它的用法:
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
/* Initializes a matrix */
double** init_matrix(int num_rows, int num_cols){
// Allocate memory for num_rows double-pointers
double** matrix = calloc(num_rows, sizeof(double*));
// return NULL if the memory couldn't allocated
if(matrix == NULL) return NULL;
// For each double-pointer (row) allocate memory for num_cols
// doubles
for(int i = 0; i < num_rows; i++){
matrix[i] = calloc(num_cols, sizeof(double));
// return NULL if the memory couldn't allocated
// and free the already allocated memory
if(matrix[i] == NULL){
for(int j = 0; j < i; j++){
free(matrix[j]);
}
free(matrix);
return NULL;
}
}
return matrix;
}
/* Fills the matrix with random double-numbers between -1 and 1 */
void randn_fill_matrix(double** matrix, int rows, int cols){
for (int i = 0; i < rows; ++i){
for (int j = 0; j < cols; ++j){
matrix[i][j] = (double) rand()/RAND_MAX*2.0-1.0;
}
}
}
/* Frees the memory allocated by the matrix */
void free_matrix(double** matrix, int rows, int cols){
for(int i = 0; i < rows; i++){
free(matrix[i]);
}
free(matrix);
}
/* Outputs the matrix to the console */
void print_matrix(double** matrix, int rows, int cols){
for(int i = 0; i < rows; i++){
for(int j = 0; j < cols; j++){
printf(" %- f ", matrix[i][j]);
}
printf("\n");
}
}
int main(){
srand(time(NULL));
int m = 3, n = 3;
double** A = init_matrix(m, n);
randn_fill_matrix(A, m, n);
print_matrix(A, m, n);
free_matrix(A, m, n);
return 0;
}
一个原因是你想要改变传递给函数的作为函数参数的指针的值,要做到这一点,你需要指针指向指针。
简单地说,当你想在函数调用之外保留(或保留)内存分配或分配的变化时,使用**。(因此,传递带有双指针arg的函数。)
这可能不是一个很好的例子,但会告诉你基本的用法:
#include <stdio.h>
#include <stdlib.h>
void allocate(int **p)
{
*p = (int *)malloc(sizeof(int));
}
int main()
{
int *p = NULL;
allocate(&p);
*p = 42;
printf("%d\n", *p);
free(p);
}
假设你有一个指针。取值为地址。 但现在你想更改地址。 你可以。通过执行pointer1 = pointer2,你给了pointer1一个pointer2的地址。 但是!如果在函数中执行此操作,并且希望结果在函数完成后仍然存在,则需要做一些额外的工作。你需要一个新的pointer3来指向pointer1。将指针3传递给函数。 这里有一个例子。先看看下面的输出,以便理解。
#include <stdio.h>
int main()
{
int c = 1;
int d = 2;
int e = 3;
int * a = &c;
int * b = &d;
int * f = &e;
int ** pp = &a; // pointer to pointer 'a'
printf("\n a's value: %x \n", a);
printf("\n b's value: %x \n", b);
printf("\n f's value: %x \n", f);
printf("\n can we change a?, lets see \n");
printf("\n a = b \n");
a = b;
printf("\n a's value is now: %x, same as 'b'... it seems we can, but can we do it in a function? lets see... \n", a);
printf("\n cant_change(a, f); \n");
cant_change(a, f);
printf("\n a's value is now: %x, Doh! same as 'b'... that function tricked us. \n", a);
printf("\n NOW! lets see if a pointer to a pointer solution can help us... remember that 'pp' point to 'a' \n");
printf("\n change(pp, f); \n");
change(pp, f);
printf("\n a's value is now: %x, YEAH! same as 'f'... that function ROCKS!!!. \n", a);
return 0;
}
void cant_change(int * x, int * z){
x = z;
printf("\n ----> value of 'a' is: %x inside function, same as 'f', BUT will it be the same outside of this function? lets see\n", x);
}
void change(int ** x, int * z){
*x = z;
printf("\n ----> value of 'a' is: %x inside function, same as 'f', BUT will it be the same outside of this function? lets see\n", *x);
}
以下是输出:(先阅读这个)
a's value: bf94c204
b's value: bf94c208
f's value: bf94c20c
can we change a?, lets see
a = b
a's value is now: bf94c208, same as 'b'... it seems we can, but can we do it in a function? lets see...
cant_change(a, f);
----> value of 'a' is: bf94c20c inside function, same as 'f', BUT will it be the same outside of this function? lets see
a's value is now: bf94c208, Doh! same as 'b'... that function tricked us.
NOW! lets see if a pointer to a pointer solution can help us... remember that 'pp' point to 'a'
change(pp, f);
----> value of 'a' is: bf94c20c inside function, same as 'f', BUT will it be the same outside of this function? lets see
a's value is now: bf94c20c, YEAH! same as 'f'... that function ROCKS!!!.
字符串是使用双指针的一个很好的例子。字符串本身是一个指针,所以任何时候你需要指向一个字符串,你就需要一个双指针。
我今天看到了一个很好的例子,从这篇博客文章,我总结如下。
假设您有一个链表中节点的结构,可能是这样
typedef struct node
{
struct node * next;
....
} node;
现在您想实现一个remove_if函数,它接受删除条件rm作为参数之一,并遍历链表:如果一个条目满足该条件(例如rm(entry)==true),它的节点将从链表中删除。最后,remove_if返回链表的头(可能与原始头不同)。
你可以写信
for (node * prev = NULL, * curr = head; curr != NULL; )
{
node * const next = curr->next;
if (rm(curr))
{
if (prev) // the node to be removed is not the head
prev->next = next;
else // remove the head
head = next;
free(curr);
}
else
prev = curr;
curr = next;
}
就像你的for循环。这里的信息是,如果没有双指针,您必须维护一个prev变量来重新组织指针,并处理两种不同的情况。
但是使用双指针,你实际上可以写
// now head is a double pointer
for (node** curr = head; *curr; )
{
node * entry = *curr;
if (rm(entry))
{
*curr = entry->next;
free(entry);
}
else
curr = &entry->next;
}
你现在不需要一个prev,因为你可以直接修改什么prev->next指向。
为了使事情更清楚,让我们稍微跟随一下代码。拆卸过程中:
如果entry ==* head:它将是*head (==*curr) =* head->next - head现在指向新标题节点的指针。您可以通过直接将head的内容更改为一个新的指针来实现这一点。 如果entry != *head:类似地,*curr是prev->next所指向的,现在指向entry->next。
无论哪种情况,您都可以使用双指针以统一的方式重新组织指针。