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
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);
}
其他回答
下面是一个非常简单的c++示例,说明如果要使用函数将指针设置为指向对象,则需要一个指针指向指针。否则,指针将继续返回null。
(一个c++的答案,但我相信在C中也是一样的)
(同样,供参考:谷歌("pass by value c++") = "默认情况下,c++中的参数是按值传递的。当实参按值传递时,实参的值被复制到函数的形参中。”)
我们想让指针b等于字符串a。
#include <iostream>
#include <string>
void Function_1(std::string* a, std::string* b) {
b = a;
std::cout << (b == nullptr); // False
}
void Function_2(std::string* a, std::string** b) {
*b = a;
std::cout << (b == nullptr); // False
}
int main() {
std::string a("Hello!");
std::string* b(nullptr);
std::cout << (b == nullptr); // True
Function_1(&a, b);
std::cout << (b == nullptr); // True
Function_2(&a, &b);
std::cout << (b == nullptr); // False
}
// Output: 10100
在Function_1(&a, b);这条线上会发生什么?
The "value" of &main::a (an address) is copied into the parameter std::string* Function_1::a. Therefore Function_1::a is a pointer to (i.e. the memory address of) the string main::a. The "value" of main::b (an address in memory) is copied into the parameter std::string* Function_1::b. Therefore there are now 2 of these addresses in memory, both null pointers. At the line b = a;, the local variable Function_1::b is then changed to equal Function_1::a (= &main::a), but the variable main::b is unchanged. After the call to Function_1, main::b is still a null pointer.
在函数_2(&a, &b);这一行发生了什么?
The treatment of the a variable is the same: within the function, Function_2::a is the address of the string main::a. But the variable b is now being passed as a pointer to a pointer. The "value" of &main::b (the address of the pointer main::b) is copied into std::string** Function_2::b. Therefore within Function_2, dereferencing this as *Function_2::b will access and modify main::b . So the line *b = a; is actually setting main::b (an address) equal to Function_2::a (= address of main::a) which is what we want.
如果你想用一个函数来修改一个东西,无论是一个对象还是一个地址(指针),你必须传递一个指向那个东西的指针。您实际传入的内容不能被修改(在调用范围内),因为创建了本地副本。
(一个例外是如果形参是一个引用,例如std::string& a.但通常这些是const。一般来说,如果你调用f(x),如果x是一个对象,你应该能够假设f不会修改x。但如果x是一个指针,那么你应该假设f可能修改x指向的对象。)
例如,你可能想要确保当你释放某个东西的内存时,你将指针设置为空。
void safeFree(void** memory) {
if (*memory) {
free(*memory);
*memory = NULL;
}
}
当你调用这个函数时,你会用指针的地址来调用它
void* myMemory = someCrazyFunctionThatAllocatesMemory();
safeFree(&myMemory);
现在myMemory被设置为NULL,任何重用它的尝试都将是非常明显的错误。
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。
为什么是双指针?
目标是使用函数改变studentA所指向的对象。
#include <stdio.h>
#include <stdlib.h>
typedef struct Person{
char * name;
} Person;
/**
* we need a ponter to a pointer, example: &studentA
*/
void change(Person ** x, Person * y){
*x = y; // since x is a pointer to a pointer, we access its value: a pointer to a Person struct.
}
void dontChange(Person * x, Person * y){
x = y;
}
int main()
{
Person * studentA = (Person *)malloc(sizeof(Person));
studentA->name = "brian";
Person * studentB = (Person *)malloc(sizeof(Person));
studentB->name = "erich";
/**
* we could have done the job as simple as this!
* but we need more work if we want to use a function to do the job!
*/
// studentA = studentB;
printf("1. studentA = %s (not changed)\n", studentA->name);
dontChange(studentA, studentB);
printf("2. studentA = %s (not changed)\n", studentA->name);
change(&studentA, studentB);
printf("3. studentA = %s (changed!)\n", studentA->name);
return 0;
}
/**
* OUTPUT:
* 1. studentA = brian (not changed)
* 2. studentA = brian (not changed)
* 3. studentA = erich (changed!)
*/
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;
}