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While pointers simplify the performance of some C programming tasks, others, such as dynamic memory allocation, cannot be performed without pointers.
Therefore, if you want to become an excellent C programmer, it is necessary to learn pointers.
1. What is a pointer?
A pointer is a variable that stores a memory address.
This memory address points to the location of another variable, enabling the program to access and manipulate the value of that variable.
Every variable has a memory address, and the memory address is 可用 & 运算符取到内存地址.
Pointer declarations use the * symbol
// p 是一个指针变量，它存储了一个地址
// *p 是指针 p 所指向的地址上存储的值
int *p;
In this statement, the variable p is a pointer of integer type. You can use the & symbol to get the address of a variable.
int n = 10;
int *p = &n;
In this statement, &n gets the address of variable n and assigns the address to pointer p. Through the pointer p, the value of the variable n can be accessed and modified.
*p = 20;
printf("%d", n); // 输出20
*In this statement, p represents the value stored in the memory address pointed to by the pointer p. After assigning *p with a value of 20, the value of variable n is also changed to 20.
Second, how to use the pointer?
Define a pointer variable;
assign the variable address to the pointer;
access the value of the address available in the pointer variable.
These are returned by using the unary operator * to return the value of the variable at the address specified by the operand.
1. Arithmetic operations on pointers
- Each time the pointer
递增points to下一个the storage unit of the element. - Each time the pointer
递减points to前一个the storage unit of the element. - The number of bytes that the pointer jumps when incrementing and decrementing depends on
指针变量数据类型长度, for example, an int is 4 bytes.
increment a pointer
#include <stdio.h>
const int MAX = 3;
int main ()
{
int var[] = {
10, 100, 200};
int i, *ptr;
/* 指针中的数组地址 */
ptr = var;
for ( i = 0; i < MAX; i++)
{
printf("存储地址：var[%d] = %p\n", i, ptr );
printf("存储值：var[%d] = %d\n", i, *ptr );
/* 指向下一个位置 */
ptr++;
}
return 0;
}
// 存储地址：var[0] = e4a298cc
// 存储值：var[0] = 10
// 存储地址：var[1] = e4a298d0
// 存储值：var[1] = 100
// 存储地址：var[2] = e4a298d4
// 存储值：var[2] = 200
Comparison of pointers
As long as the address pointed to by the variable pointer is less than or equal to the address &var[MAX - 1] of the last element of the array, the variable pointer is incremented.
#include <stdio.h>
const int MAX = 3;
int main ()
{
int var[] = {
10, 100, 200};
int i, *ptr;
/* 指针中第一个元素的地址*/
ptr = &var;
i = 0;
while ( ptr <= &var[MAX - 1] )
{
printf("存储地址：var[%d] = %p\n", i, ptr );
printf("存储值：var[%d] = %d\n", i, *ptr );
/* 指向下一个位置 */
ptr++;
i++;
}
return 0;
}
2. Array of pointers
ptr is declared as an array consisting of MAX integer pointers.
Therefore, each element in ptr is a pointer to an int value.
int *ptr[MAX];
The following example uses three integers, which will be stored in an array of pointers, as follows:
#include <stdio.h>
const int MAX = 3;
int main ()
{
int var[] = {
10, 100, 200};
int i, *ptr[MAX];
for ( i = 0; i < MAX; i++)
{
ptr[i] = &var[i]; /* 赋值为整数的地址 */
}
return 0;
}
3. Pointer to Pointer
A pointer to a pointer is a form of multi-level indirection. When we define a pointer to pointer, the first pointer contains the address of the second pointer, which points to the location containing the actual value.
Pointer to pointer of type int:
int a = 100;
int *p1 = &a;
int **p2 = &p1;
The C language does not limit the number of levels of pointers. For each additional level of pointers, an asterisk * must be added when defining pointer variables.
p1 is a first-level pointer, pointing to common type data, with one * when defined;
p2 is a second-level pointer, pointing to first-level pointer p1, with two * when defined.
4. Pass the pointer to the function
#include <stdio.h>
void increment(int *ptr) {
(*ptr)++;
}
int main() {
int num = 5;
printf("Before increment: %d\n", num);
increment(&num);
printf("After increment: %d\n", num);
return 0;
}
5. Return pointer from function
// todo