hello , 今天给大家分享一道简单C语言试题，温故知新。
问题：给定两个指针p1 和 p2（不为空）, p1 = p2 和 *p1 = *p2 是一样的吗？
考察知识点：指针理解、左值、右值。
思路
对于这道题，容易出现误区：看表象，两者输出结果一致，认为是一样的操作。
p1 = p2：很好理解，将p1 指向p2,即p1 和 p2 同时指向p2地址处，覆盖了原来p1的值。
*p1 = *p2：表示将p2 指向的内容 赋值给p1 指向的内容，指针本身的地址不变。
p1 = p2
源代码：改变的是地址
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
typedef struct _student {
int id;
int age;
char name[32];
float score;
} student_t;
static void dump_info(student_t *pstu)
{
printf("INFO(%s): student->id = %d\n", __func__, pstu->id);
printf("INFO(%s): student->age = %d\n", __func__, pstu->age);
printf("INFO(%s): student->name = %s\n", __func__, pstu->name);
printf("INFO(%s): student->score = %f\n", __func__, pstu->score);
}
int main()
{
student_t *p1 = NULL;
student_t *p2 = NULL;
p1 = (student_t*)malloc(sizeof(student_t));
if (!p1) {
printf("ERROR(%s):malloc for p1 error\n", __func__);
return -1;
}
p2 = (student_t*)malloc(sizeof(student_t));
if (!p1) {
printf("ERROR(%s):malloc for p1 error\n", __func__);
return -1;
}
p1->id = 1;
p1->age = 21;
strcpy(p1->name, "stu1");
p1->score = 90.0;
p2->id = 2;
p2->age = 25;
strcpy(p1->name, "stu2");
p2->score = 100.0;
printf("INFO(%s): $$$$$$$$$$$ before $$$$$$$$$$ \n", __func__);
printf("INFO(%s): p1 = %p \n",__func__, p1);
printf("INFO(%s): p2 = %p \n",__func__, p2);
dump_info(p1);
dump_info(p2);
p1 = p2;
printf("INFO(%s): $$$$$$$$$$$ p1 = p2 $$$$$$$$$$ \n", __func__);
printf("INFO(%s): p1 = %p \n",__func__, p1);
printf("INFO(%s): p2 = %p \n",__func__, p2);
dump_info(p1);
dump_info(p2);
return 0;
}
输出结果：p1 的地址被覆盖了，变成p2.
INFO(main): $$$$$$$$$$$ before $$$$$$$$$$
INFO(main): p1 = 0xd65010
INFO(main): p2 = 0xd65050
INFO(dump_info): student->id = 1
INFO(dump_info): student->age = 21
INFO(dump_info): student->name = stu2
INFO(dump_info): student->score = 90.000000
INFO(dump_info): student->id = 2
INFO(dump_info): student->age = 25
INFO(dump_info): student->name =
INFO(dump_info): student->score = 100.000000
INFO(main): $$$$$$$$$$$ p1 = p2 $$$$$$$$$$
INFO(main): p1 = 0xd65050
INFO(main): p2 = 0xd65050
INFO(dump_info): student->id = 2
INFO(dump_info): student->age = 25
INFO(dump_info): student->name =
INFO(dump_info): student->score = 100.000000
INFO(dump_info): student->id = 2
INFO(dump_info): student->age = 25
INFO(dump_info): student->name =
INFO(dump_info): student->score = 100.000000
*p1 = *p2
源代码：改变的是指向的内容
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
typedef struct _student {
int id;
int age;
char name[32];
float score;
} student_t;
static void dump_info(student_t *pstu)
{
printf("INFO(%s): student->id = %d\n", __func__, pstu->id);
printf("INFO(%s): student->age = %d\n", __func__, pstu->age);
printf("INFO(%s): student->name = %s\n", __func__, pstu->name);
printf("INFO(%s): student->score = %f\n", __func__, pstu->score);
}
int main()
{
student_t *p1 = NULL;
student_t *p2 = NULL;
p1 = (student_t*)malloc(sizeof(student_t));
if (!p1) {
printf("ERROR(%s):malloc for p1 error\n", __func__);
return -1;
}
p2 = (student_t*)malloc(sizeof(student_t));
if (!p1) {
printf("ERROR(%s):malloc for p1 error\n", __func__);
return -1;
}
p1->id = 1;
p1->age = 21;
strcpy(p1->name, "stu1");
p1->score = 90.0;
p2->id = 2;
p2->age = 25;
strcpy(p1->name, "stu2");
p2->score = 100.0;
printf("INFO(%s): $$$$$$$$$$$ before $$$$$$$$$$ \n", __func__);
printf("INFO(%s): p1 = %p \n",__func__, p1);
printf("INFO(%s): p2 = %p \n",__func__, p2);
dump_info(p1);
dump_info(p2);
*p1 = *p2;
printf("INFO(%s): $$$$$$$$$$$ p1 = p2 $$$$$$$$$$ \n", __func__);
printf("INFO(%s): p1 = %p \n",__func__, p1);
printf("INFO(%s): p2 = %p \n",__func__, p2);
dump_info(p1);
dump_info(p2);
return 0;
}
输出结果：p1 和 p2 的地址不变，p1 指向的内容被覆盖了
INFO(main): $$$$$$$$$$$ before $$$$$$$$$$
INFO(main): p1 = 0x20d2010
INFO(main): p2 = 0x20d2050
INFO(dump_info): student->id = 1
INFO(dump_info): student->age = 21
INFO(dump_info): student->name = stu2
INFO(dump_info): student->score = 90.000000
INFO(dump_info): student->id = 2
INFO(dump_info): student->age = 25
INFO(dump_info): student->name =
INFO(dump_info): student->score = 100.000000
INFO(main): $$$$$$$$$$$ p1 = p2 $$$$$$$$$$
INFO(main): p1 = 0x20d2010
INFO(main): p2 = 0x20d2050
INFO(dump_info): student->id = 2
INFO(dump_info): student->age = 25
INFO(dump_info): student->name =
INFO(dump_info): student->score = 100.000000
INFO(dump_info): student->id = 2
INFO(dump_info): student->age = 25
INFO(dump_info): student->name =
INFO(dump_info): student->score = 100.000000
总结
本文简单温故一下知识的用法。对于p1 = p2 和 *p1 = *p2,只需要把握变化的地方就行。
- p1 = p2: 改变的是指针地址。所以指向内容也跟着变了。
-
*p1 = *p2：改变的是指针指向的内容，指针本身地址不变。
所以两者结果虽一致，但本身操作并不一致。
