C语言题解：p1 = p2 与 *p1 = *p2

2023-05-07 261

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简介： C语言题解：p1 = p2 与 *p1 = *p2

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