二级指针用法示例

1、二级指针的第一种内存模型：指针数组

#include <stdlib.h>#include <string.h>#include <stdio.h>void main21()    //不适用二级指针的常规做法{inti = 0, j = 0;intnum = 0;char*tmp = NULL;//数组 数组中的每一个元素是指针 指针数组char *myArray[] = {"aaaaaa", "ccccc", "bbbbbb", "111111"};//打印num = sizeof(myArray)/sizeof(myArray[0]);printf("排序之前\n");for (i=0; i<num; i++){//printf("%s \n", myArray[i]);printf("%s \n", *(myArray+i) );}//排序for (i=0; i<num; i++){for (j=i; j<num; j++){if (strcmp(myArray[i], myArray[j]) > 0 ){tmp = myArray[i];  //注意  交换的是数组元素 交换的是指针的值myArray[i] = myArray[j];myArray[j] = tmp;}}}printf("排序之后\n");for (i=0; i<num; i++){//printf("%s \n", myArray[i]);printf("%s \n", *(myArray+i) );}printf("hello...\n");system("pause");return ;}void printMyArray11(char **myArray, int num)//数组中的每一个元素是指针(字符串常量的首地址)，所以使用的是二级指针 {int i = 0;for (i=0; i<num; i++){//printf("%s \n", myArray[i]);printf("%s \n", *(myArray+i) );}}void sortMyArray11(char **myArray, int num)//数组中的每一个元素是指针(字符串常量的首地址) ，所以使用的是二级指针{int i =0 , j = 0;char *tmp = NULL;//排序for (i=0; i<num; i++){for (j=i; j<num; j++){if (strcmp(myArray[i], myArray[j]) > 0 ){tmp = myArray[i];  //注意：交换的是数组元素(字符串常量的首地址)，交换的是指针的值                    //改变的是指针的指向myArray[i] = myArray[j];myArray[j] = tmp;}}}}void main(){inti = 0, j = 0;intnum = 0;char*tmp = NULL;//数组 数组中的每一个元素是指针(字符串常量的首地址) 指针数组char *myArray[] = {"aaaaaa", "ccccc", "bbbbbb", "111111"};//打印num = sizeof(myArray)/sizeof(myArray[0]);printf("排序之前\n");printMyArray11(myArray, num);sortMyArray11(myArray, num);printf("排序之后\n");printMyArray11(myArray, num);printf("hello...\n");system("pause");return ;}

2、二级指针的第二种内存模型：二维数组

#include <stdlib.h>#include <string.h>#include <stdio.h>//打印  排序 //封装成函数void main31(){int i = 0, j = 0;int num = 4;char myBuf[30];char tmpBuf[30];char myArray[10][30] = {"aaaaaa", "ccccc", "bbbbbbb", "1111111111111"};//打印 printf("排序之前\n");for (i=0; i<num; i++){printf("%s\n", myArray[i]);}for (i=0; i<num; i++){for (j=i+1; j<num; j++){if (strcmp (myArray[i],  myArray[j]) > 0){strcpy(tmpBuf, myArray[i]);  //交换的是内存块strcpy(myArray[i], myArray[j]);strcpy(myArray[j], tmpBuf);}}}//打印 printf("排序之后\n");for (i=0; i<num; i++){printf("%s\n", myArray[i]);}printf("hello...\n");system("pause");return ;}//问题的本质是：dm03_二级指针做输入_第2种内存模型(二维数组) 的 myArray + 1             // dm03_二级指针做输入_第1种内存模型(指针数组)   myArray + 1 不一样 ；//指针的步长不一样  指针所指向的内存空间的数据类不一样 。void printMyArray02_err(char **myArray, int num)//错误打印模型，涉及到多维数组退化为指针的细节{int i = 0;for (i=0; i<num; i++){//printf("%s \n", myArray[i]);printf("%s \n", *(myArray+i) );  //见问题本质}}void printMyArray02(char myArray[10][30], int num)//正确打印模型{int i = 0;for (i=0; i<num; i++){//printf("%s \n", myArray[i]);printf("%s \n", *(myArray+i) );  //见问题本质}}//交换的是内存块!void sortMyArray02(char myArray[10][30], int num){int i, j = 0;char tmpBuf[30];for (i=0; i<num; i++){for (j=i+1; j<num; j++){if (strcmp (myArray[i],  myArray[j]) > 0){strcpy(tmpBuf, myArray[i]);  //交换的是内存块strcpy(myArray[i], myArray[j]);strcpy(myArray[j], tmpBuf);}}}}//打印  排序 //封装成函数void main333(){int i = 0, j = 0;int num = 4;char myBuf[30];char tmpBuf[30];char myArray[10][30] = {"aaaaaa", "ccccc", "bbbbbbb", "1111111111111"};//myarray：编译器只会关心：有10行 ，每行30列。目的是：//myarray+1：多维数组名的本质，+1以后步长是多少，跳30个单元！{int len1 = sizeof(myarray);   // 300int len2 = sizeof(myarray[0]);//30 int size = len1/len2;         //10 ，实际上求出来的是多少行，和一维对比printf("len1:%d , len2:%d , size:%d \n", len1, len2, size);}//打印 printf("排序之前\n");printMyArray02(myArray, num);sortMyArray02(myArray, num);//打印 printf("排序之后\n");printMyArray02(myArray, num);printf("hello...\n");system("pause");return ;}

3、二级指针的第三种内存模型：扔开C编译器，手工打造二维内存

#include <stdlib.h>#include <string.h>#include <stdio.h>//直接写void main41(){int i = 0, j = 0;char **p2 = NULL;int num = 5;char *tmp = NULL;//辅助下面交换指针char tmpbuf[100];//辅助下面交换内存//自己打造内存p2 = (char **)malloc(sizeof(char *) * num);//char **里面装char *for (i=0; i<num; i++){p2[i] = (char *)malloc(sizeof(char)  * 100  ); //char buf[100];       //char *里面装char数据sprintf(p2[i], "%d%d%d", i+1, i+1, i+1);       //以"%d%d%d"的格式打印字符串到p2}//排序之前 printf("排序之前：\n");for (i=0; i<num; i++){printf("%s \n", p2[i]);}//排序 交换的是：指针/*for (i=0; i<num; i++){for (j=i+1; j<num; j++){if (strcmp( p2[i] , p2[j]) < 0){tmp = p2[i];p2[i] = p2[j];p2[j] = tmp;}}}*///排序 交换的是：内存for (i=0; i<num; i++){for (j=i+1; j<num; j++){if (strcmp( p2[i] , p2[j]) < 0){strcpy(tmpbuf, p2[i]);strcpy( p2[i], p2[j]);strcpy( p2[j], tmpbuf);}}}//排序之后 printf("排序之后：\n");for (i=0; i<num; i++){printf("%s \n", p2[i]);}//释放内存for(i=0; i<num; i++){if (p2[i] != NULL){free(p2[i]);p2[i] = NULL;}}if (p2!=NULL) {free(p2);}printf("hello...\n");system("pause");return ;}//改写、封装//分配内存char **getMem41(int num){int i = 0;char **p2 = NULL;p2 = (char **)malloc(sizeof(char *) * num);if (p2 == NULL){return NULL;}for (i=0; i<num; i++){p2[i] = (char *)malloc(sizeof(char)  * 100  ); //char buf[100];sprintf(p2[i], "%d%d%d", i+1, i+1, i+1);}return p2;}//打印void printMyArray03(char **myArray, int num)    //二级指针{int i = 0;for (i=0; i<num; i++){//printf("%s \n", myArray[i]);printf("%s \n", *(myArray+i) );}}//排序void sortMyArray03(char **myArray, int num)     //二级指针{int i =0 , j = 0;char *tmp = NULL;//排序for (i=0; i<num; i++){for (j=i; j<num; j++){if (strcmp(myArray[i], myArray[j]) > 0 ){tmp = myArray[i];  //注意  交换的是数组元素 交换的是指针的值 //改变指针的指向myArray[i] = myArray[j];myArray[j] = tmp;}}}}//释放内存void getMem41_Free(char **p2, int num)    //二级指针{int i = 0;//释放内存for(i=0; i<num; i++){if (p2[i] != NULL){free(p2[i]);p2[i] = NULL;}}if (p2!=NULL) {free(p2);}}void main(){int i = 0, j = 0;char **p2 = NULL;int num = 5;char *tmp = NULL;char tmpbuf[100];p2 = getMem41(num);//排序之前 printf("排序之前\n");printMyArray03(p2, num);sortMyArray03(p2, num);//排序之前 printf("排序之后\n");printMyArray03(p2, num);getMem41_Free(p2,  num); //p2是一个野指针return ;}

4、总览二级指针的三种内存模型：

#define  _CRT_SECURE_NO_WARNINGS #include <stdlib.h>#include <string.h>#include <stdio.h>void main2(){int i = 0;//指针数组char *   p1[] = {"123", "456", "789"};//二维数组char p2[3][4]  = {"123", "456", "789"};//手工二维内存char **p3 = (char **)malloc(3 * sizeof(char *)); //int array[3];for (i=0; i<3; i++){p3[i] = (char *)malloc(10*sizeof(char)); //char buf[10]sprintf(p3[i], "%d%d%d", i, i, i);}}

5、三种内存模型综合演练：

/*int sort(char *p[], int count, char **p, int *ncount);int sort(char *p[], int count, char(*p)[30], int *ncount);int sort(char(*p)[30], int ncount, char **p, int *ncount);//把第一种内存模型和第二种内存模型结果copy到第三种内存模型中，并排序、打印char **sort(char **p1, int num1, char(*p)[30], int num2, int *num3);*/#include"stdio.h"#include"stdlib.h"#include"string.h"int sort(char **myp1, int num1, char (*myp2)[30], int num2, int ***myp3, int *num3){int i = 0, j = 0, k = 0;int tmplen = 0;char *tmpP = NULL;//辅助指针变量，以便排序使用//分配内存，以供拷贝char **p3 = NULL;p3 = (char **)malloc((num1 + num2)*sizeof(char *));  //里面装的是指针if (p3 == NULL){return -1;}//拷贝for (i = 0; i < num1; i++){tmplen = strlen(myp1[i]) + 1;p3[i] = (char *)malloc(tmplen *sizeof(char));//挂内存，指向内存空间if (p3[i] == NULL){return -2;}strcpy(p3[i], myp1[i]);//赋值，拷贝}for (j = 0; j < num2; j++, i++){tmplen = strlen(myp2[j]) + 1;p3[i] = (char*)malloc(tmplen*sizeof(char));//挂内存，指向内存空间if (p3[i] == NULL){return -3;}strcpy(p3[i], myp2[j]);//赋值，拷贝}//排序tmplen = num1 + num2;for (i = 0; i < tmplen; i++){for (j = i + 1; j < tmplen; j++){if (strcmp(p3[i], p3[j])>0){tmpP = p3[i];p3[i] = p3[j];p3[j] = tmpP;}}}//间接赋值*num3 = tmplen;*myp3 = p3;return 0;}void sortFree1(char **p, int len)//第一种释放内存方法，仅释放内存{int i = 0;if (p == NULL){return;}for (i = 0; i < len; i++){free(p[i]);}free(p);}void sortFree2(char ***myp, int len)//第二种释放内存的方法，把二级指针指向的二维内存释放掉的同时间接地修改了实参的值{int i = 0;char **p = NULL;if (myp == NULL){return;}p = *myp; //还原成二级指针if (p == NULL){return;}for (i = 0; i < len; i++){free(p[i]);}free(p);*myp = NULL;    //myp是实参的地址     //间接赋值是指针存在的最大意义}int main(){int ret = 0;char *p1[] = { "aaaa", "ssssss", "dddddd" };//指针数组char buf2[10][30] = { "11111", "2222", "33333333" };//二维数组char **p3 = NULL;int len1, len2, len3;len1 = sizeof(p1) / sizeof(*p1);len2 = 3;ret = sort(p1, len1, buf2, len2, &p3, &len3);if (ret != 0){printf("func sort() err!");}for (int i = 0; i < len3; i++){printf("%s\n", p3[i]);}//sortFree1(p3, len3);sortFree2(&p3, len3);//释放内存return ret;}