#define  _CRT_SECURE_NO_WARNINGS

#include <stdlib.h>

#include <string.h>

#include <stdio.h>

void main71()

{

    char a[] = "i am a student";

    char b[64];

    int  i = 0;

    for (i=0; *(a+i) != '\0'; i++)

    {

        *(b+i) = *(a+i);

    }

    //0没有copy到b的buf中.

    b[i] = '\0'; //重要

    printf("a:%s \n", a);

    printf("b:%s \n", b);

    system("pause");

    return ;

}

//字符串copy函数技术推演

//字符串copy函数

//form形参 形参to 的值 不停的在变化....

//不断的修改了from和to的指向

void copy_str21(char *from, char *to)

{

    for (; *from!='\0'; from++, to++)

    {

         *to = *from;

    }

    *to = '\0';

    return ;

}

//*操作 和++的操作

//++ 优先级高

void copy_str22(char *from, char *to)

{

    for (; *from!='\0';)

    {

        *to++ = *from++;  //  先 *to = *from;  再from++, to++

    }

    *to = '\0'; //

    return ;

}

void copy_str23(char *from, char *to)

{

    while( (*to = *from) != '\0' )

    {

        from ++;

        to ++;

    }

}

void copy_str24(char *from , char *to)

{

    while ( (*to++ = *from++) != '\0')

    {

        ;

    }

}

void copy_str25(char *from , char *to)

{

    //*(0) = 'a';

    while ( (*to++ = *from++) )

    {

        ;

    }

}

void copy_str25_err(char *from , char *to)

{

    //*(0) = 'a';

    while ( (*to++ = *from++) )

    {

        ;

    }

    printf("from:%s \n", from);

}

//不要轻易改变形参的值, 要引入一个辅助的指针变量. 把形参给接过来.....

int copy_str26_good(char *from , char *to)

{

    //*(0) = 'a';

    char *tmpfrom = from;

    char *tmpto = to;

    if ( from == NULL || to == NULL)

    {

        return -1;

    }

    while ( *tmpto++ = *tmpfrom++ ) ;  //空语句

    printf("from:%s \n", from);

}

int main111()

{

    int ret = 0;

    char *from = "abcd";

    char buf2[100];

    //copy_str21(from, buf2);

    //copy_str22(from,buf2);

    //copy_str23(from, buf2);

    //copy_str24(from, buf2);

    //copy_str25(from ,buf2);

    //printf("buf2:%s \n", buf2);

    {

        //错误案例

        char *myto = NULL;  //要分配内存

        //copy_str25(from,myto );

    }

    {

        char *myto = NULL;  //要分配内存

        ret = copy_str26_good(from, myto);

        if (ret != 0)

        {

            printf("func copy_str26_good() err:%d  ", ret);

            return ret;

        }

    }

    system("pause");

    return ret;

}

int main777()

{

    int ret = 0;

    char *from = "abcd";

    char buf2[100]; 

    printf("copy_str25_err begin\n");

    copy_str25_err(from, buf2);

    copy_str26_good(from, buf2);

    printf("copy_str25_err end\n");

    return 0;

}