2013-07-05 15:47:19
本函数给出了几种strcat与strncat的实现，有ugly implementation，也有good implementation。并参考标准库中的implementation，最后给出了比较好的implementation。
注意以下几点：
对于while (*cp++)，要注意循环结束时，指针指向的位置是否是预期的，如下面的：
while ( *cp )
cp++;
与
while (*cp++)
;
cp--;
的效果是一样的。
在第二种写法中就要注意在结束循环后，对cp减1，才能指向字符串结束符的位置。
while (*cp++ != '\0');可以用while ( *cp++ );代替
同样while ( (*cp++ = *src++) != '\0');可用while ( *cp++ = *src++ );代替
_strncat_1可实现与标准库函数即_strncat_2同样的功能，可以使得在count大于、小于以及等于source长度时，加上字符串结束符，且加入了输入合法性检查；但_strncat_1的写法更为简洁
小结：
标准库函数并没有输入合法性检查，这将输入合法性检查的任务推给了函数的调用者。
对于strcat函数，好的implementation要考虑一下几点：
- 函数src参数应为const，dst参数为非const，count为size_t类型；
- 函数要返回dst的地址，以方便嵌套使用该函数；
- 确定dst要有字符串结束符；
- 注意输入合法性检查注意输入合法性检查。
对于strncpy函数，除了以上几点外，好的implementation还要考虑：
当source的长度小于count时，应该怎么办？
标准库函数的做法是，将source的所有有效字符复制完成后，再加一个字符串结束符；当source的长度大于火等于count时，将source的count个有效字符复制完成后，再加一个字符串结束符
代码：
#include <iostream> using namespace std;
#define SIZE 100 /***
*char *strcat(dst, src) - concatenate (append) one string to another
*
*Purpose:
* Concatenates src onto the end of dest. Assumes enough
* space in dest.
*
*Entry:
* char *dst - string to which "src" is to be appended
* const char *src - string to be appended to the end of "dst"
*
*Exit:
* The address of "dst"
*
*Exceptions:
*
*******************************************************************************/ //代码写的比较笨拙
//while (*cp++ != '\0');可以用while ( *cp++ );代替
//同样while ( (*cp++ = *src++) != '\0');可用while ( *cp++ = *src++ );代替
char * _strcat_1(char *dst,char *src)
{
if (NULL == dst || NULL == src)
{
return dst;
}
char *cp = dst;
while (*cp++ != '\0');
cp--; //指向'\0' while ( (*cp++ = *src++) != '\0');
//*StrFront = '\0'; //加上字符串结束符,不需要，while结束时，已经加上了结束符
return ( dst );
} //标准库函数给出的implementation
char * _strcat_2(char *dst,const char *src)
{
char *cp = dst; while ( *cp )
cp++; while ( *cp++ = *src++ )
; return ( dst );
} //标准库函数给出的implementation，加上输入合法性检查
//好的implementation要考虑一下几点：
//1）函数src参数应为const，dst参数为非const
//2）注意输入合法性检查
char * _strcat_3(char *dst,const char *src)
{
if (NULL == dst || NULL == src)
{
return dst;
} char *cp = dst; while ( *cp )
cp++; while ( *cp++ = *src++ )
; return ( dst );
}
/***
*char *strncat(front, back, count) - append count chars of back onto front
*
*Purpose:
* Appends at most count characters of the string back onto the
* end of front, and ALWAYS terminates with a null character.
* If count is greater than the length of back, the length of back
* is used instead. (Unlike strncpy, this routine does not pad out
* to count characters).
*
*Entry:
* char *front - string to append onto
* char *back - string to append
* unsigned count - count of max characters to append
*
*Exit:
* returns a pointer to string appended onto (front).
*
*Uses:
*
*Exceptions:
*
*******************************************************************************/ //标准库函数给出的implementation，加上输入合法性检查
//好的implementation要考虑一下几点：
//1）函数src参数应为const，dst参数为非const
//2）注意输入合法性检查
char * _strncat_1(char *dst,const char *src,size_t count)
{
if (NULL == dst || NULL == src)
{
return dst;
} char *cp = dst; while ( *cp )
cp++; /*while ( count-- && *cp++ = *src++ )
;
*/
while ( count-- && (*cp++ = *src++) ) //注意要加括号
; return ( dst );
} //标准库函数的implementation
//_strncat_1可实现与该函数同样的功能，且加入了输入合法性检查
//_strncat_1的写法更为简洁
char * _strncat_2 (
char * front,
const char * back,
size_t count
)
{
char *start = front; while (*front++)
;
front--; //将front指向字符串结束符 while (count--)
if (!(*front++ = *back++)) //在back的长度小于count时，直接返回，此时front已有字符串结束符
return(start); *front = '\0'; //对于back的长度大于count时，加上字符串结束符
return(start);
}
//测试程序
int main()
{
char src_1[SIZE] = "hello ";
char dst_1[SIZE] = "world!";
size_t count = ;
//_strcat
cout<<"test _strcat_1..."<<endl;
cout<<"the dst string is : "<<dst_1<<endl;
cout<<"the src string is : "<<src_1<<endl;
cout<<"the count is : "<<count<<endl;
_strcat_1(dst_1,src_1);
cout<<"the cat result is : "<<dst_1<<endl;
cout<<"(return pointer)the cat result is : "<<_strcat_1(dst_1,src_1)<<endl; cout<<"test _strcat_2..."<<endl;
cout<<"the dst string is : "<<dst_1<<endl;
cout<<"the src string is : "<<src_1<<endl;
cout<<"the count is : "<<count<<endl;
_strcat_2(dst_1,src_1);
cout<<"the cat result is : "<<dst_1<<endl;
cout<<"(return pointer)the cat result is : "<<_strcat_2(dst_1,src_1)<<endl; cout<<"test _strcat_3..."<<endl;
cout<<"the dst string is : "<<dst_1<<endl;
cout<<"the src string is : "<<src_1<<endl;
cout<<"the count is : "<<count<<endl;
_strcat_3(dst_1,src_1);
cout<<"the cat result is : "<<dst_1<<endl;
cout<<"(return pointer)the cat result is : "<<_strcat_3(dst_1,src_1)<<endl; //_strncat_1
char src_2[SIZE] = "happy birthday!";
char dst_2[SIZE] = "baby,";
count = ;
cout<<"test _strncat_1..."<<endl;
cout<<"the dst string is : "<<dst_2<<endl;
cout<<"the src string is : "<<src_2<<endl;
cout<<"the count is : "<<count<<endl;
_strncat_1(dst_2,src_2,count);
cout<<"the cat result is : "<<dst_2<<endl;
cout<<"(return pointer)the cat result is : "<<_strncat_1(dst_2,src_2,count)<<endl; count = ;
cout<<"test _strncat_1..."<<endl;
cout<<"the dst string is : "<<dst_2<<endl;
cout<<"the src string is : "<<src_2<<endl;
cout<<"the count is : "<<count<<endl;
_strncat_1(dst_2,src_2,count);
cout<<"the cat result is : "<<dst_2<<endl;
cout<<"(return pointer)the cat result is : "<<_strncat_1(dst_2,src_2,count)<<endl; //_strncat_2
char src_3[SIZE] = "happy birthday!";
char dst_3[SIZE] = "baby,";
count = ;
cout<<"test _strncat_2..."<<endl;
cout<<"the dst string is : "<<dst_3<<endl;
cout<<"the src string is : "<<src_3<<endl;
cout<<"the count is : "<<count<<endl;
_strncat_1(dst_3,src_3,count);
cout<<"the cat result is : "<<dst_3<<endl;
cout<<"(return pointer)the cat result is : "<<_strncat_1(dst_3,src_3,count)<<endl; count = ;
cout<<"test _strncat_2..."<<endl;
cout<<"the dst string is : "<<dst_3<<endl;
cout<<"the src string is : "<<src_3<<endl;
cout<<"the count is : "<<count<<endl;
_strncat_1(dst_3,src_3,count);
cout<<"the cat result is : "<<dst_3<<endl;
cout<<"(return pointer)the cat result is : "<<_strncat_1(dst_3,src_3,count)<<endl; return ;
}
运行结果：
test _strcat_1...
the dst string is : world!
the src string is : hello
the count is :
the cat result is : world!hello
(return pointer)the cat result is : world!hello hello
test _strcat_2...
the dst string is : world!hello hello
the src string is : hello
the count is :
the cat result is : world!hello hello hello
(return pointer)the cat result is : world!hello hello hello hello
test _strcat_3...
the dst string is : world!hello hello hello hello
the src string is : hello
the count is :
the cat result is : world!hello hello hello hello hello
(return pointer)the cat result is : world!hello hello hello hello hello hello
test _strncat_1...
the dst string is : baby,
the src string is : happy birthday!
the count is :
the cat result is : baby,happ
(return pointer)the cat result is : baby,happhapp
test _strncat_1...
the dst string is : baby,happhapp
the src string is : happy birthday!
the count is :
the cat result is : baby,happhapphappy birthday!
(return pointer)the cat result is : baby,happhapphappy birthday!happy birthday!
test _strncat_2...
the dst string is : baby,
the src string is : happy birthday!
the count is :
the cat result is : baby,happ
(return pointer)the cat result is : baby,happhapp
test _strncat_2...
the dst string is : baby,happhapp
the src string is : happy birthday!
the count is :
the cat result is : baby,happhapphappy birthday!
(return pointer)the cat result is : baby,happhapphappy birthday!happy birthday!
请按任意键继续. . .