strcat与strncat的C/C++实现

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：

1. 函数src参数应为const，dst参数为非const，count为size_t类型；
2. 函数要返回dst的地址，以方便嵌套使用该函数；
3. 确定dst要有字符串结束符；
4. 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. . .