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++ != '');可以用while ( *cp++ );代替

同样while ( (*cp++ = *src++) != '');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++ != '');可以用while ( *cp++ );代替
//同样while ( (*cp++ = *src++) != '');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++ != '');
    cp--;                //指向''
    
    while ( (*cp++ = *src++) != '');
    //*StrFront = '';    //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 = '';   //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 = 4;
    //_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 = 4;
    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 = 30;
    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 = 4;
    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 = 30;
    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 0;
}

运行结果：

 test _strcat_1...
the dst string is : world!
the src string is : hello
the count is : 4
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 : 4
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 : 4
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 : 4
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 : 30
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 : 4
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 : 30
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. . .