试写atoi函数

int atoi(const char* strNum);

在接口中用到了const char*，想了半天，到底是指针是const还是指向const char的指针呢，在The C++ Programming Language里面给出过一个助记的方法：把一个声明从右向左读。因此const char*是指后者。

 

atoi函数库文件编写：

[root@Shentar ~/myprogs/cpp/atoi]# cat atoi.cpp

#include <string.h>
#include <stdio.h>

#define MAX_NUM_LEN 10
int atoi(const char* strNum)
{
    int sum = 0;
    bool bnegative = false;
    size_t len = 0;
    if (NULL == strNum)
    {
        throw "argument error!";
    }

    if ('-' == *strNum)
    {
        bnegative = true;
        strNum++;
    }
    else if ('+' == *strNum)
    {
        strNum++;
    }
    else
    {
        /*do nothing;*/
    }

    len = strnlen(strNum, MAX_NUM_LEN);
    if (len <= 0)
    {
        throw  "the input str is empty!\n";
    }

    unsigned char digitnum;
    for (int i = 0; i != len; i++)
    {
        digitnum = *strNum - 0x30;
        if (digitnum > 9 || digitnum < 0)
        {
            printf("char %c is not a digit.\n", *strNum);
            if (i == 0)
            {
                throw "digit error!";
            }
            else
            {
                break;
            }
        }

        sum = sum * 10 + digitnum;
        strNum++;
    }

    if (bnegative)
    {
        sum = -sum;
    }

    return sum;
}

 

测试程序：

[root@Shentar ~/myprogs/cpp/atoi]# cat testatoi.cpp

#include <stdio.h>


int atoi(const char* str);

int main(int argc, char** argv)
{
    if (argc < 2)
    {
        printf("usage: testmain numberchar\n");
        return -1;
    }

    try
    {
        printf("the input str [%s] was converted to int [%d].\n", argv[1], atoi(argv[1]));
    }
    catch(const char* errstr)
    {
        printf("%s\n", errstr);
        return -1;    
    }

    return 0;
}

lib:
    g++ -o libatoi.so -shared atoi.cpp

test:lib
    g++ -o testatoi testatoi.cpp -L. -latoi 

all:test

clean:
    rm *.o *.so *.out -rf

测试结果：

[root@Shentar ~/myprogs/cpp/atoi]# ./testatoi f
char f is not a digit.
digit error!
[root@Shentar ~/myprogs/cpp/atoi]# ./testatoi 1
the input str [1] was converted to int [1].
[root@Shentar ~/myprogs/cpp/atoi]# ./testatoi -11
the input str [-11] was converted to int [-11].
[root@Shentar ~/myprogs/cpp/atoi]# ./testatoi -21
the input str [-21] was converted to int [-21].
[root@Shentar ~/myprogs/cpp/atoi]# ./testatoi -20981
the input str [-20981] was converted to int [-20981].
[root@Shentar ~/myprogs/cpp/atoi]# ./testatoi 20981
the input str [20981] was converted to int [20981].
[root@Shentar ~/myprogs/cpp/atoi]# ./testatoi a20981
char a is not a digit.
digit error!
[root@Shentar ~/myprogs/cpp/atoi]# ./testatoi 2a0981
char a is not a digit.
the input str [2a0981] was converted to int [2].
[root@Shentar ~/myprogs/cpp/atoi]# ./testatoi 2ab0981
char a is not a digit.
the input str [2ab0981] was converted to int [2].
[root@Shentar ~/myprogs/cpp/atoi]# ./testatoi 2ab-0981
char a is not a digit.
the input str [2ab-0981] was converted to int [2].
[root@Shentar ~/myprogs/cpp/atoi]# ./testatoi -2ab-0981
char a is not a digit.
the input str [-2ab-0981] was converted to int [-2].
[root@Shentar ~/myprogs/cpp/atoi]# ./testatoi -ab-0981
char a is not a digit.
digit error!
[root@Shentar ~/myprogs/cpp/atoi]# ./testatoi 
usage: testmain numberchar