字符串转整数

本方法借鉴自glibc，可以过滤如下情况：

前缀空格

非法输入

正负号

16进制8进制

溢出

1234L

#include <iostream>
using namespace std;

#define LONG_MAX 2147483647L
#define LONG_MIN -2147483648L

long StrToInt(const string &str){
    unsigned long result = 0;//如果不用unsigned long，那么后面无法判断溢出（当发生溢出时，总是负数）
    int radix = 0;
    int sign = 1;

    string::const_iterator it = str.begin();

    //1 filter preceding blank
    while (*it == ' ' ||  *it == '\t')
        it++;

    //2 judge sign
    if (*it == '+'){
        sign = 1;
        it++;
    }
    else if (*it == '-'){
        sign = 0;
        it++;
    }

    //3 judge invalid input on first char
    if (*it < '0' || *it > '9'){
        cout<<"err input!"<<endl;
        return 0;
    }

    //4 judge radix
    radix = 10;
    if (*it == '0' && it+1 != str.end()){
        it++;
        if (*it == 'x' || *it == 'X'){
            it++;
            radix = 16;
        }
        else
            radix = 8;
    }

    //5 proceed each char one time
    while (it != str.end()){
        unsigned long digit = *it - '0';
        //5.1 invalid input
        if (*it < '0' || *it > '9'){
            //5.2 long excludes, ex: 1234L
            if ((*it=='l'||*it=='L')&&(it+1==str.end()))
                break;
            else{
                cout<<"err input!"<<endl;
                return 0;
            }
        }
        //5.3 overflow
        if (result > LONG_MAX/10 || 
            sign>0 ? (result == LONG_MAX/10) && (digit < LONG_MAX%10) : 
            (result == (LONG_MAX+1)/10) && (digit < (LONG_MAX+1)%10)){
            cout<<"overflow!"<<endl;
            if (sign)
                result = LONG_MAX;
            else
                result = LONG_MIN;
            return result;
        }

        result *= radix;
        result += digit;
        it++;
    }

    //6 add sign
    if (sign == 0)
        result = 0 - result;
    return result;
}

int main()
{
    string strs[10];
    strs[0] = "123";
    strs[1] = "0";
    strs[2] = "abc";
    strs[3] = "!#$%";
    strs[4] = "0x5";
    strs[5] = "-5L";
    strs[6] = "-0x999999";
    strs[7] = "-099999";
    strs[8] = "1234%&s";
    strs[9] = "-0x81had";

    for (int i = 0; i < 10; i++){
        long tmp = 0;
        cout<<StrToInt(strs[i])<<endl;
    }

    return 0;
}

EOF