(C++)UrlEncode的标准实现

关于UrlEncode的实现(C++)，网上有很多不同的版本，对需要编码的字符集的选取并不统一。那么到底有没有标准呢？答案是有的，参见wiki

    绝对不编码的，只有字母、数字、短横线(-)、下划线(_)、点(.)和波浪号(~)，其他字符要视情况而定，所以一般性的urlencode只需保留上述字符不进行编码。

    下面给出实现：

    unsigned char ToHex(unsigned char x)   
    {   
        return  x > 9 ? x + 55 : x + 48;   
    }  
      
    unsigned char FromHex(unsigned char x)   
    {   
        unsigned char y;  
        if (x >= 'A' && x <= 'Z') y = x - 'A' + 10;  
        else if (x >= 'a' && x <= 'z') y = x - 'a' + 10;  
        else if (x >= '0' && x <= '9') y = x - '0';  
        else assert(0);  
        return y;  
    }  
      
    std::string UrlEncode(const std::string& str)  
    {  
        std::string strTemp = "";  
        size_t length = str.length();  
        for (size_t i = 0; i < length; i++)  
        {  
            if (isalnum((unsigned char)str[i]) ||   
                (str[i] == '-') ||  
                (str[i] == '_') ||   
                (str[i] == '.') ||   
                (str[i] == '~'))  
                strTemp += str[i];  
            else if (str[i] == ' ')  
                strTemp += "+";  
            else  
            {  
                strTemp += '%';  
                strTemp += ToHex((unsigned char)str[i] >> 4);  
                strTemp += ToHex((unsigned char)str[i] % 16);  
            }  
        }  
        return strTemp;  
    }  
      
    std::string UrlDecode(const std::string& str)  
    {  
        std::string strTemp = "";  
        size_t length = str.length();  
        for (size_t i = 0; i < length; i++)  
        {  
            if (str[i] == '+') strTemp += ' ';  
            else if (str[i] == '%')  
            {  
                assert(i + 2 < length);  
                unsigned char high = FromHex((unsigned char)str[++i]);  
                unsigned char low = FromHex((unsigned char)str[++i]);  
                strTemp += high*16 + low;  
            }  
            else strTemp += str[i];  
        }  
        return strTemp;  
    }