C++写的UrlEncode和UrlDecode

  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;
 }