URI Encoding and Decoding


Here are two functions of URI encoding and decoding. They use std::string as the argument and return type.

A URI is represented as a sequence of characters, not as a sequence of octets. That is because URI might be "transported" by means that are not through a computer network, e.g., printed on paper, read over the radio, etc.—RFC2396

UriEncode() maps octets to characters, such as:

"\0\1\2" -> "%00%01%02"
"~ABCD"  -> "%7EABCD"

Each octet except alphanum is converted to "% HEX HEX". UriDecode() converts them back.

Code Snippets


std::string UriEncode(const std::string & sSrc)
   const char DEC2HEX[16 + 1] = "0123456789ABCDEF";
   const unsigned char * pSrc = (const unsigned char *)sSrc.c_str();
   const int SRC_LEN = sSrc.length();
   unsigned char * const pStart = new unsigned char[SRC_LEN * 3];
   unsigned char * pEnd = pStart;
   const unsigned char * const SRC_END = pSrc + SRC_LEN;

   for (; pSrc < SRC_END; ++pSrc)
      if (SAFE[*pSrc]) 
         *pEnd++ = *pSrc;
         // escape this char
         *pEnd++ = '%';
         *pEnd++ = DEC2HEX[*pSrc >> 4];
         *pEnd++ = DEC2HEX[*pSrc & 0x0F];

   std::string sResult((char *)pStart, (char *)pEnd);
   delete [] pStart;
   return sResult;


std::string UriDecode(const std::string & sSrc)
   // Note from RFC1630: "Sequences which start with a percent
   // sign but are not followed by two hexadecimal characters
   // (0-9, A-F) are reserved for future extension"

   const unsigned char * pSrc = (const unsigned char *)sSrc.c_str();
   const int SRC_LEN = sSrc.length();
   const unsigned char * const SRC_END = pSrc + SRC_LEN;
   // last decodable '%' 
   const unsigned char * const SRC_LAST_DEC = SRC_END - 2;

   char * const pStart = new char[SRC_LEN];
   char * pEnd = pStart;

   while (pSrc < SRC_LAST_DEC)
      if (*pSrc == '%')
         char dec1, dec2;
         if (-1 != (dec1 = HEX2DEC[*(pSrc + 1)])
            && -1 != (dec2 = HEX2DEC[*(pSrc + 2)]))
            *pEnd++ = (dec1 << 4) + dec2;
            pSrc += 3;

      *pEnd++ = *pSrc++;

   // the last 2- chars
   while (pSrc < SRC_END)
      *pEnd++ = *pSrc++;

   std::string sResult(pStart, pEnd);
   delete [] pStart;
   return sResult;

Usage Example

Just copy the source codes or external link the functions to use them.

int main()
   extern std::string UriEncode(const std::string & sSrc);
   extern std::string UriDecode(const std::string & sSrc);
   const std::string ORG("\0\1\2", 3);
   const std::string ENC("%00%01%02");
   assert(UriEncode(ORG) == ENC);
   assert(UriDecode(ENC) == ORG);
   return 0;

Differences from Other Implementations

Following are other implementations on URI/URL encoding and decoding.

This implementation differs from the above in these ways because it:

  • Has a decode function.
  • Encodes a buffer. Also, it supports encoding a char buffer, including '\0'. Example:
  • "ABC\0ABC" -> "ABC%00ABC"
  • Runs faster because it uses a array to do the mapping.
  • Is portable. It doesn't use MFC CString.



  • Programmer

    Posted by Tim on 02/25/2014 09:14am

    Note to other users: The HEX2DEC array used in the decoding function is in the .cpp in the download file.

  • Or we could encode like this

    Posted by Anonymous on 12/02/2012 07:25pm

    99% of the time the hex codes will be double digit in reality std::string URLEscape(char*url) { std::ostringstream s; for (;*url;s

  • License

    Posted by Cem Kalyoncu on 05/26/2012 10:50am

    Pretty good, is it possible to specify the license? MIT/BSD/PD/Apache licenses would be most useful.

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