string file2str(string filename)
{
   int length;
   char * buffer;

   ifstream is;
   is.open (filename.c_str(), ios::binary );

   // get length of file:
   is.seekg (0, ios::end);
   length = is.tellg();
   is.seekg (0, ios::beg);

   // allocate memory:
   buffer = new char [length];
 
   // read data as a block:
   is.read (buffer,length);
   
   is.close();
   string retval(buffer,length);  // null characters have no effect  
   return retval;
}

Nine times out of ten, reading the whole file into memory at once and then parsing it with a stringstream is over 5-10 times faster than doing the reading and parsing at the same time with an ifstream, because the ifstream ends up sending a lot of little read requests to the hard drive instead of one long sequential read, and the one long sequential read is going to be a lot faster on almost every machine.