XMLURL problem!

Hi...
  Need some help on XMLURL - -
I am using c++ implementation of xerces taken from apache.

My requirement is to build a XMLURL with a base string and a relative
string..
Let me say if my base string is c:\temp\base.xml and relative string is
rel.xml, I wish
to create a XMLURL as c:\temp\rel.xml
For this...i have tried the following 2 steps - 
1. Using the constructor of XMLURL
2. Using function - makeRelativeTo().
But both of them are throwing MalFormedURLException...

Cud anyone help me in this to do this functionality....
Any urgent help is highly appreciated..
Thnx
manoj