Re: How to: optional CString parameter

Though not quite the same as NULL, you could use:

virtual CString Get(
      CString strURL, // If you don't modify, use a const reference.
      const CString& strErrorMsg = CString());

But I wouldn't use default parameters with a virtual function. A better
approach would be:

CString Get(
      CString strURL,
      const CString& strErrorMsg = CString())
{
   return Get_impl(strURL, strErrorMsg);
}

virtual CString Get_impl(
      const CString& strURL,
      const CString& strErrorMsg);

The other reason to use this technique is to avoid overloading virtual
functions, which reduces the burden on derived classes that want to
override the virtual function.