The TokenBuffer probably needs a reset as well. I figure now. I'm
working on a patch for this based on some stuff Bogdan Mitu send me. Give
these snippets a spin if you want:

Add this method to CircularQueu.hpp
/// Clear the queue
inline void clear( void )
{
   m_offset = 0;
   storage.clear();
}

Add this one to InputBuffer.hpp and TokenBuffer.hpp

/// Reset the input buffer to empty state
virtual inline void reset( void )
{
   nMarkers = 0;
   markerOffset = 0;
   numToConsume = 0;
   queue.clear();
}

Add this one to LexerSharedInputState:

virtual void initialize( ANTLR_USE_NAMESPACE(std)istream& in, const char* file = "" )
{
   column = 1;
   line = 1;
   tokenStartColumn = 1;
   tokenStartLine = 1;
   guessing = 0;
   filename = file;
   if( input && inputResponsible )
      delete input;

   input = new CharBuffer(in);
   inputResponsible = true;
}

And to ParserSharedInputState:

// Reset the ParserInputState and the underlying TokenBuffer
void reset( void )
{
   input->reset();
   guessing = 0;
}

-------
And change the last bit of your program to:

std::istringstream y("C=-{SC=ROOT},C=*{SC=$},C=*{ER=100{}}
");

L.getInputState().initialize(y);
P.getInputState().reset();

parser.startRule();

This is pretty much untested so beware =) It might be that
P.getInputState is declared const in that case create the
ParserSharedINputstate outside the parser and initalize the parser with it
on creation.

I guess I'll put this in 2.7.2.