Union Type Object problem

[Enrico Oliosi]

Hi,
I have to do a parser in Bison and I need to use in %union{} construct
some type Object.
I have this example-class Expression:

class Expression {
public:
  virtual ~Expression () {}
  //  It's necessary because we need to clone objects without
  // knowing the exact type.
  virtual Expression *clone () = 0;
  // The value represented by the expression
  virtual int value () = 0;
};

this is a very simple class, but when I define the Bison Type in
union construct in this way:

%union{
  Expression *exp;
  int num;
}

the compiler give me this error:

> > error: syntax error before `*' token

I compile this file Bison like this:
bash> bison -d parser.y 
bash> flex     parser.lex
bash> g++  -DCPLUSPLUS parser.tab.c lex.yy.c -lfl -o parser

Thanks a lot.
Enrico