C ++, Union vs Class Inheritance

I have a specific problem and am stuck which approach is better than the other:

An enumerated structure that defines the data in a member of an OR union Class group with inheritance

An example code is given below:

Union based structure

  typedef union TokenValue{
   bool bValue;
   long lvalue;
   double dvalue;
   std::string svalue;
   }

  class Token{
  public:
   TokenType type;
   TokenValue value;
   };

Class inheritance

class TokenBase{
public:
  TokenType type;
  };



class TokenNumber: public TokenBase{
public:
bool isInt;
long lvalue;
double dvalue;
};

class TokenString: public TokenBase{
 public:
  std::string svalue;
 };