template<typename Curve>
class SpotCurve1D
{
public:
  typedef SpotCurveTraits<Curve> CurveTraits;
  typedef typename CurveTraits::tType tType;
  typedef typename CurveTraits::fType fType;

  fType GetSpot(tType) const;
  

  template<typename Interp, typename Extrap>
  fType GetSpot(tType, 
                typename boost::call_traits<Interp>::param_type= Interp(),
                typename boost::call_traits<Extrap>::param_type= Extrap() ) const;
...
};

template<typename Curve>
inline typename SpotCurveTraits<Curve>::fType
SpotCurve1D<Curve>::GetSpot(tType t) const
{
  return ...;
}

template<typename Curve>
template<typename Interp, typename Extrap>
inline typename SpotCurveTraits<Curve>::fType
SpotCurve1D<Curve>::GetSpot(tType t, 
                            typename boost::call_traits<Interp>::param_type i= Interp(),
                            typename boost::call_traits<Extrap>::param_type e= Extrap() ) const
{
.... 
}