Listing 10: The modified classes and their use for calculating the Gauss distribution

class Literal {
public:   Literal(double v) : _val(v) {}
          double eval(double) const { return _val; }
private:  const double _val;
};

class Identity {
public:   double eval(double d) const { return d; }
};

template <class ExprT1,class ExprT2, class BinOp>
class BinExpr {
public:   double eval(double d) const 
          { return _op(_expr1.eval(d),_expr2.eval(d)); }
};

...

template <class ExprT1,class ExprT2, class BinOp>
class BinaryExpr {
public:
    BinaryExpr(ExprT1 e1, ExprT2 e2,BinOp op=BinOp())
       : _expr1(e1),_expr2(e2),_op(op) {}
    double eval() const 
    { return _op(_expr1.eval(),_expr2.eval()); }
private:
    exprTraits<ExprT1>::expr_type _expr1;
    exprTraits<ExprT2>::expr_type _expr2;
    BinOp  _op;
};
— End of Listing —