c++ must take exactly one argument

I'm a new c++ learner.Now,I have a problem in my code, but I don't know how to debug it.Please help me.It's form.h file.

using namespace std;

class Form
{
friend ostream& operator << (ostream&, const Form&);
int prc; 
int wdt;
int fmt;
double val;

public:
explicit Form(int p=6):prc(p)
{
  fmt = 0;
  wdt = 0;
}

//Bound_form operator()(double d) const;

Form& scientific()
{
 fmt = ios_base::scientific;
 return *this;
 }

Form& precision(int p) 
{
 prc = p;
 return *this;
}
Form& fill(char);

std::ostream& operator<<(std::ostream& os, const Form& bf)
{
  ostringstream s;
  s.precision(bf.prc);
  s.setf(static_cast<ios_base::fmtflags>(bf.fmt), ios_base::floatfield);
  s.setf(bf.fmt,ios_base::floatfield);

  s << bf.val;
 return os << s.str() ;
 }
};

When i compile, it says: enter image description here