Operator overloading: calling friend function from member function

I have a Student class. I want to overload + operator so I can add a double variable to class. Here is Student class:

class Student { private: std::string firstName; double grade; public: Student(const std::string &firstName, double grade); double getGrade() const; friend Student operator+(double grade, const Student &student); Student operator+(double grade) const; }; 

And implementation:

Student::Student(const std::string &firstName, double grade) { this->firstName = firstName; this->grade = grade; } double Student::getGrade() const { return grade; } Student operator+(double grade, const Student &student) { return Student(student.firstName, student.grade + grade); } Student Student::operator+(double grade) const { return operator+(grade, *this); } 

double + Student is done via friend function and Student + double goes through member function. When I compile I get this:

error: no matching function for call to ‘Student::operator+(double&, const Student&) const’ return operator+(grade, *this); ^ note: candidate is: note: Student Student::operator+(double) const Student Student::operator+(double grade) const { ^ note: candidate expects 1 argument, 2 provided 

Why I can't call a friend function from a member function?

[UPDATE]

However when I overload << operator, I can call it from member function without pre-pending ::.

friend std::ostream &operator<<(std::ostream &os, const Student &student); 

and the implementation:

std::ostream &operator<<(std::ostream &os, const Student &student) { os << student.grade; return os; }