Question

I have the following piece of code:

 class Student {
 public:
     Student(){}
     void display() const{}
     friend istream& operator>>(istream& is, Student& s){return is;}
     friend ostream& operator<<(ostream& os, const Student& s){return os; }
 };
int main()
{
    Student st;
    cin >> st;
    cout << st;

    getch();
    return 0;
}

I have tried myself when omitting the friend keywords to make the operators become the member function of the Student class, then the compiler would produce “binary 'operator >>' has too many parameters“. I have read some document saying that happened because all member functions always receive an implicit parameter “this” (that’s why all member functions can access private variables).
Based on that explanation, I have tried as follows:

class Student {
 public:
     Student(){}
     void display() const{}
     istream& operator>>(istream& is){return is;}
     ostream& operator<<(ostream& os){return os; }
 };
int main()
{
    Student st;
    cin >> st;
    cout << st;

    getch();
    return 0;
}

And got the error message: “error C2679: binary '>>' : no operator found which takes a right-hand operand of type 'Student' (or there is no acceptable conversion)“

Can anyone give me a clear explanation, please?

Answer 1

You can’t say that the function is a friend function, and then include the function in-line. The friend keyword implies that the function is not defined in the class, but it can access all the private and protected variables of the class. Change your code to:

class Student {
  public:
    Student(){}
    void display() const{}
    friend istream& operator>>(istream& is, Student& s);
    friend ostream& operator<<(ostream& os, const Student& s);
};

istream& operator >>(istream& is, Student& s) { return is; }
ostream& operator <<(ostream& os, const Student& s) { return os; }

Look at http://www.java2s.com/Code/Cpp/Overload/Overloadstreamoperator.htm for another example.

With << and >>, the left hand operand is always a file stream, so you cannot overload them within your actual class (it’d technically have to go in the file stream class).