Question

I am testing some C++ code related to overloading IO operators. The code as follows:

class Student {
 private:
     int no;
 public:
     Student(int n)
     {
         this->no = n;
     }
     int getNo() const {
         return this->no;
     }
     friend istream& operator>>(istream& is, Student& s);
     friend ostream& operator<<(ostream& os, const Student& s);
 };
 ostream& operator<<(ostream& os, const Student& s){
     cout << s.getNo(); // use cout instead of os
     return os;
}
istream& operator>>(istream& is, Student& s) 
{
    cin >> s.no; // use cin instead of is
    return is;
}

However, inside the << and >>, I can use:

 ostream& operator<<(ostream& os, const Student& s){
     os << s.getNo(); // use os instead of cout
     return os;
}
istream& operator>>(istream& is, Student& s) 
{
    is >> s.no; // use is instead of cin
    return is;
}

In <<, I use os object instead of cout and the similarity for >> operator. So, I am curious to know if there is any difference of that?

Answer 1

The difference is obvious, is/os are input/output streams while cin/cout are the standard input/output streams. cin/cout are instances of i/o streams, not synonyms.

The point being, there are streams other than standard input/output, such as files, string streams, and whatever custom implementation you can think of. If you use cin/cout in your streaming operators, ignoring the stream they should read/write to, then you’ll end up with

file_stream << some_student;

printing to the standard output, rather than to the file where its supposed to.