Question

What is the main purpose of overloading operators in C++?

In the code below, << and >> are overloaded; what is the advantage to doing so?

#include <iostream>
#include <string>

using namespace std;

class  book {
    string name,gvari;
    double cost;
    int year;
    public:
    book(){};

    book(string a, string b, double c, int d) { a=name;b=gvari;c=cost;d=year; }
    ~book() {}
    double setprice(double a) { return a=cost; }
    friend ostream& operator <<(ostream& , book&);
    void printbook(){
        cout<<"wignis saxeli "<<name<<endl;
        cout<<"wignis avtori "<<gvari<<endl;
        cout<<"girebuleba "<<cost<<endl;
        cout<<"weli "<<year<<endl;
    }
};

ostream& operator <<(ostream& out, book& a){
    out<<"wignis saxeli "<<a.name<<endl;
    out<<"wignis avtori "<<a.gvari<<endl;
    out<<"girebuleba "<<a.cost<<endl;
    out<<"weli "<<a.year<<endl;
    return out;
}

class library_card : public book {
    string nomeri;
    int raod;
    public:
    library_card(){};
    library_card( string a, int b){a=nomeri;b=raod;}
    ~library_card() {};
    void printcard(){
        cout<<"katalogis nomeri "<<nomeri<<endl;
        cout<<"gacemis raodenoba "<<raod<<endl;
    }
    friend ostream& operator <<(ostream& , library_card&);
};

ostream& operator <<(ostream& out, library_card& b) {
    out<<"katalogis nomeri "<<b.nomeri<<endl;
    out<<"gacemis raodenoba "<<b.raod<<endl;
    return out;
}


int main() {
    book A("robizon kruno","giorgi",15,1992);
    library_card B("910CPP",123);
    A.printbook();
    B.printbook();
    A.setprice(15);
    B.printbook();

    system("pause");
    return 0;
}

Answer 1

It doesn’t ever have to be used; it’s just a convenience, a way of letting user-defined types act more like built-in types.

For example, if you overload operator<<, you can stream books the same way as integers and strings:

cout << "Book #" << n << " is " << books[n] << endl;

If you don’t, you’d have to write the same thing like this:

cout << "Book #" << n << " is ";
books[n].printbook();
cout << endl;

Similarly, if you create a Fraction class and give it an operator+, you can use fractions the same way you use integers, and so on.

It’s sometimes a tough design choice whether your class should act like a native type in one way or another (for example, does string’s operator+ make sense?), but the point is that C++ gives you the choice.