Thread: Insertion Operator overloading works on some objects but not on others

Hi all,

I have overloaded an insertion operator which works on the object within which it is defined (see line 47).

But when I use this insertion operator on an object that contains the other object within which the operator is defined, I get a compilation error "no match for operator <<" on line 50.

What I mean is, calling the overloaded operator on an object within which it is defined works. But calling it on a statement which returns that obejct (line 50) doesnt work. Which is strange because the statement on line 50 looks to me like it returns a Date object and that it should work.

Any clues as to how to fix this?

Code:

#include<iostream>


using namespace std;


class Date{
public:
    friend ostream& operator <<(ostream& out, Date& d){
        out << d.day << "/" << d.month << "/" << d.year << endl;
    }
    Date(int y, int m, int d) : year(y), month(m), day(d){


    };
private:
    int year, month, day;
};


class Publication{
public:
    Publication(const string& p, const Date& d, const string& t) : publisher(p), date(d), title(t){


    };
    Date GetDate() const {
        return date;
    };
    string GetPublisher(){
        return publisher;
    };
    string GetTitle(){
        return title;
    };


private:
    string publisher;
    Date date;
    string title;
};


int main (){
    Date date1(2018,10,23);
    cout << date1;//THIS WORKS
    Publication publication1("MegaPublishers", date1, "HowToBeAMillionare");
    cout << publication1.GetPublisher();//THIS ALSO WORKS
    cout << publication1.GetDate();//BUT THIS DOESNT
    return 0;
}

This:

Code:

friend ostream& operator <<(ostream& out, Date& d){

should have been:

Code:

friend ostream& operator <<(ostream& out, const Date& d){

as the act of printing an object does not change its observable state.

Also, this:

Code:

string GetPublisher(){

should have been:

Code:

string GetPublisher() const{

and likewise for GetTitle. For some reason you got it right for GetDate.