overloading shift left and shift right operator (cin and cout)

Question

I've already made a Point class here . Everything works fine when I write

cout << p1 << endl; //which p is a Point

but when I have two objects of Point and write

cout << (p1 + p2) << endl; //or (p1 - p2) and etc...

i get errors. You can see the errors here. I don't know the reason. please help.

Answer 1

Your issue is that you are attempting to pass an rvalue into a function which accepts a non-const lvalue reference. This is invalid . To fix the issue, just take the Point argument by const reference:

ostream &operator<<(ostream &output, const Point &p);

Answer 2

The error should come from the output operator signature: instead of having:

ostream &operator<<(ostream &output, Point &p){
    output << '(' << p._x << ", " << p._y << ')';
    return output;
}

you should have:

ostream &operator<<(ostream &output, const Point &p) { // notice const here
    output << '(' << p._x << ", " << p._y << ')';
    return output;
}

This is because (p1 + p2) returns a temporary and that needs to bind to a const reference.

Answer 3

Here is corrected code

You need to add const specifier, like this

ostream &operator<<(ostream&, const Point&);

It's offtopic, but your input is would not work with output, since you read two doubles separated by space, but output it parentheses and comma.