C++ operators and arguments

Question

Hi!

Let's say I have a class Point:

class Point {
    int x, y;
public:
    Point& operator+=(const Point &p) { x=p.x; y=p.y; return *this; }
};

Why can I not call this as such:

Point p1;
p1 += Point(10,10);

And is there any way to do this, while still having a reference as the argument?

Answer 1

Why can I not call this as such:

Because you forgot to declare a matching constructor. Other than that, this call looks fine.

(Also, the code inside your operator += is wrong: it overwrites the values instead of doing additions).

Answer 2

You operator code is perfectly fine. You need to create a constructor which takes two ints:

class Point {
public:
  Point() : x( 0 ), y( 0 ) { }
  Point( int _x, int _y ) : x( _x ), y( _y ) { }
// rest of the code
};

Note that if you declare a constructor which takes some arguments then in order to make instantiation such as Point x; you need to declare a default constructor yourself.

P.S. Just read Konrad's answer. Yes, you might want to use += rather than = for your members. :)

Answer 3

Here's the code you need:

class Point {
    int x,y;
public:
    Point(int x=0,int y=0) : x(x), y(y) {}
    Point& operator+=(const Point&p) {x+=p.x;y+=p.y;return *this;}
};

As Konrad pointed out, you need a constructor. Also you need to explicitly perform the additions inside your operator overload.