Operator function: I'm not following the flow.

I'm a little confused on how the program reads the math operations for this line:

Code:

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Point point4 = point1 + point2 + point3

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It seems to me that the way the code is set up it can only add two point objects at a time.

Code:

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Point operator+(const Point &pt) {return add(pt);}

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How does the operator function know how to add the sum of point1 and point2 to point3. Is this where the inline function

Code:

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{return add(pt)}

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comes in?

I know this is very basic to some. I'm having a hard time understanding this.

Complete code is below:

Code:

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//this program sets three Point objects to non-negative, then adds them creating
//the 4th Point object.

#include <iostream>
using namespace std;

class Point {
private:            // Data members (private)
    int x, y;
public:              // Constructors
    Point() {};
    Point(int new_x, int new_y) {set(new_x, new_y);}
    Point(const Point &src) {set(src.x, src.y);}

// Operations

    Point add(const Point &pt);
    Point operator+(const Point &pt) {return add(pt);}
   
// Other member functions

    void set(int new_x, int new_y);
    int get_x() const {return x;}
    int get_y() const {return y;}
};

int main() {

    Point point1(20, 20);
    Point point2(0, 5);
    Point point3(-10, 25);
    Point point4 = point1 + point2 + point3;

    cout << "The point is " << point4.get_x();
    cout << ", " << point4.get_y() << "." << endl;

    system("PAUSE");
    return 0;
}

void Point::set(int new_x, int new_y) {
    if (new_x < 0)
        new_x *= -1;
    if (new_y < 0)
        new_y *= -1;
    x = new_x;
    y = new_y;
}

Point Point::add(const Point &pt) {
    Point new_pt;
    new_pt.x = x + pt.x;
    new_pt.y = y + pt.y;
    return new_pt;
}

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Just the same way the compiler does

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a = 3 + 7 + 11;

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You can only add two numbers at a time.

Let me see if I'm understanding correctly.

Code:

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Point point4 = point1 + point2 + point3;

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This has two function calls to:

Code:

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Point operator+(const Point &pt) {return add(pt);}

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The first one I understand how that works. In the second call what would be "this object"? Would that be the new_pt from

Code:

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{return add(pt);}

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Yes. Incidentally, your member operator+ should be declared const since it does not change the observable state of the current object.

Quote:

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Originally Posted by laserlight

Yes. Incidentally, your member operator+ should be declared const since it does not change the observable state of the current object.

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I thought it was

Code:

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Point operator+(const Point &pt) {return add(pt);}

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Quote:

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Originally Posted by tabl3six

I thought it was

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That declares the parameter as a const reference. I am talking about:

Code:

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Point operator+(const Point &pt) const
{
    return add(pt);
}

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Of course, add should be declared const as well.

so the final statement should be:

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Point operator+(const Point &pt) const {return const add(pt);}

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No. That would likely be a syntax error. Declaring add as const means:

Code:

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Point add(const Point &pt) const;

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No const inside the curlicues.

I think I understand thanks for the help.