Operator function: I'm not following the flow.

This is a discussion on Operator function: I'm not following the flow. within the C++ Programming forums, part of the General Programming Boards category; I'm a little confused on how the program reads the math operations for this line: Code: Point point4 = point1 ...

  1. #1
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    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:
    Point point4 = point1 + point2 + point3
    It seems to me that the way the code is set up it can only add two point objects at a time.

    Code:
    Point operator+(const Point &pt) {return add(pt);}
    How does the operator function know how to add the sum of point1 and point2 to point3. Is this where the inline function
    Code:
    {return add(pt)}
    comes in?

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

    Complete code is below:

    Code:
    //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;
    }

  2. #2
    and the Hat of Guessing tabstop's Avatar
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    Just the same way the compiler does
    Code:
    a = 3 + 7 + 11;
    You can only add two numbers at a time.

  3. #3
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    Let me see if I'm understanding correctly.
    Code:
    Point point4 = point1 + point2 + point3;
    This has two function calls to:
    Code:
    Point operator+(const Point &pt) {return add(pt);}
    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:
    {return add(pt);}

  4. #4
    C++ Witch laserlight's Avatar
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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 Originally Posted by Bjarne Stroustrup (2000-10-14)
    I get maybe two dozen requests for help with some sort of programming or design problem every day. Most have more sense than to send me hundreds of lines of code. If they do, I ask them to find the smallest example that exhibits the problem and send me that. Mostly, they then find the error themselves. "Finding the smallest program that demonstrates the error" is a powerful debugging tool.
    Look up a C++ Reference and learn How To Ask Questions The Smart Way

  5. #5
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    Quote Originally Posted by laserlight View Post
    Yes. Incidentally, your member operator+ should be declared const since it does not change the observable state of the current object.
    I thought it was
    Code:
    Point operator+(const Point &pt) {return add(pt);}

  6. #6
    C++ Witch laserlight's Avatar
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    Quote Originally Posted by tabl3six
    I thought it was
    That declares the parameter as a const reference. I am talking about:
    Code:
    Point operator+(const Point &pt) const
    {
        return add(pt);
    }
    Of course, add should be declared const as well.
    Quote Originally Posted by Bjarne Stroustrup (2000-10-14)
    I get maybe two dozen requests for help with some sort of programming or design problem every day. Most have more sense than to send me hundreds of lines of code. If they do, I ask them to find the smallest example that exhibits the problem and send me that. Mostly, they then find the error themselves. "Finding the smallest program that demonstrates the error" is a powerful debugging tool.
    Look up a C++ Reference and learn How To Ask Questions The Smart Way

  7. #7
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    so the final statement should be:
    Code:
    Point operator+(const Point &pt) const {return const add(pt);}

  8. #8
    C++ Witch laserlight's Avatar
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    No. That would likely be a syntax error. Declaring add as const means:
    Code:
    Point add(const Point &pt) const;
    Quote Originally Posted by Bjarne Stroustrup (2000-10-14)
    I get maybe two dozen requests for help with some sort of programming or design problem every day. Most have more sense than to send me hundreds of lines of code. If they do, I ask them to find the smallest example that exhibits the problem and send me that. Mostly, they then find the error themselves. "Finding the smallest program that demonstrates the error" is a powerful debugging tool.
    Look up a C++ Reference and learn How To Ask Questions The Smart Way

  9. #9
    and the Hat of Guessing tabstop's Avatar
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    No const inside the curlicues.

  10. #10
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    I think I understand thanks for the help.

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