C++, Printout (<<) operator overloading

Question

I am a beginner at C++ and I am trying to make a program that uses 2 points, adds them together to a line. Then adds a line together with a point, and gets a polygon.

I am trying, unsuccessfully, to overload the operator << so that I can print out my line:

#include <iostream>
using namespace std;

class Line {
private: 
    OnePoint onevalue; 
    OnePoint twovalue; 
public: 
    Line(OnePoint a, OnePoint b) {
        onevalue = a; 
        twovalue = b; 
    }

ostream& operator<<(ostream& print, Line& linje){ // Error right here. 
    print << linje.onevalue << ',' << linje.twovalue; // Too many 
    return print; // parameters for this type of function
}    
};

class OnePoint {
private: 
    double xvalue; 
    double yvalue; 
public:
    OnePoint(double x = 0.0, double y = 0.0) {
    xvalue = x;
    yvalue = y;
}

friend ostream& operator<<(ostream& printh, OnePoint& cPoint) {
     printh << "(" << cPoint.xvalue << ',' << cPoint.yvalue << ")";
     return printh;
}

void Plus(OnePoint a) {
    xvalue = xvalue + a.xvalue; 
    yvalue = yvalue + a.yvalue; 
}

void Minus(OnePoint b) {     
    xvalue = xvalue + b.xvalue;
    yvalue = yvalue + b.yvalue;
}

OnePoint Plustwo(OnePoint a) {
     return (xvalue + a.xvalue, yvalue - a.yvalue); 
}

void Change(double a, double b) {
      xvalue += a;
      yvalue += b; 
}

void Print(OnePoint b) {
      cout << xvalue << "," << yvalue << endl; 
}

OnePoint operator+(OnePoint a) {
       OnePoint temp; 
       temp.xvalue = xvalue + a.xvalue; 
       temp.yvalue = yvalue + a.yvalue; 
       return temp; 
}        
};

//--------------------------------------------------------------------        
int main(){  
    OnePoint a(3.0, 3.0); 
    OnePoint b(1.0, 1.0);  
    OnePoint d(1.0, 4.0);
    OnePoint c; 

    c = a + b + d;         
    c.Print(c);        
}

Edit:

I can now cout my OnePoint, but I cannot cout Line.

See also: http://stackoverflow.com/questions/236801/should-operator-be-implemented-as-a-friend-or-as-a-member-function (similar, though not an exact duplicate) — anderas, Sep 24 '15 at 13:38
you should start considering separating your code into header file `.h` (where you keep your class/member declarations) and a `.cpp` file where you implement them. — Ziezi, Sep 26 '15 at 16:11

score 1 · Answer 1 · answered Sep 24 '15 at 13:37

1

As a member function, the operator gets an extra hidden parameter for the this pointer.

You can make it a free function by declaring it a friend of the class:

class Line {
  // other members
public: 

  friend ostream& operator<<(ostream& print, Line& linje){
    print << linje.onevalue << ',' << linje.twovalue; 
    return print;             
  }

};

answered Sep 24 '15 at 13:37

Bo Persson

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I still don't get what's wrong, as it still doesn't work to write: OnePoint a(3.0, 3.0); OnePoint b(1.0, 1.0); OnePoint d(1.0, 4.0); Line d(a, b); cout << d; – David Lund Sep 24 '15 at 14:09
It solves the problem for `Line`. Now you have to solve the same problem for `OnePoint`. – Bo Persson Sep 24 '15 at 16:04
So I added the same to my code for my class OnePoint. It still doesn't work to cout a "line". I can cout a "onepoint" though. – David Lund Sep 26 '15 at 14:58
Oh, I got it. OnePoint needed to be above the Line class. – David Lund Sep 26 '15 at 15:31

C++, Printout (<<) operator overloading

Edit:

1 Answers1