Calling constructor with multiple arguments

Question

I have

Triangle::Triangle()
{
    A = NULL;
    B = NULL;
    C = NULL;
}
Triangle::Triangle(Point& X,Point& Y, Point& Z)
{
    A = new Point;
    *A = X;
    B = new Point;
    *B = Y;
    C = new Point;
    *C = Z;
}

and 

istream& operator>>(istream& in, Triangle& T)
{
    Point X,Y,Z;
    in>>X>>Y>>Z;
    Triangle T(X,Y,Z);  
    return in;
}

Where Point is another class which defines a point with coordonates X and Y. I don't know how to call the constructor with multiple arguments in the overloaded function. Can you help me?

Any reason you're using pointers for the class's points and taking the constructor arguments by non-const reference? — Qaz
– Qaz, Commented Mar 2, 2013 at 15:15
You shouldn't use a constructor in that function, not if you're passing in a Triangle by reference. — Beta
– Beta, Commented Mar 2, 2013 at 15:17

ruben2020 · Accepted Answer · 2013-03-02 16:04:06Z

3

This is how you do it:

Point px;
Point py;
Point pz;
Triangle trig(px, py, pz);

trig will be the object, which is an instance of class Triangle and the above would call the constructor with 3 Point arguments.

Another way is for pointers:

 Triangle *pTrig = new Triangle(pX, pY, pZ);

In addition, I suggest, that this would be better:

Triangle::Triangle()
   : A(NULL), B(NULL), C(NULL)
{
}

Triangle::Triangle(const Point& X,const Point& Y, const Point& Z)
 : A(new Point(X)), B(new Point(Y)), C(new Point(Z))
{
}

Assuming that Point has a copy constructor.

You want to call it from inside the operator>> function to update argument T, but this won't work, because you cannot call the constructor on something that's already constructed. Instead, what you need is to implement an assignment operator. Please see http://en.wikipedia.org/wiki/Assignment_operator_%28C%2B%2B%29 for more information.

Then you can do T = Triangle(X,Y,Z);

To implement the assignment operator, you can do this:

Triangle& Triangle::operator= (const Triangle& other)
{
    if (this != &other) // protect against invalid self-assignment
    {
        if (A != NULL) delete A;
        if (B != NULL) delete B;
        if (C != NULL) delete C;
        A = new Point(other.A);
        B = new Point(other.B);
        C = new Point(other.C);
    }
    return *this;
}

Assuming Point has copy constructors. To implement copy constructors, please see http://en.wikipedia.org/wiki/Copy_constructor

A copy constructor looks like the following, but you need to do it for Point:

Triangle& Triangle::Triangle(const Triangle& other)
  : A(new Point(other.A)), B(new Point(other.B)), C(new Point(other.C))
{
}
}

edited Mar 2, 2013 at 16:04

answered Mar 2, 2013 at 15:18

ruben2020

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2 Comments

ruben2020 Over a year ago

Ahh... now I understand... you want to call it inside the operator>> function. No... that does not work. Let me update my answer.

user2116010 Over a year ago

Great!!It's working and you've done the assignment operator function too. Great! Thank you very much!

Yasser Mohseni · Accepted Answer · 2013-03-02 15:25:32Z

0

The first two constructors are overrides for default constructor. The third function is operator overloading which overloads >> operator. You just need to create an object of Triangle class as follow:

Triangle tr(x,y,z);

or

Triangle* tr = new Triangle(x,y,z);

Where x, y, and z are objects of Point class.

By the way, as you can see in your operator overloading (the third function), you are already creating an object of class Triangle (Triangle T(X,Y,Z);).

answered Mar 2, 2013 at 15:25

Yasser Mohseni

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1 Comment

user2116010 Over a year ago

Yes but the argument of the operator overloading is the first constructor and won't pass the correct values for X,Y and Z

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