Implementing operator<< for a derived class - c++

Question

So I have a base class (Shape) and three derived classes, Circle, Rectangle and Square (Square is derived from Rectangle) I'm attempting to implement operator<< which just calls the correct display function for what called it. However, I don't think I have the syntax correct. Here's a snippet--where have I gone wrong?
class Shape
{
public:
Shape(double w = 0, double h = 0, double r = 0)
{
width = w;
height = h;
radius = r;
}
virtual double area() = 0;
virtual void display() = 0;
protected:
double width;
double height;
double radius;
};
ostream & operator<<(ostream & out, const Shape & s)
{
s.display(out);
return out;
}
class Rectangle : public Shape
{
public:
Rectangle(double w, double h) : Shape(w, h)
{
}
virtual double area() { return width * height; }
virtual void display()
{
cout << "Width of rectangle: " << width << endl;
cout << "Height of rectangle: " << height << endl;
cout << "Area of rectangle: " << this->area() << endl;
}
};

Answer 1

You're calling display like this:
s.display( out );
But display is defined as:
vritual void display() = 0;
The function was declared and defined taking no parameters. It should take a reference to std::ostream as a parameter:
virtual void display(std::ostream &) = 0;
It should also be a const method as you're passing in a const object through the operator << overload:
virtual void display(std::ostream &) const = 0;
Don't forget that in the definition of display you should be writing to the ostream object, not specifically std::cout.
Here is a compiling program on Ideone.

Answer 2

You have a number of problems here. Firstly, let's deal with the printing problem:
ostream & operator<<(ostream & out, const Shape & s)
Here, you're passing a const Shape. This means you can only call const methods on the s you pass in. However, you haven't marked any of the methods in the base (or derived) class as const. Neither area nor display should change the state of the object. Secondly, you're trying to call s.display(out), that is, passing an ostream& to display. The function signature you have doesn't reflect this. So putting that all together we get:
virtual double area() const = 0;
virtual void display(ostream& out) const = 0;
You've also got some other problems - a base class that doesn't declare a virtual destructor. If you're planning on using a class polymorphically, it must have a virtual destructor:
virtual ~Shape() { }
You also need to amend your methods in the derived class:
double area() const { return width * height; }
void display(ostream& out) const
{
out << "Width of rectangle: " << width << endl;
out << "Height of rectangle: " << height << endl;
out << "Area of rectangle: " << area() << endl;
}
Note that display in Rectangle was always printing to cout beforehand.

Answer 3

You almost got it right, here is the working solution:
#include <iostream>
using std::cout;
using std::endl;
using std::ostream;
class Shape
{
public:
Shape(double w = 0, double h = 0, double r = 0)
{
width = w;
height = h;
radius = r;
}
virtual ~Shape() {} // Recommended!
virtual double area() const = 0;
virtual void display(ostream & out) const = 0;
protected:
double width;
double height;
double radius;
};
ostream & operator<<(ostream & out, const Shape & s)
{
// Since `s` is `const`, then `display` method should be `const` too.
s.display(out);
return out;
}
class Rectangle : public Shape
{
public:
Rectangle(double w, double h) : Shape(w, h)
{
}
virtual double area() const { return width * height; }
virtual void display(ostream & out) const
{
// Since `display` method is `const`, then `area` method should be
// `const` too.
out << "Width of rectangle: " << width << endl;
out << "Height of rectangle: " << height << endl;
out << "Area of rectangle: " << this->area() << endl;
}
};
void main() {
Rectangle r(1, 2);
cout << r << endl;
}
Please, pay attention to the const qualifiers which enforce const-correctness of class methods. I've added some useful comments so that you can follow the logic smoothly. Take it as a rule of thumb, if the method does not modify class members, then you should declare it const.