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Overloading +
OO in C++ - beyond the basics example from a Well House Consultants training course
More on OO in C++ - beyond the basics [link]

This example is described in the following article(s):
   • C++ - putting the language elements together into a program - [link]
   • Printing objects in C++ - [link]

Source code: shapes_03.cpp Module: C233
#include <iostream>
using namespace std;

/* An extension of the demonstration of a polymorphic array of various
types of shape on the heap, adding in addition opeators showing how we
can overload "+" for one / each of our object types. The demonstration
also defines a method called "add" on Circles, which illustrates what's
really going on in the method definition for "+", and then calls it up
using the standard OO notation rather than the icing provided by the use
of the language's operator. */

class Shape {
                float width;
                float height;
                // bool cached; Later ;-)
                virtual float getarea() {} ; };
class Circle: public Shape {
                Circle(float radius);
                float getarea() ;
                Circle operator+(Circle);
                Circle add(Circle); };
class Rectangle: public Shape {
                Rectangle(float width, float height);
                float getarea() ; };
class Square: public Rectangle {
                Square(float size); };

Square::Square (float size) : Rectangle(size, size) { }
Rectangle::Rectangle(float width, float height) {
        this->width = width; this->height = height; }
Circle::Circle (float radius) {
        this->width = this->height = radius * 2.0; }

float Circle::getarea() { return width * width * 3.14159265 / 4.0; }
float Rectangle::getarea() { return width * height ; }

Circle Circle::operator+(Circle that) {
        return(Circle((this->width + that.width)/2.0));
Circle Circle::add(Circle that) {
        return(Circle((this->width + that.width)/2.0));

int main (int argc, char **argv) {

        Shape * papers[5];

        // Put the objects on the heap

        papers[0] = new Circle(15.0);
        papers[1] = new Rectangle(34.0,2.0);
        papers[2] = new Circle(2.3) ;
        papers[3] = new Square(3.33);
        papers[4] = new Rectangle(5.61,7.92);

        for (int k=0; k<5; k++) {
                Shape * current = papers[k];
                cout << "Area is " << current->getarea() << endl;

        Circle * left = new Circle(12.5);
        Circle * right = new Circle(16.6);

        // Circle all = (*left).operator+(*right);
        // Above is a nasty thing to try to write, so ...
        Circle all = *left + *right;
        cout << "ADDED to get " << all.getarea() << endl;

        // Same thing ... with an "add" method rather than operator+
        Circle eall = (*left).add(*right);
        cout << "ADDED to get " << eall.getarea() << endl;


/* Sample Output

munchkin:c5 grahamellis$ ./shapes_03
Area is 706.858
Area is 68
Area is 16.619
Area is 11.0889
Area is 44.4312
ADDED to get 2660.33
ADDED to get 2660.33
munchkin:c5 grahamellis$


Learn about this subject
This module and example are covered on the following public courses:
 * Learning to program in C and C++
 * C++ for C Programmers
 * C and C++ Programming
 * Learning to program in C and C++
 * C and C++ Programming
Also available on on site courses for larger groups

Books covering this topic
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Other Examples
This example comes from our "OO in C++ - beyond the basics" training module. You'll find a description of the topic and some other closely related examples on the "OO in C++ - beyond the basics" module index page.

Full description of the source code
You can learn more about this example on the training courses listed on this page, on which you'll be given a full set of training notes.

Many other training modules are available for download (for limited use) from our download centre under an Open Training Notes License.

Other resources
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