All the files for the shape demo on one page! Although it's much easier
to write a short program all in one file, as the program grows you'll
need to split it into elements to be maintained on different cycles, tested
one at a time, and shared on an "as needed" basis between lots of programs.

This example file contains the source of a whole series of files, here as
a single page on the web so that you can cut and paste each of them without
having to keep following link after link to get to the next one!

// ---------------- circle.h -----------------

#ifndef CIRCLE

#include "shape.h"

class Circle: public Shape {
        public:
                Circle(float radius);
                float getarea() ; };

#define CIRCLE
#endif

// ---------------- rectangle.h -----------------

#ifndef RECTANGLE

#include "shape.h"

class Rectangle: public Shape {
        public:
                Rectangle(float width, float height);
                float getarea() ; };

#define RECTANGLE
#endif

// ---------------- shape.h -----------------

#ifndef SHAPE

class Shape {
        public:
                float width;
                float height;
                // bool cached; Later ;-)
                virtual float getarea() {} ; };

#define SHAPE
#endif

// ---------------- square.h -----------------

#ifndef SQUARE

#include "rectangle.h"

class Square: public Rectangle {
        public:
                Square(float size);
                void addframe(const float amount);
                } ;
#define SQUARE
#endif

// ---------------- circle.cpp -----------------

#include "circle.h"

Circle::Circle (float radius) {
        this->width = this->height = radius * 2.0; }
float Circle::getarea() { return width * width * 3.14159265 / 4.0; }

// ---------------- rectangle.cpp -----------------

#include "rectangle.h"

Rectangle::Rectangle(float width, float height) {
        this->width = width; this->height = height; }
float Rectangle::getarea() { return width * height ; }

// ---------------- square.cpp -----------------

#include "square.h"

Square::Square (float size) : Rectangle(size, size) { }

void Square::addframe(const float wid) {
        width += wid;
        height += wid;
        }

// ---------------- shape.cpp -----------------

#include "shape.h"

// Yeah, right! Base class has no code at present in this demo

// ---------------- shape_main.cpp -----------------

#include <iostream>
using namespace std;

#include "circle.h"
#include "rectangle.h"
#include "square.h"

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);

        ((Square *)papers[3])->addframe(2.5);

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

}

// ---------------- makefile -----------------

HEADS=shape.h circle.h square.h rectangle.h

hereitis: shape_main.o circle.o shape.o square.o rectangle.o $(HEADS)
        g++ -o hereitis shape_main.o circle.o shape.o square.o rectangle.o

shape_main.o: shape_main.cpp $(HEADS)
        g++ -c shape_main.cpp

shape.o: shape.cpp $(HEADS)
        g++ -c shape.cpp

square.o: square.cpp $(HEADS)
        g++ -c square.cpp

circle.o: circle.cpp $(HEADS)
        g++ -c circle.cpp

rectangle.o: rectangle.cpp $(HEADS)
        g++ -c rectangle.cpp

clean:
        @echo "Cleaning up ..."
        @rm -rf *.o
        @rm hereitis test

test: hereitis
        ./hereitis
        @touch test

// ----------- And some test runs of all of that! -----------

munchkin:c5 grahamellis$ make clean
Cleaning up ...
munchkin:c5 grahamellis$ make
g++ -c shape_main.cpp
g++ -c circle.cpp
g++ -c shape.cpp
g++ -c square.cpp
g++ -c rectangle.cpp
g++ -o hereitis shape_main.o circle.o shape.o square.o rectangle.o
munchkin:c5 grahamellis$ make test
./hereitis
Area is 706.858
Area is 68
Area is 16.619
Area is 33.9889
Area is 44.4312
munchkin:c5 grahamellis$