6.s096 2013

Lecture 6

Date: January 24, 2013
Topics: C++ Inheritance
Assignment: Assignment 6

- Lecture Notes

Download the slides as a PDF

- Lab Exercises

Take a look at this example code:

#include <stdio.h>

class Shape {
	virtual ~Shape();
	virtual void draw() = 0;

class Circle : public Shape {
	virtual ~Circle();
	virtual void draw();

Shape::~Shape() {
	printf("shape destructor\n");

// void Shape::draw() {
//	printf("Shape::draw\n");
// }

Circle::~Circle() {
	printf("circle destructor\n");

void Circle::draw() {

int main() {
	Shape *shape = new Circle;
	delete shape;

	return 0;

Put it in a file named lab6.cpp then compile it like this:

$ g++ -Wall lab6.cpp -o lab6
$ ./lab6
circle destructor
shape destructor

Verify your understanding of how the virtual keyword and method overriding work by performing a few experiments:

  1. Remove the virtual keyword from each location individually, recompiling and running each time to see how the output changes. Can you predict what will and will not work?
  2. Try making Shape::draw non-pure by removing = 0 from its declaration.
  3. Try changing shape (in main()) from a pointer to a stack-allocated variable.