1. C++ Programming Inheritance
Design
Example classes
Virtual functions
Pure virtual functions
Abstract classes

2. Relationships between Classes "Is a"
The relationship "Is a" ( "Inherits from")
A class (A) can inherit the state (variables) and behaviour (methods) of another class (B)
We say class A is a base class (or a generalization, or a super-type) of class B and class B is a class derived from Class A (or a specialization, or a subtype)
For example, leptons, hadrons, bosons are all particles, but belong to different groups, each with their special properties
Particle
Lepton
Hadron
Boson

3. Example
To demonstrate the design and implementation of the inheritence we will develop classes that describe typical family pets
Let's try to find common characteristics typical of our pets - cats and dogs
Each one has a name
They eat - we give them food
They “speak” - dogs bark, cats meow
Specific behaviours:
Cats hunt mice
Dogs guard home

4. Design ...
Dog
name
Eat()
Speak()
Cat
name
Eat()
Speak()
Pet
name
Eat()
Speak()
The common data members and functions can be regrouped in a base class Pet
(The specific behaviors will be considered later)
Dog
Cat

5. Classes (definition) Declaration of the inheritance relationship
#include <string>
class Pet {
public:
Pet(std::string name);
void Eat();
void Speak();
std::string Name();
private:
std::string m_name; };
#include “Pet.h”
#include <string> 
class Dog : public Pet {
public:
Dog(std::string name); };
Declaration of the inheritance relationship
#include “Pet.h”
#include <string> 
class Cat : public Pet {
public:
Cat(std::string name); };
Pet
name
Eat()
Speak()
Dog
Cat

6. Classes (implementation)
#include "Pet.h"
#include <iostream>
Pet::Pet(std::string name)
: m_name(name) { }
void Pet::Eat() {
std::cout << m_name << " eats" << std::endl;
void Pet::Speak() {
std::cout << m_name << " speaks" << std::endl;
std::string Pet::Name() {
return m_name;
#include "Dog.h"
#include <iostream>
Dog::Dog(std::string name)
: Pet(name) { }
#include "Cat.h"
#include <iostream>
Cat::Cat(std::string name)
: Pet(name) { }
Pet
name
Eat()
Speak()
Initialisation of the base class
Dog
Cat

7. Classes (use of) ... main1.cxx
#include "Dog.h"
#include "Cat.h"
int main() {
Dog dog("Lassie");
Cat cat("Tilly");
Cat cat2("Tom");
// When you arrive at home pets welcome you
dog.Speak();
cat.Speak();
cat2.Speak();
// We have to feed them
dog.Eat();
cat.Eat();
cat2.Eat(); }
main1.cxx
Declaration of objects with their specific types (by derived classes)
Calling functions defined in the base class
[localhost]> ./atHome1
Lassie speaks
Tilly speaks
Tom speaks
Lassie eats
Tilly eats
Tom eats

8. ... Classes (use of) main.cxx
The existence of the base class allows us to use our objects in generic way
#include "Dog.h"
#include "Cat.h"
#include <vector>
int main() {
// The collection of our pets
std::vector<Pet*> ourPets;
ourPets.push_back(new Dog("Lassie"));
ourPets.push_back(new Cat("Tilly"));
ourPets.push_back(new Cat("Tom"));
// When you arrive at home pets welcome you
std::vector<Pet*>::iterator it;
for ( it=ourPets.begin(); it != ourPets.end(); it++ ) {
Pet* pet = *it;
pet->Speak(); }
/// We have to feed them
(*it)->Eat();
Using objects by their base type (Pet *)
[localhost]> ./atHome
Lassie speaks
Tilly speaks
Tom speaks
Lassie eats
Tilly eats
Tom eats

9. Specialization of functions ...
We will make our model more precise:
The Dog eats the granules ProPlan
The Cat eat the cans Whiskas
The Dog barks
The Cat meow
Modelling:
Pet
name
Eat()
Speak()
Each derived class redefines the functions of the base class
Dog
Eat()
Speak()
Cat
Eat()
Speak()

10. ... Specialization of functions
We will add these methods to the classes Dog and Cat
For the correct function to be called, it must be declared as being virtual
The "correct" function = the function defined for the class whose dynamic type corresponds to the object
The object defined by reference Pet1 is of Dog type, Pet2 of Cat type
The virtual function acts as an interface to the functions defined in the base class and all the derived classes
#include <string>
class Pet {
public:
Pet(std::string name);
virtual void Eat();
virtual void Speak();
private:
std::string m_name; };
#include “Pet.h”
#include <string> 
class Dog : public Pet {
public:
Dog(std::string name);
virtual void Eat();
virtual void Speak(); };
#include “Pet.h”
#include <string> 
class Cat : public Pet {
public:
Cat(std::string name);
virtual void Eat();
virtual void Speak(); };
It is not obligatory to re-declare virtual functions in derived classes, but it's good practice to do so
La déclaration de la relation de héritage

11. ... Specialization of functions ...
In implementing these functions, there is nothing special:
Dog::Eat() {
std::cout << Name() << " eats the granules ProPlan " << std::endl; }
void Dog::Speak() {
std::cout << Name() << ": woof, woof !! " << std::endl;
Cat::Eat() {
std::cout << Name() << " eats the cans Whiskas “ << std::endl; }
void Cat::Speak() {
std::cout << Name() << ": meow, meow !! " << std::endl;

12. ... Specialization of functions
We compiles and run the two programs again:
[localhost] > ./atHome1
Lassie: woof, woof !!!
Tilly: meow, meow !!!
Tom: meow, meow !!!
Lassie eats the granules ProPlan
Tilly eats the cans Whiskas
...
If objects were declared with their specific types (through derived classes)
If objects were declared with their generic type (through a base class)
[localhost]> ./atHome
Lassie speaks
Tilly speaks
Tom speaks
Lassie eats
Tilly eats
...
[localhost] > ./atHome
Lassie: woof, woof !!!
Tilly: meow, meow !!!
Tom: meow, meow !!!
Lassie eats the granules ProPlan
Tilly eats the cans Whiskas
...
When functions are not declared virtual, it is the function of the declared type that is called, without taking into account the actual type of object
When functions are declared virtual, it is the function of the real type of object, which is called

13. If You Buy a Canary We add a new class derived from the class Pet:
#include “Pet.h”
#include <string> 
class Canary : public Pet {
public:
Canary(std::string name); };
#include "Canary.h"
#include <iostream>
Canary::Canary(std::string name)
: Pet(name) { }
The Canary class does not specify the methods of the base class
main4.cxx
// ...
#include "Canary.h"
int main() {
Canary* canary = new Canary("Robbie");
ourPets.push_back(canary); }
[localhost]> ./atHome
Lassie: woof, woof !!!
Tilly: meow, meow !!!
Tom: meow, meow !!!
Robbie speaks
Lassie eats the granules ProPlan
Tilly eats the cans Whiskas
Tom eats the cans Whiskas
Robbie eats

14. Pure Virtual Functions ...
We are going to re-examine the requirements of our program
Imagine that we leave our pets to our neighbour and ask them to replace us during our holidays
Our program will help him know the needs of our pets:
But what does Robbie eat ?
Our base class enabled us avoid to specify the needs of our canary
[localhost]> ./atHome
Lassie: woof, woof !!!
Tilly: meow, meow !!!
Tom: meow, meow !!!
Robbie speaks
Lassie eats the granules ProPlan
Tilly eats the cans Whiskas
Tom eats the cans Whiskas
Robbie eats

15. ... Pure Virtual Functions ...
The base class can require all derived classes to implement a function:
To do this the function must be declared as a pure virtual function
#include <string>
class Pet {
public:
Pet(std::string name);
virtual void Eat() = 0;
virtual void Speak();
private:
std::string m_name; };
void Pet::Eat() {
std::cout << m_name << " eats" << std::endl; }
The pure virtual functions have no implementation

16. ... Pure Virtual Functions
We try to compile our program ... and it does not compile!
It is impossible to create an instance:
of the class that has not defined all inherited pure virtual functions
Classes whose instance cannot be created are called abstract classes
[localhost] > g++ -I. *.cxx -o atHome4
main4.cxx: In function ‘int main()’:
main4.cxx:10: error: cannot allocate an object of abstract type ‘Canary’
Canary.h:4: note: because the following virtual functions are pure within ‘Canary’:
Pet.h:10: note: virtual void Pet::Eat()
Pet.h:11: note: virtual void Pet::Speak()

17. Specific functions ... We will consider specific behaviors:
The cat chases the mouse
Dog guards house
Dog
name
Eat()
Speak()
Guard()
Cat
name
Eat()
Speak()
HuntMice
Pet
name
Eat()
Speak()
The specific functions are added only in derived classes
Dog
Guard()
Cat
HuntMice()

18. ... Specific functions ... main1.cxx main.cxx
#include "Dog.h"
#include "Cat.h"
int main() {
Dog dog("Lassie");
Cat cat("Tilly");
Cat cat2("Tom");
// When you arrive at home pets welcome you
dog.Speak();
...
// We have to feed them
dog.Eat();
// And everyone continues in their work
dog.Guard();
cat.HuntMice(); }
main1.cxx
main.cxx
#include "Dog.h"
#include "Cat.h"
#include <vector>
int main() {
// The collection of our pets
std::vector<Pet*> ourPets;
ourPets.push_back(new Dog("Lassie"));
...
// When you arrive at home pets welcome you
// We have to feed them
// And everyone continues in their work
for ( it=ourPets.begin(); it != ourPets.end(); it++ ) {
Pet* pet = *it;
Dog* dog = dynamic_cast<Dog*>(pet);
If ( dog ) dog->Guard();
Cat* cat = dynamic_cast<Cat*>(pet);
If ( cat ) cat->HuntMice(); }
The specific functions can not be called in a generic way\

19. Conclusions The class inheritance allows us
Avoid duplication of common code
By a common implementation in the base class
Process objects in a generic way
Collections of objects by the type of base class
Call functions with the common interface
By declaring virtual functions
Forcing Derived classes implement the interfaces required
Declaring pure virtual functions