1. Derived Classes

2. C++ 2 Outline  Definition  Virtual functions  Virtual base classes  Abstract classes. Pure virtual functions.

3. C++ 3 Definition class Derived : list-of-base-classes { // new data member and member functions };  The list of base classes is formed from: public base_class protected base_class private base_class

4. C++ 4 Example (base class list) class ClassName : public C_1, …, public C_n { // … };  Class ClassName is derived from: C_1,..., C_n.

5. C++ 5 Access Control In the base class Base class access specifier In the derived class public protectedpublicprotected privatepublicno access publicprotected privateprotectedno access publicprivate protectedprivate no access

6. C++ 6 The Constructor of a Derived Class  Derived classes don’t inherit constructors and destructors.  The constructor of the derived class: ClassName(list-of-parameters) : C_1(list1),..., C_n(list_n) { // … }

7. C++ 7 Example #include using namespace std; class Base { public: void f1(); void f2(); };

8. C++ 8 The Derived Class class Derived : public Base { public: void f1(); };  Override only the f1 function.  Function f2 will be inherited from Base.

9. C++ 9 The Member Functions of the Base Class void Base::f1() { cout << "Base: f1\n"; } void Base::f2() { cout << "Base: f2\n"; f1(); }

10. C++ 10 Member Function of the Derived Class void Derived::f1() { cout << "Derived: f1\n"; }

11. C++ 11 The main function int main() { Derived d; d.f2(); }  The selection of the f1 function has been done in compile time. Base: f2 Base: f1 Output:

12. C++ 12 Virtual functions class Base { public: virtual void f1(); void f2(); };  If function f1 is declared as virtual, then the selection of the file will be done in running-time.  We have to place only one virtual keyword in front of the declaration of the f1 function, in the base class.  In this case all inherited f1 functions will be considered virtual.

13. C++ 13 The main Function int main() { Derived d; d.f2(); } Base: f2 Derived: f1 Output:

14. C++ 14 Virtual Base Classes  In case of multiple inheritance a derived class can inherit multiple issues of a data member. Animal DomesticMammal Dog

15. C++ 15 The Animal Class #include using namespace std; class Animal { protected: char name[20]; public: Animal(char* n); };

16. C++ 16 The Mammal Class class Mammal : public Animal { protected: int weight; public: Mammal(char* n, int w); };

17. C++ 17 The Domestic Class class Domestic : public Animal { protected: int comportment; public: Domestic(char* n, int c); };

18. C++ 18 The Dog Class class Dog : public Mammal, public Domestic { protected: bool bark; public: Dog(char* n, int w, int c, bool b); void Display(); };

19. C++ 19 Constructor of the Animal Class Animal::Animal(char* n) { strcpy(name, n); }

20. C++ 20 Other Constructors Mammal::Mammal(char* n, int w): Animal(n) { weight = w; } Domestic::Domestic(char* n, int c): Animal(n) { comportment = c; }

21. C++ 21 Constructor of the Dog Class Dog::Dog(char* n, int w, int c, bool b): Mammal(n, w), Domestic(n, c) { bark = b; }

22. C++ 22 The Display Member Function void Dog::Display() { cout << "name (mammal): " << Mammal::name << endl; cout << "name (domestic): " << Domestic::name << endl;

23. C++ 23 The Display Member Function cout << "weight: " << weight << endl; cout << "comportment: " << comportment << endl; if ( bark ) cout << "barking\n"; else cout << "no barking"; }

24. C++ 24 The main Function int main() { Dog v("Hungarian Vizsla", 12, 9, true); v.Display(); }  In the Display member function we can’t access the name data member simply, because this data member was inherited in two different way.

25. C++ 25 Output name (mammal): Hungarian Vizsla name (domestic): Hungarian Vizsla weight: 12 comportment: 9 barking  We can access the name data member in the Dog class only by using the scope operator.

26. C++ 26 Virtual Base Class  If we would like to have only one issue of the name data member we have to use virtual base classes.  Thus, we have to place the virtual keyword in the base class list in front of the class (if we intend to make that base class virtual).

27. C++ 27 The Mammal Class class Mammal : public virtual Animal { protected: int weight; public: Mammal(char* n, int w); };

28. C++ 28 The Domestic Class class Domestic : public virtual Animal { protected: int comportment; public: Domestic(char* n, int c); };

29. C++ 29 Constructor of the Dog Class Dog::Dog(char* n, int w, int c, bool b): Animal(n), Mammal(n, w), Domestic(n, c) { bark = b; }  Mammal and Domestic doesn’t call Animal automatically.

30. C++ 30 The Display Member Function void Dog::Display() { cout << "name (mammal): " << name << endl; cout << "weight: " << weight << endl; cout << "comportment: " << comportment << endl; if ( bark ) cout << "barking\n"; else cout << "no barking"; }

31. C++ 31 The main Function int main() { Dog v("Hungarian Vizsla", 12, 9, true); v.Display(); }  We can access the name data member without using the scope operator.

32. C++ 32 Output name: Hungarian Vizsla weight: 12 comportment: 9 barking

33. C++ 33 Abstract Classes. Pure Virtual Functions  A base class can have some known features, but we are not able to define them, only in the derived class.  In this case we declare a virtual function, but we don’t define it in the base class.  If a virtual member function is declared in the base class, but isn’t defined, we call it a pure virtual function.

34. C++ 34 Declaration of Pure Virtual Functions  Pure virtual functions are declared in the regular way, but the declaration ends with =0. This means, that we don’t want to define the function right now.  If a class contains at least one pure virtual function, then we name it abstract class.  No instance of an abstract class can be defined.

35. C++ 35 Overriding the Pure Virtual Functions  We have to override all pure virtual functions in the derived class.  In other case the derived class will be also abstract.

36. C++ 36 Example Animal Dove Bear Horse

37. C++ 37 The Animal Class #include using namespace std; class Animal { protected: double weight; double age; double speed; public:

38. C++ 38 The Animal Class Animal( double w, double a, double s); virtual double average_weight() = 0; virtual double average_age() = 0; virtual double average_speed() = 0; int fat() { return weight > average_weight(); } int fast() { return speed > average_speed(); } int young() { return 2 * age < average_age(); } void display(); };

39. C++ 39 Constructor of the Animal Class Animal::Animal( double w, double a, double s) { weight = w; age = a; speed = s; }

40. C++ 40 The display Member Function void Animal::display() { cout << ( fat() ? "fat, " : "thin, " ); cout << ( young() ? "young, " : "old, " ); cout << ( fast() ? "fast" : "slow" ) << endl; }

41. C++ 41 The Dove Class class Dove : public Animal { public: Dove( double w, double a, double s): Animal(w, a, s) {} double average_weight() { return 0.5; } double average_age() { return 6; } double average_speed() { return 90; } };

42. C++ 42 The Bear Class class Bear: public Animal { public: Bear( double w, double a, double s): Animal(w, a, s) {} double average_weight() { return 450; } double average_age() { return 43; } double average_speed() { return 40; } };

43. C++ 43 The Horse Class class Horse: public Animal { public: Horse( double w, double a, double s): Animal(w, a, s) {} double average_weight() { return 1000; } double average_age() { return 36; } double average_speed() { return 60; } };

44. C++ 44 The main function void main() { Dove d(0.6, 1, 80); Bear b(500, 40, 46); Horse h(900, 8, 70); d.display(); b.display(); h.display(); }

45. C++ 45 Output fat, young, slow fat, old, fast thin, young, fast