Introduction to Programming

Transcription

1 Introduction to Programming Summer Term 2015 Dr. Adrian Kacso, Univ. Siegen Tel.: 0271/ , Office: H-B 8406 State: June 17, 2015 Betriebssysteme / verteilte Systeme Introduction to Programming (1) i Introduction to Programming Summer Term Inheritance and Polymorphism Betriebssysteme / verteilte Systeme Introduction to Programming (1) 257

2 9 Inheritance and Polymorphism Contents: Inheritance Polymorphism Abstract classes Yet another example Betriebssysteme / verteilte Systeme Introduction to Programming (1) Inheritance Reminder: classes A class defines a new type: a collection of variables (data members, attributes) a set of related operations (methods, member functions) they define the interface to objects of the class i.e., the data members can (usually) be accessed only via these methods An object is an instance (a variable) of a class Betriebssysteme / verteilte Systeme Introduction to Programming (1) 259

3 9.1 Inheritance Often Classes are quite similar Assume we need classes for cats and dogs: Cat age Dog age Attributes weight weight Methods eat() Why are these classes so similar? breed eat() Speak() Speak() purr() wagtail() Because both cats and dogs are special kinds of mammals! the common attributes / methods are those of mammals Betriebssysteme / verteilte Systeme Introduction to Programming (1) Inheritance Refinement / specialization Cat purr() Mammal age weight eat() Speak() Dog breed wagtail() Class Mammal defines the common attributes and methods Cat and Dog are derived from Mammal They inherit all attributes and methods of Mammal they also can define additional ones Betriebssysteme / verteilte Systeme Introduction to Programming (1) 261

4 9.1 Inheritance Extending the hierarchy of animal classes Animal age weight eat() Mammal Speak() nursebaby() Fish spawn() Cat purr() Dog breed wagtail() Betriebssysteme / verteilte Systeme Introduction to Programming (1) Inheritance Relations between classes Mammal inherits from Animal, and Dog inherits from Mammal A Dog is a Mammal which is an Animal Animal is a (direct) base class (or superclass) of Mammal, Animal and Mammal are base classes (or superclasses) of Dog Mammal is a derived class (or subclass) of Animal, Dog and Mammal are derived classes (or subclasses) of Animal Betriebssysteme / verteilte Systeme Introduction to Programming (1) 263

5 9.1 Inheritance Uses of inheritance Conceptual Modelling: define a hierarchy of classes that represent the concepts of the data specialization / generalization, e.g., cats and dogs as special kinds of mammals this mostly defines hierarchies of interfaces Code re-use: take a given class, and extend it by a little code to get what you need Betriebssysteme / verteilte Systeme Introduction to Programming (1) Inheritance A Mammal class class Mammal { public: Mammal(); ~Mammal(); int getage() const; void setage(int); int getweight() const; void setweight(int); void Speak() const; protected: // allow access by derived classes int itsage; int itsweight; ; Betriebssysteme / verteilte Systeme Introduction to Programming (1) 265

6 9.1 Inheritance Private vs. protected vs. public Class members can be private, protected, or public: private members can be accessed only within methods of the class itself protected members can be accessed from within methods of the class itself, and of its derived classes public members can also be accessed from other classes ( code outside the class itself ) Betriebssysteme / verteilte Systeme Introduction to Programming (1) Inheritance A Cat class class Cat : public Mammal { public: Cat(); ~Cat(); void purr() const; ; public class inheritance means that the public members of the base class are also public for the derived class (there is also private and protected inheritance, but we will not use it in this course) Betriebssysteme / verteilte Systeme Introduction to Programming (1) 267

7 9.1 Inheritance A Dog class enum Breed { YORKIE, DANDIE, SHETLAND, DOBERMAN, LABRADOR ; class Dog : public Mammal { public: Dog(); Dog(Breed breed); ~Dog(); Breed getbreed() const; void wagtail() const; protected: Breed itsbreed; ; Betriebssysteme / verteilte Systeme Introduction to Programming (1) Inheritance The interface of Dog (accumulated) The following methods can be called for a Dog object: Inherited from Mammal Defined by Dog int getage() const; Dog(); void setage(int); Dog(Breed breed); int getweight() const; ~Dog(); void setweight(int); Breed getbreed() const; void Speak() const; void wagtail() const; The constructor and destructor is not inherited!! Betriebssysteme / verteilte Systeme Introduction to Programming (1) 269

8 9.1 Inheritance The data members of Dog (accumulated) A Dog object has the following data members: Inherited from Mammal Defined by Dog int itsage; Breed itsbreed; int itsweight; Betriebssysteme / verteilte Systeme Introduction to Programming (1) Inheritance Constructors and destructors What happens when a Dog is created? first, the Mammal constructor is called it initializes the data members inherited from Mammal then, the Dog constructor is called it initializes the data members added by Dog And when a Dog is deleted? first, the Dog destructor is called, then, the Mammal destructor Betriebssysteme / verteilte Systeme Introduction to Programming (1) 271

9 9.1 Inheritance Passing arguments to base constructors Assume we have the following constructors for Mammal: Mammal(); Mammal(int age); And we want the following ones for Dog: Dog(); Dog(int age, Breed breed); Dog(int age, int weight, Breed breed); How to do this? Betriebssysteme / verteilte Systeme Introduction to Programming (1) Inheritance Passing arguments to base constructors The Mammal constructors could be implemented like this: Mammal::Mammal() : itsage(1), itsweight(5) { Mammal::Mammal(int age) : itsage(age), itsweight(5) { Betriebssysteme / verteilte Systeme Introduction to Programming (1) 273

10 9.1 Inheritance Passing arguments to base constructors Dog::Dog() : Mammal(), // could be omitted itsbreed(yorkie) { Dog::Dog(int age, BREED breed) : Mammal(age), // base class initialization itsbreed(breed) { Dog::Dog(int age, int weight, BREED breed) : Mammal(age), // itsweight(weight), compiler error: class Dog itsweight? itsbreed(breed) { itsweight = weight; // cannot initialize itsweight outside!! Betriebssysteme / verteilte Systeme Introduction to Programming (1) Inheritance Overriding methods Implementation of the Speak method in Mammal: void Mammal::Speak() { cout << "Mammal sound" << endl; Mammal m; m.speak(); Mammal sound Dog d; d.speak(); Mammal sound Dog inherits the implementation of Speak but shouldn t a dog say Woof? Dog should implement the Speak method differently Betriebssysteme / verteilte Systeme Introduction to Programming (1) 275

11 9.1 Inheritance Overriding methods This can be done by overriding the method: class Dog : public Mammal { public: void Speak() const; ; // redefine the method // in class Dog void Dog::Speak() const // and provide a new { // implementation cout << "Woof!" << endl; Dog d; d.speak(); Woof! Betriebssysteme / verteilte Systeme Introduction to Programming (1) Inheritance Overriding vs. overloading Overloading define multiple methods with the same name, but different signature Overriding define a method in a derived class with the same name and signature as in the base class Caution: overriding means hiding! if you override one of many, overloaded methods (with the same name), you hide all of them! Betriebssysteme / verteilte Systeme Introduction to Programming (1) 277

12 9.1 Inheritance Hiding, calling base methods void Mammal::Move(int distance) { void Mammal::Move(int distance, int angle) { void Dog::Move(int distance) { Now, with Dog d; you can not call d.move(2,4) anymore But, you can call d.mammal::move(2,4) And even d.mammal::move(2), which calls Mammal s implementation of Move(int distance) Betriebssysteme / verteilte Systeme Introduction to Programming (1) Polymorphism Often we want an object to do something, without knowing the exact class of the object E.g., if the Cat class implements void Cat::Speak() const { cout << "Meow!" << endl; can we do something like Mammal *mypets[3]; int i; mypets[0] = new Dog(DOBERMAN); mypets[1] = new Cat(); mypets[2] = new Dog(LABRADOR); cin >> i; mypets[i]->speak(); // ==> Woof! or Meow! Betriebssysteme / verteilte Systeme Introduction to Programming (1) 279

13 9.2 Polymorphism Pointers, references and inheritance If Sub is a subclass of Super, then the following assignments are allowed: Sub sub; Super *p = Super &r = sub; I.e., a super class pointer (reference) can also point (refer) to an object of a subclass but not the other way round! Quiz: what happens in this case? Sub sub; Super super; super = sub; Betriebssysteme / verteilte Systeme Introduction to Programming (1) Polymorphism Virtual methods In C++, if you want the polymorphism to work, the base class must declare the methods in question as being virtual! class Mammal { public: virtual void Speak() const; ; Otherwise, in our example, you would just get Mammal sound i.e., the method of class Mammal is invoked Betriebssysteme / verteilte Systeme Introduction to Programming (1) 281

14 9.2 Polymorphism How does it work? Dog object A Dog object in memory: itsage itsweight itsbreed Mammal part An object of a derived class consists of data members of the base class additional data members of the derived class Betriebssysteme / verteilte Systeme Introduction to Programming (1) Polymorphism How does it work? Dog *d Dog object (seen by d) A Dog object in memory: itsage itsweight itsbreed Mammal part A derived class pointer sees everything Betriebssysteme / verteilte Systeme Introduction to Programming (1) 283

15 9.2 Polymorphism How does it work? Mammal *m Dog *d Dog object (seen by d) A Dog object in memory: itsage itsweight itsbreed Mammal part (seen by m) A base class pointer sees just the base class part at the beginning Betriebssysteme / verteilte Systeme Introduction to Programming (1) Polymorphism How does it work? Mammal *m Dog *d A Dog object in memory: vptr itsage itsweight itsbreed v table Speak() Move() Dog::Speak() { cout << "Woof!" Mammal::Move() { The object contains a pointer to a virtual method table this table has an entry for each virtual method, which points to the proper implementation Thus, each object knows its virtual methods Betriebssysteme / verteilte Systeme Introduction to Programming (1) 285

16 9.2 Polymorphism How does it work? (now with a Cat object) Mammal *m Cat *c A Cat object in memory: vptr itsage itsweight v table Speak() Move() Cat::Speak() { cout << "Meow! Mammal::Move() { In a Cat object s virtual method table, the entry for Speak() points to Cat::Speak() Thus, m->speak() prints: Moew! Betriebssysteme / verteilte Systeme Introduction to Programming (1) Polymorphism How does it work? (now with a Mammal object) Mammal *m A Mammal object in memory: v table vptr itsage itsweight Speak() Move() Mammal::Speak() { cout << "Mamm Mammal::Move() { In a Mammal object s virtual method table, the entry for Speak() points to Mammal::Speak() Thus, m->speak() prints: Mammal sound Betriebssysteme / verteilte Systeme Introduction to Programming (1) 287

17 9.2 Polymorphism Virtual destructors When an object is destroyed, we always want to call the destructor of the object s actual class e.g., call ~Dog(), even with a pointer to Mammal In principle, destructors should always be declared as virtual but virtual makes objects larger and method calls slower Rule of thumb: if a class contains a virtual method, the destructor also should be virtual (assumption: in this case, you refer to objects via a base class pointer or reference) Betriebssysteme / verteilte Systeme Introduction to Programming (1) Abstract Classes Reminder: the drawing example Remember the classes of our drawing program example: Point Line Rectangle draw() move() rotate() getx() gety() draw() move() rotate() draw() move() rotate() Ideally, we would like to use the methods draw(), move(), and rotate() in a polymorphic way, e.g.: drawingelements[i]->draw(); Betriebssysteme / verteilte Systeme Introduction to Programming (1) 289

18 9.3 Abstract Classes A class hierarchy for the drawing example DrawingElement draw() move() rotate() Point getx() gety() Line Rectangle But, implementing draw(), move(), and rotate() in the base class doesn t really make sense! Betriebssysteme / verteilte Systeme Introduction to Programming (1) Abstract Classes The animal hierarchy revisited Mammal Speak() nursebaby() Animal age weight eat() Fish spawn() Animal and Mammal provide useful interfaces But implementing the methods only makes sense for the bottommost classes think e.g. of this artificial Mammal sound in Mammal::Speak() Cat purr() Dog breed wagtail() Animal or Mammal objects also don t make too much sense Betriebssysteme / verteilte Systeme Introduction to Programming (1) 291

19 9.3 Abstract Classes Pure virtual methods Instead of providing an interface and its implementation, a base class also can provide only the interface: class Mammal { public: Mammal(); virtual ~Mammal(); virtual void Speak() const = 0; // pure virtual ; That is, we leave the implementation to the derived classes: void Dog::Speak() const { cout << "Woof!\n"; Betriebssysteme / verteilte Systeme Introduction to Programming (1) Abstract Classes Abstract classes In C++, a class is abstract, if it has at least one pure virtual method abstract C++ classes may also implement some methods An abstract class can not be instantiated you cannot create an object of an abstract class But (of course) polymorphism still works if AC is an abstract class, you can still use pointers (or references) to AC to point to objects of (non-abstract) subclasses of AC (In Java, interfaces serve a similar purpose) Betriebssysteme / verteilte Systeme Introduction to Programming (1) 293

20 9.3 Abstract Classes Hierarchies of abstract / concrete classes We can define Animal and Mammal as abstract classes they now simply provide common interfaces for all their subclasses If we derive Dog and Cat from Mammal, we make these classes concrete, so we can have Dog objects that implement the Mammal interface Betriebssysteme / verteilte Systeme Introduction to Programming (1) Abstract Classes The abstract / concrete hierarchy of animals Mammal Speak() nursebaby() Animal age weight eat() Fish spawn() Abstract Classes (define interfaces) Cat purr() Dog breed wagtail() Concrete Classes (provide implementations) Betriebssysteme / verteilte Systeme Introduction to Programming (1) 295

21 9.4 Yet Another Example A hierarchy of graphics elements From a previous exam: Define an abstract class Element for graphics elements. The abstract class shall have a method circumference that returns the circumference of the object as a double value. Betriebssysteme / verteilte Systeme Introduction to Programming (1) Yet Another Example Class Element class Element { public: virtual ~Element() { virtual double circumference() const = 0; ; Notes: No need to declare a constructor the compiler will create an empty one for us We are using a short form with ~Element(): you can also define a method s body inside the class declaration not recommended for real programming! Betriebssysteme / verteilte Systeme Introduction to Programming (1) 297

22 9.4 Yet Another Example Derive real classes Implement three classes of graphics elements which are derived from Element: 1. Circle, defined via its radius r. 2. Rectangle, defined via the lengths of its sides a and b. 3. Square, derived from Rectangle, using a = b. For each class, you need to implement a constructor, the method Circumference, and the data members. (Remark: In general, deriving a square class from a rectangle class is a questionable approach: although each square is a rectangle, not every operation on a rectangle is allowed on a square, e.g., set the length of one side ) Betriebssysteme / verteilte Systeme Introduction to Programming (1) Yet Another Example Class Circle class Circle : public Element { public: Circle(double radius) : itsradius(radius) { double circumference() const { return 2 * PI * itsradius; private: double itsradius; ; Betriebssysteme / verteilte Systeme Introduction to Programming (1) 299

23 9.4 Yet Another Example Class Rectangle class Rectangle : public Element { public: Rectangle(double a, double b) : itsa(a), itsb(b) { double circumference() const { return 2 * (itsa + itsb); private: double itsa, itsb; ; Betriebssysteme / verteilte Systeme Introduction to Programming (1) Yet Another Example Class Square class Square : public Rectangle { public: Square(double a) : Rectangle(a,a) { ; Betriebssysteme / verteilte Systeme Introduction to Programming (1) 301

24 9.4 Yet Another Example Use the Element interface Implement a function: double sum(element const *array[], unsigned int len) that computes the sum of the circumferences of the Element objects in the array. len gives the length of the array. Betriebssysteme / verteilte Systeme Introduction to Programming (1) Yet Another Example Function sum() double sum(element const *array[], unsigned int len) { double retval = 0.0; //unsigned int i; for ( unsigned int i=0; i < len; i++) { retval += array[i]->circumference(); return retval; Betriebssysteme / verteilte Systeme Introduction to Programming (1) 303

25 9.5 Summary Inheritance Cat purr() Mammal age weight eat() Speak() Dog breed wagtail() Class Mammal defines the common attributes and methods Cat and Dog are derived from Mammal They inherit all attributes and methods of Mammal they also can define additional ones Betriebssysteme / verteilte Systeme Introduction to Programming (1) Summary A Cat class derived from Mammal class Mammal { public: Mammal(); ~Mammal(); int getage() const; protected: int itsage; int itsweight; ; // allow access by derived classes class Cat : public Mammal { // public members remain public public: Cat(); ~Cat(); void purr() const; ; Betriebssysteme / verteilte Systeme Introduction to Programming (1) 305

26 9.5 Summary Polymorphism Often we want an object to do something, without knowing the exact class of the object E.g., we can write Mammal *mypets[3]; int i; mypets[0] = new Dog(DOBERMAN); mypets[1] = new Cat(); mypets[2] = new Dog(LABRADOR); cin >> i; mypets[i]->speak(); // ==> Woof! or Meow! Caveat: This only works if Mammal declares the Speak-Method as virtual! Betriebssysteme / verteilte Systeme Introduction to Programming (1) Summary How do virtual methods work? Mammal *m Dog *d A Dog object in memory: vptr itsage itsweight itsbreed v table Speak() Move() Dog::Speak() { cout << "Woof!" Mammal::Move() { The object contains a pointer to a virtual method table This table has an entry for each virtual method, which points to the proper implementation Thus, each object knows its virtual methods Betriebssysteme / verteilte Systeme Introduction to Programming (1) 307

27 9.5 Summary Pure virtual methods Instead of providing an interface and its implementation, a base class also can provide only the interface: class Mammal { public: Mammal(); virtual ~Mammal(); virtual void Speak() const = 0; // pure virtual ; That is, we leave the implementation to the derived classes: void Dog::Speak() const { cout << "Woof!\n"; Betriebssysteme / verteilte Systeme Introduction to Programming (1) Summary Reminder: abstract classes In C++, a class is abstract, if it has at least one pure virtual method abstract C++ classes may also implement some methods An abstract class can not be instantiated you cannot create an object of an abstract class But (of course) polymorphism still works if AC is an abstract class, you can still use pointers (or references) to AC to point to objects of (non-abstract) subclasses of AC (In Java, interfaces serve a similar purpose) Betriebssysteme / verteilte Systeme Introduction to Programming (1) 309