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# Lecture03 inheritance

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### Lecture03 inheritance

1. 1. InheritanceTCP1201 OOPDS
2. 2. Learninig Objectives  To understand inheritance terminology  To understand Object Relationship Diagrams (ORD)  To understand how to define subclasses  To understand multiple inheritance  To understand diamond inheritance and virtual inheritance  To understand the call order of constructor and destructor in inheritance  To understand function overriding  To understand protected access privilege  To understand upcasting & downcastingTCP1201 OOPDS
3. 3. Inheritance By nature we commonly group objects that have a common attributes and behaviors into classes (categories)., e.g. animals, vehicles, and human. Under each of these classes there can be 0, 1, or more subclasses (subcategories). TCP1201 OOPDS
4. 4. Example 1/2 Under the animal class, we can create three subclasses: bird, mammal and fish. Animal, bird, mammal and fish are classes. Bird, mammal and fish are subclasses of animal. Eagle, parrot, whale, monkey, goldfish and shark are instances/objects. (They can actually be sub-subclasses.) TCP1201 OOPDS
5. 5. The "is-a" Relationship Note the following statements: – Eagle is a bird. Parrot is a bird. Both are also animals. – Cat is a mammal. Monkey is a mammal. Both are also animals. – Goldfish is a fish. Shark is a fish. Both are also animals. A bird is different from a mammal, a mammal is different from a fish, and a fish is different from a bird. Even though they are all different among one another, they are the same in regard that they are all animals. TCP1201 OOPDS
6. 6. Example 2/2 Under the Human class, we can create 2 subclasses: Lecturer and Student. Sharaf and Manoj are instances of Lecturer. Tom and John are instances of Student. Sharaf, Manoj, Tom and John are also Humans. TCP1201 OOPDS
7. 7. Inheritance Terminology The "is-a" relationship between a superclass and its subclasses is commonly referred to as inheritance. We say that a subclass "inherits from"/"derives from" a superclass. Example: Bird inherits from Animal. Subclass inherits all the characteristics (attributes and methods) from its superclass. – All objects of Bird have a common set of attributes and methods of Bird, and also inherit a common set of attributes and behaviors from Animal. – Example: If animals have skin, then birds also have skin. Inheritance promotes code reuse and simplifies code maintenance in the long run. TCP1201 OOPDS
8. 8. Definition of Inheritance Inheritance is the mechanism which allows a class B to inherit attributes and methods Superclass/ of a class A. We say "B inherits from A". Base class/ Objects of class B have direct access to Parent class non-private attributes and methods of class A. Is-a If class B inherits from class A, then A is called the superclass/base class/parent Subclass/ class) of B. B is called the Derived class/ Child class subclass/derived class/child class) of A. Superclass is more general than subclass – Subclass is more specific than superclass TCP1201 OOPDS
9. 9. Inheritance in UML Class Diagram From the following UML Class Diagram – We know that Human is the superclass. – Lecturer and Student are subclasses of Human class. Human - name: string + Human (name: string) + speak (sentence: string): void Lecturer Student - room: string - CGPA: double + Lecturer (name: string, + Student (name:string, room: string) CGPA:double) TCP1201 OOPDS
10. 10. Superclass: Human The C++ Definition for the superclass Human: class Human { string name; public: Human(string name) : name(name) {} void speak(string sentence) { cout << "My name is " << name << ". " << sentence << endl; } }; TCP1201 OOPDS
11. 11. Defining Subclasses To indicate that a subclass inherits from superclass, we use a single colon ":", followed by an access privilege keyword (usually public) and the name of the superclass in the declaration. class subclass : public superclass : <constructor initialization list> { ... }; In most cases, subclass constructor should initialize superclass attributes. TCP1201 OOPDS
12. 12. Subclass Lecturer Hence, the declaration for Lecturer is as follows: class Lecturer : public Human { string room; public: Lecturer (string name, // name is for initializing // superclass attribute. string room); }; TCP1201 OOPDS
13. 13. Initializing Superclass from Subclass Wrong way of initializing superclass from subclass: Lecturer (string name, string room) { this->name = name; // Compile-error. this->room = room; } or Lecturer (string name, string room) : name(name), room(room) {} // Compile-error. The reason is name is a private attribute of superclass hence cannot be directly accessed by subclass. TCP1201 OOPDS 13
14. 14. Calling Superclass Constructor The correct way of initializing superclass is to invoke superclass constructor at subclass constructor initializer list. Lecturer (string name, string room) : Human(name) { // Correct. this->room = room; "calls" the Human constructor with } the argument name. or Lecturer (string name, string room) : Human(name), room(room) { } // Correct. Note that we are reusing existing code (Human constructor). TCP1201 OOPDS
15. 15. Inheritance Exampleclass Human { class Student : public Human { string name; double CGPA; public: public: Human (string name) : name(name) {} Student (string name, double void speak (string sentence) { CGPA) cout << "My name is " : Human(name), CGPA(CGPA) {} << name << ". " }; << sentence << endl; } int main() {};class Lecturer : public Human { Human h("Hugo"); string room; Lecturer l("Lee", "BR1111"); public: Student* s = new Student Lecturer (string name, ("Sarah", 3.99); string room) h.speak ("Ha"); : Human(name), room(room) {} l.speak ("Hi"); // Human::speak()}; s->speak ("Ho");// Human::speak()Output: delete s;My name is Hugo. Ha }My name is Lee. HiMy name is Sarah. Ho TCP1201 OOPDS
16. 16. Simpler Code Maintenance Now assume that both Lecturer and Student need a new attribute called address. Instead of adding the address at subclasses Lecturer and Student directly, we should add the address at superclass Human because the attribute will be inherited by both Lecturer and Student. The C++ Definition for the superclass Human: class Human { string name; string address; public: Human(string name, string address = "") : name(name), address(address) {} ... TCP1201 OOPDS
17. 17. Multiple Levels of Inheritance We can have more than one level of inheritance. For example, the Student class can be the superclass of LocalStudent and ForeignStudent. (Ignore the attribute address we discuss in previous slide.) Human - name: string + Human (name: string) + speak (sentence: string): void Lecturer Student - room: string - CGPA: double + Lecturer (name: string, + Student (name: string, room: string) CGPA: double) LocalStudent ForeignStudent - icno: string - passportno: string + LocalStudent (name: string, + ForeignStudent (name: string, CGPA: double, icno: string) CGPA: double, passportno: string) TCP1201 OOPDS
18. 18. Multiple Levels of Inheritance A subclass inherits the attributes and methods of all of its superclasses: – A ForeignStudent object will, therefore, inherit: • all of the attributes and methods of Human class, • plus the attributes and methods of Student class, • plus the attributes and methods of ForeignStudent class. TCP1201 OOPDS
19. 19. Multiple Levels of Inheritance A subclass constructor can invoke only the immediate superclass constructor, not the super-superclass constructor, unless virtual inheritance is used.class LocalStudent : public Student { string icno; public: LocalStudent (string name, double CGPA, string icno) : Human(name), // Compile error, attempt to call // super-superclass constructor. Student(name, CGPA), icno(icno) {}}; TCP1201 OOPDS
20. 20. Multiple Levels of Inheritance Sample constructor for LocalStudent and ForeignStudent. class LocalStudent : public Student { string icno; public: LocalStudent (string name, double CGPA, string icno) : Student(name, CGPA), // Call immediate superclass constructor, okay icno(icno) {} }; class ForeignStudent : public Student { string passportno; public: ForeignStudent (string name, double CGPA, string passportno) : Student(name, CGPA), passportno(passportno) {} // okay }; TCP1201 OOPDS
21. 21. Multiple Inheritance Multiple inheritance is NOT the same as "multiple levels of Inheritance". Multiple inheritance occurs when a class has 2 or more direct/immediate superclasses. Use comma "," to separate surperclasses. class C : public A, public B { public: C(…) : A(…), B(…) // Constructor initialization list. {} }; A B Constructor Constructor for class A for class B C TCP1201 OOPDS
22. 22. Diamond Inheritance Problem Multiple inheritance may introduce diamond inheritance problem, which arises when 2 classes B1 and B2 inherit from A, and class C inherits from both B1 and B2. The pronlem is C would have duplicate sets (2 sets) of the members inherited from A, which might not be desirable. If one set of members of A is preferred at C, we can use virtual inheritance to avoid duplicate sets. A B1 B2 C TCP1201 OOPDS
23. 23. Virtual Inheritance Diamond Inheritance Problemclass A { ... };class B1: public A { ... };class B2: public A { ... }; Aclass C: public B1, public B2 { public: C(...) : B1(...), B2 (...) { }}; B1 B2 Solution: virtual inheritanceclass A {};class B1: virtual public A {};class B2: virtual public A {}; Cclass C: public B1, public B2 { public: C(...) : A(...), B1(...), B2 (...) {} // Must call super-superclass constructor.}; TCP1201 OOPDS
24. 24. protected Access Privilege Recall from the previous lecture the following classes: class Human { string name; // private ... }; class Lecturer : public Human { string room; public: Lecturer (string name, string room) : Human(name), room(room) {} // name is private in Human ... }; The reason we have to initialize the attribute name via constructor initialization list is subclass cannot access superclass private members. TCP1201 OOPDS
25. 25. protected Access Privilege If a class member is declared as protected, then it is accessible to the class itself and its subclasses. class Human { protected: string name; ... }; class Lecturer : public Human { string room; public: Lecturer (string name, string room) { this->name = name; // Fine since name is protected. ... }; TCP1201 OOPDS
26. 26. protected Access Privilege However, same as private member, a protected member of a superclass is not accessible at subclass constructor initilizer list. class Human { protected: string name; ... }; class Lecturer : public Human { string room; public: Lecturer (string name, string room) { : name(name), // Error, name is not accessible here. room(room) { } ... }; TCP1201 OOPDS
27. 27. protected Access Privilege We use hash symbol "#" to denote protected access privilege in UML Class Diagram. Human # name: string + Human (name: string) + speak (sentence: string): void TCP1201 OOPDS
28. 28. Accessing Superclass Private Attribute Subclass can access/modify superclass private attributes indirectly via the public/protected get/set methods provided by superclass.class Super { int main() { int z; Sub s(3); public: cout << s.getZ() void setZ (int z) { << endl; this->z = z; s.setZ (33); // Fine } cout << s.getZ() int getZ() const { return z; } << endl;}; }class Sub: public Super { public: Sub(int z) { Output: setZ (z); // superclaas setZ 3 } 33}; TCP1201 OOPDS
29. 29. Function Overriding A subclass can override a superclass method by supplying a new version of that method with the same signature. When the method is invoked in the subclass, the subclass version is automatically selected. class Human { public: void eat() { cout << "Eatingn"; } void speak() { cout << "Im a humann"; } }; class Student : public Human { public: void speak() { // Override superclass speak(). cout << "Im a studentn"; } }; TCP1201 OOPDS
30. 30. Constructor vs. Destructor Call Order in Inheritance When creating instances of a subclass (or derived class), the constructors finish its execution from the superclass(es) and moving downwards towards the subclass itself. When destroying instances of a subclass, the destructors finish its execution from the subclass itself and moving upwards towards the superclass(es). The call order of destructors is the reverse of the call order of constructors. TCP1201 OOPDS
31. 31. Constructor vs. Destructor Call Orderclass Human { class Student : public Human { string name; int id; public: public: Human(string name) : name(name) { Student (int id, string name) cout << "Human " << name : Human(name), id(id) { << " created.n"; } cout << "Student " << id ~Human() { << " created.n"; } cout << "Human " << name ~Student() { << " destroyed.n"; } cout << "Student " << id}; << " destroyed.n"; } };Output:Human Michael created. int main() {Student 111 created. // create s1Human Kelly created. Student s1 (111, "Michael");Student 222 created. Student *s2 = new StudentStudent 222 destroyed. (222, "Kelly"); // create s2Human Kelly destroyed. delete s2; // destroy s2Student 111 destroyed. } // destroy s1Human Michael destroyed. TCP1201 OOPDS
32. 32. Function Overridingclass Human { public: void eat() { cout << "Eatingn"; } void speak() { cout << "Im a humann"; }};class Student : public Human { public: void speak() { // Override superclass speak(). cout << "Im a studentn"; }};int main() { Human* h = new Human; Student* s = new Student; Output: h->speak(); // call Human::speak() s->speak(); // call Student::speak() Im a human h->eat(); // call Human::eat() Im a student s->eat(); // call Human::eat() Eating delete h; Eating delete s;} TCP1201 OOPDS
33. 33. Upcasting and Downcasting An object of subclass can be treated as an object/pointer/reference of its superclass, and vice versa. Upcasting: casting a subclass object as superclass. – Can be implicit. – Widely used in polymorphism (Lecture 4). Downcasting: casting a superclass object as subclass. – Must be explicit. – Rarely used, and is error-proned. TCP1201 OOPDS
34. 34. Upcasting and Downcastingclass Super { };class Sub : public Super { };int main() { Super a; // a Super object. Sub b; // a Sub object. Sub* pb = &b; // Sub pointer points to Sub object. Super* pa = pb;// Super pointer points to Sub object, // ok, implicit upcasting.pa = &b; // Ok, implicit upcasting.Super& ra = b; // Super reference references a Sub object, // ok, implicit upcasting.pa = &a; // Super pointer points to Super object.pb = pa; // Sub pointer points to Super object, // compile-error, implicit downcasting.pb = (Sub*) pa; // Ok, explicit downcasting.Sub& rb1 = a; // Compile-error, implicit downcasting.Sub& rb2 = (Sub&) a; // Ok, explicit downcasting. pa = dynamic_cast<Super*>(&b); // Ok, explicit upcasting. pa = static_cast<Super*>(&b); // Ok, explicit upcasting.} TCP1201 OOPDS
35. 35. Benefits of Upcasting Upcasting allows us to use an array of superclass pointers to point to objects from subclasses. Another benefit is we can pass subclass object to a function parameter of type superclass pointer/reference. class Human { // Superclass public: void eat() { cout << "Eatingn"; } }; class Lecturer : public Human { }; // Subclass class Student : public Human { }; // Subclass void callEat (Human & h) { // Superclass reference. h.eat(); } int main() { Human* h[3] = { new Human, new Lecturer, new Student}; for (int i = 0; i < 3; i++) { h[i]->eat(); callEat (*h[i]); delete h[i]; } } TCP1201 OOPDS