Inheritance Joe Meehean

Object Oriented Programming Objects state (data) behavior (methods) identity (allocation of memory) Class objects definition defines interface objects are explicitly created (instantiated) from a class

Advantages of OOP Encapsulation related data is stored and modified together implementation separate from interface implementation can easily be modified Abstraction objects viewed by functionality they provide not how they provide it makes building complex software easier

Abstraction Example Roommate offers to drive to beach Car (Abstraction) runs passenger seats cargo space

Abstraction Example

The problem in C++ Say we have a Book class, it has author title retail price calculatePrice method We want to add used books more owners = cheaper could add an owners count to Book We also want to add limited edition books, they have signed adds 20% to price 1 st add 20% to price limited editions should not decrease based on number of owners 6

The problem in C++ Book class is getting pretty complex really represents a lot more than just new books We would like to say a UsedBook is a Book, but we want to calculate its price differently Share all the things UsedBook has in common with Book author title retail price Don’t share the things it does not price calculation 7

Inheritance Can do this using Inheritance sometimes called an “is-a” relationship UsedBook is a Book Book is the base class UsedBook is the derived classes UsedBook is derived from Book sometimes called extends UsedBook extends Book 8

Inheritance Derived classes automatically have all the member data of the base class all the public methods of the base class Derived classes can selectively replace the methods of the base class e.g., calculatePrice() add their own member data add their own member functions 9

Base class Defining a base class exactly the same as any other class Virtual methods methods that can be replaced by derived class syntax: virtual type1 name(type2 arg1, type2 arg2, …) e.g., virtual double calculatePrice() virtual only goes before method declaration, not definition Protected members new access label protected can be accessed by derived classes cannot be accessed by public 10

Base class 11 class Book{ protected: string author_; string title_; double retail_; public: Book(string author, string title, double retail); virtual ~Book(); virtual double calculatePrice(); virtual string getAuthor(); virtual string getTitle(); }

Base class 12 double Book::calculatePrice(){ return retail_; } string Book::getAuthor(){ return author_; } string Book::getTitle(){ return title_; }

Derived class Defining a derived class need to list base class in definition syntax: class classname : access-label base-class { e.g., class UsedBook : public Book { can replace virtual methods of base class, just redefine method e.g., double calculatePrice() can add member variables, just like any other class 13

Derived class Instantiated (an instance of) derived class has two parts base derived access base members like defined right in the derived class If derived class doesn’t redefine virtual functions it still has those functions defined by base class 14

Derived class 15 class UsedBook : public Book{ protected: int prev_owners_; public: UsedBook(string author, string title, double retail, int prev_owners); virtual ~UsedBook(); virtual double calculatePrice(); }

Derived class 16 double UsedBook::calculatePrice(){ double price = retail_; price -= (retail_/5) * prev_owners; return price; }

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Types of inheritance Type of inheritance class UsedBook : access-label Book { can be public, private, or protected Public inheritance members of the base class retain access levels private members still private public members still public any public method defined by base class, also automatically defined for derived class 18

Types of inheritance Type of inheritance class UsedBook : access-label Book { can be public, private, or protected Protected inheritance public members of the base class now protected protected members of the base class still protected private members of the base class still private any public/protected/private method defined by base class automatically defined for the derived class but, can be called by the derived class only (and classes that extend derived class) 19

Types of inheritance Type of inheritance class UsedBook : access-label Book { can be public, private, or protected Private inheritance public & protected members of the base class now private private members of the base class still private any public/protected/private method defined by base class automatically defined for the derived class but, can be called by the derived class only 20

Dynamic Binding Can refer to objects of derived type by base type e.g., Base *p = new Derived(); Does not throw away any part of the derived class Derived class is just pointed at using a base pointer 21 UsedBook ub(...); Book *pb = &ub; //... pb = new LimitedEdition(...);

Dynamic Binding What happens when we call a method on an object using a base pointer? Base method is virtual & defined by derived class, the derived method is called Base method is virtual & not defined by derived class, the base method is called Base method is not virtual & defined by derived class, the base method is called This magic is referred to as dynamic binding 22

The problem Interface set of methods provided by a class What if we just want to define an interface? methods that should be provided by every derived class should not be able to get an instance of base class base class is so abstract, an instance doesn’t make sense 23

Abstract class class that cannot be instantiated How do we make one? define a class with a pure virtual function Pure virtual function function declared but explicitly not defined in base class syntax: virtual type1 name(type2 arg1, type3 arg2, …) = 0 24

Abstract class 25 class BlackJackPlayer { public: virtual bool hold(int hand_total) = 0; };

Abstract class 26 class HoldAt17 : public BlackJackPlayer{ bool hold(int hand_total); }; bool HoldAt17::hold(int hand_total){ return hand_total >= 17; }

Abstract class 27 class HumanPlayer : public BlackJackPlayer{ bool hold(int hand_total); }; bool HumanPlayer::hold(int hand_total){ cout << “Total: “ << hand_total << endl; cout << “Enter h to Hold:”; char key_press; cin >> key_press; return key_press == ‘h’; }

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Inheritance & Copy Control Derived-class synthesized default constructor initializes data members of derived class initializes data members of base class Derived-class default constructor automatically invokes base class default constructor 29 UsedBook::UsedBook() : prev_owners_(0){} // uses Book() to initialize author, title, etc..

Inheritance & Copy Control Passing arguments to the base-class constructor member initialization list may only initialize derived member data initialize base-class member data using base-class constructor 30 UsedBook::UsedBook(string author, string title, double retail, int owners) : prev_owners_(owners), Book(author, title, retail) { }

Inheritance & Copy Control Derived copy constructor if derived class defines copy constructor should copy derived member data use base-class copy constructor to initialize base-class data 31 UsedBook::UsedBook(const UsedBook& orig) : prev_owners_(orig.prev_owners_), Book(orig) { }

Inheritance & Copy Control Derived assignment operator if derived class defines an assignment operator use base-class assignment operator to assign base-class data should clean-up and reinitialize derived member data 32 UsedBook& UsedBook::operator =(const UsedBook& rhs) { if( this != &rhs ){ // call base-class assignment operator Book::operator=(rhs); // assign derived member data prev_owners_ = rhs.prev_owners; } return *this; }

Inheritance & Copy Control Derived destructor define only if derived member data needs explicit cleanup never has to cleanup base-class member data DO NOT CALL BASE CLASS DESTRUCTOR 33 UsedBook::~UsedBook() { // nothing to do }

Inheritance & Copy Control Base-class destructor MUST be virtual otherwise wrong destructor may be called 34 Book *pBook = new UsedBook(...);... delete pBook;

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