Inheritance

2. Inheritance ITK 169 Fall 2003

3. Base Class • Also called the Parent Class • This is the class that later classes will be build on. • Suppose the Red Bird Rec Center decided to keep track of those members who play table tennis. • Consider the base class Player listed on the next slide.

4. class Player { private: string fName, lName; bool hasTable; public: // default and special constructor in one using default values… Player(const string& first = "empty", const string& last = "empty", bool table = false); void set_fName(const string& first) { fName = first; } void set_lName(const string& last) { lName = last; } void set_hasTable(bool table) { hasTable = table; } string get_fullnameLF()const { return (lName + ", " + fName); } string get_fullnameFL()const { return (fName + " " + lName); } bool get_hasTable()const { return hasTable; } void printNameFL(ostream& outs) { outs << fName << " " << lName; } friend ostream& operator <<(ostream& outs, const Player& aPlayer); };

5. Derived Class • Some members have played in local table tennis tournaments. • They would like the class to include the point ratings they’ve earned. • Rather than start from scratch, you can derive a class from the Player class. • RatedPlayer will be the derived class.

6. Notice - :public Player class RatedPlayer : public Player { private: unsigned int rating; public: // default and special constructor in one using default values… RatedPlayer(unsigned int rt = 0, const string& first = "empty", const string& last = "empty", bool table = false); // constructor to add a rating to an existing player RatedPlayer(unsigned int rate, const Player& aPlayer); void set_rating(unsigned int num) { rating = num; } unsigned int get_rating() { return rating; } };

7. Public Derivation • In the class header : public Player is added to indicate that • RatedPlayer is derived from Player, • and that Player is a public base class. • With public derivation, all public members of the base class become public members of the derived class. • The private portions of the base class become part of the derived class but they can only be accessed through public and protected methods of the base class. • (More on protected methods later.)

8. So… • An object of type RatedPlayer has in store: • 3 data members of Player • fName, lName; hasTable; • plus a rating. • An object of type RatedPlayercan use the methods of the base type and its own class. • An object of type Player has only 3 data members and can only access Player methods.

9. Derived Requirements • A derived class can add additional data members and member functions as needed. • A derived class needs its own constructors. • Note the constructors must provide data for the any new data member(s) AND the base data members.

10. Access Considerations • A derived class does not have direct access to the private members of the base class. • It has to work through the base class methods. • In particular, the derived constructors have to use the base class constructors.

11. Derived Constructors • Each constructor includes a call to a Player constructor at the end of the parameter list - note the use of a : RatedPlayer::RatedPlayer(unsigned int rate, const string& first, const string& last, bool table) : Player(first, last, table) { rating = rate; } RatedPlayer::RatedPlayer(unsigned int rate, const Player& aPlayer) : Player(aPlayer), rating(rate) { } This is call an initializer list. Note: We can have more than just the Player constructor call here.

12. Key Points to Remember • The base class object is constructed first. • The derived class constructor should pass base class information to a base class constructor via a member initialization list. • The derived class constructor should initialize those data members that were added to the derived class.

13. Using our 2 Classes Player mickey("Mickey", "Mouse"); RatedPlayer daffy(10, "Daffy", "Duck"); cout << mickey << " plays occasionally for fun.\n"; cout << daffy << " is a serious tournament player.\n"; daffy.printNameFL(cout); cout << " is rated " << daffy.get_rating( ) << " out of 50, in the city tournaments.\n";

14. Private Members • Why would a derived class NOT have direct access to private data members? • What about private functions? • Is there a “go around”? • Is it a good idea to allow direct access to private members (data or functions)?