9 Understanding Inheritance ObjectOriented Programming Using C Second

9 Understanding Inheritance Object-Oriented Programming Using C++ Second Edition 1

9 Objectives • In this chapter, you will learn: • About inheritance and its benefits • How to create a derived class • About restrictions imposed with inheritance • How to choose a class access specifier • How to override and overload parent class functions within a child class • How to use a constructor initialization list 2

9 Objectives • In this chapter, you will learn: • How to provide for a base class constructor within a derived class • How to override the class access specifier • How to use multiple inheritance • About the special problems posed by multiple inheritance • How to use virtual inheritance 3

9 Understanding Inheritance • Objects used in computer programs also are easier to understand if you can place them within a hierarchy of inheritance • Suppose you have written several programs with a class named Student • The First. Year. Student class inherits from the Student class, or is derived from it • The Student class is called a parent class, base class, superclass, or ancestor; the First. Year. Student class is called a child class, derived class, subclass, or descendant • When you override a function, you substitute one version of the function for another 4

9 Understanding the Advantages Provided by Inheritance • Inheritance is considered a basic building block of object-oriented programming • Programs in which you derive new classes from existing classes offer several advantages: – You save time because much of the code needed for your class is already written – You save additional time because the existing code has already been tested—that is, you know it works correctly; it is reliable 5

9 Understanding the Advantages Provided by Inheritance • Advantages of using derived classes: – You save even more time because you already understand how the base class works, and you can concentrate on the complexity added as a result of your extensions to the class – In a derived class, you can extend and revise a parent class without corrupting the existing parent class features – If other classes have been derived from the parent class, the parent class is even more reliable—the more its code has been used, the more likely it is that logical errors have already been found and fixed 6

9 Creating a Derived Class • The class, named Person, is shown in Figure 9 -1 • It contains three fields, and two member functions that set and display the data field values 7

9 Creating a Derived Class • To create a derived class, you include the following elements in the order listed: – – – – – Keyword class Derived class name Colon Class access specifier, either public, private, or protected Base class name Opening brace Class definition statements Closing brace Semicolon 8

9 Creating a Derived Class • The Customer class shown in Figure 9 -2 contains all the members of Person because it inherits them 9

9 The Customer Class 10

9 Creating a Derived Class Ex 9 -1 11

9 Creating a Derived Class • In the set of steps outlined on pages 317 to 319 of the textbook, you create two classes, Car and Convertible • The Car class serves as a base class and includes features common to all cars 12

9 Ex 9 -2 Creating a Derived Class • You create the Convertible class and a demonstration program in the steps referred to on pages 319 and 320 of the textbook 13

9 Understanding Inheritance Restrictions • Member accessibilities of the members of the base class in the derived classes 14

9 Understanding Inheritance Restrictions • The protected specifier allows members to be used by class member functions and by derived classes, but not by other parts of a program 15

9 Choosing the Class Access Specifier • C++ programmers usually use the public access specifier for inheritance • If a derived class uses the public access specifier, then the following statements are true: – Base class members that are public remain public in the derived class – Base class members that are protected remain protected in the derived class – Base class members that are private are inaccessible in the derived class 16

9 Choosing the Class Access Specifier • If a derived class uses the protected access specifier, then the following statements are true: – Base class members that are public become protected in the derived class – Base class members that are protected remain protected in the derived class – Base class members that are private are inaccessible in the derived class 17

9 Choosing the Class Access Specifier • If a derived class uses the private access specifier, then the following statements are true: – Base class members that are public become private in the derived class – Base class members that are protected become private in the derived class – Base class members that are private are inaccessible in the derived class 18

9 Choosing the Class Access Specifier 19

9 Choosing the Class Access Specifier 20

9 Choosing the Class Access Specifier • A main() program that instantiates an Introvert object cannot access any of the four data items directly; all four fields are private to the Introvert class 21

9 Choosing the Class Access Specifier 22

9 Choosing the Class Access Specifier • To summarize the use of three class access specifiers with class data members: – If a class has private data members, they can be used only by member functions of that class – If a class has protected data members, they can be used by member functions of that class and by member functions of derived classes – If a class has public data members, they can be used by member functions of that class, by member functions of derived classes, and by any other functions’ including the main() function of a program 23

9 Choosing the Class Access Specifier • With one generation of inheritance, there are nine possible combinations, as shown in Table 9 -1 • Remember the following: – Private data can be accessed only by a class’s member functions (or friend functions), and not by any functions in derived classes – If a class serves as a base class, most often its data members are protected, and its member functions are public – The access specifier in derived classes is most often public, so that the derived class can refer to all non-private data and functions of the base class – When a class is derived from another derived class, the newly derived class never has any more liberal access to a base class member than does its immediate predecessor 24

9 Summary of Child Class Member Access Rules Ex 9 -3 check errors 25

9 Overriding and Overloading Parent Class Functions • When a new class is derived from an existing class, the derived class has access to non-private member functions in the base class • The new class also can have its own member functions • Those functions can have names that are identical to the function names in the base class • Any child class function with the same name and argument list as the parent overrides the parent function; • No overloading across scopes. 26

9 Overriding and Overloading Parent Class Functions 27

9 Overriding and Overloading Parent Class Functions 28

9 Overriding and Overloading Parent Class Functions Ex 9 -4 29

9 Overriding and Overloading Parent Class Functions • The output in Figure 9 -21 shows that even though you set fields for a Person and an Employee by using separate functions that require separate argument lists, you use the same output. Data() function that exists within the parent class for both a Person and an Employee 30

9 Overriding and Overloading Parent Class Functions Ex 9 -4 31

9 Overriding and Overloading Parent Class Functions • A derived class object can be assigned to a base class object, as in a. Person = worker; • The assignment causes each data member to be copied from worker to a. Person, and leaves off any data for which the base class doesn’t have members • The reverse assignment cannot take place without writing a specialized function • When any class member function is called, the steps shown on page 333 of the textbook take place 32

9 Overriding and Overloading Parent Class Functions Ex 9 -5 • In the set of steps outlined on pages 333 and 334 of the textbook, you create a Race. Car class as a child of the Car class you created earlier • A Race. Car has all the attributes of a Car, but its maximum allowed speed is higher • Overriding a base class member function with a derived member function demonstrates the concept of polymorphism 33

9 Using Constructor Initialization Lists • Many classes use a constructor function to provide initial values for fields when a class object is created; that is, many functions simply contain a series of assignment statements • As an alternative to the separate prototype and implementation of the constructor shown in Figure 925, you can implement the constructor within the declaration section for the class as an inline function 34

9 The Item Class 35

9 Using Constructor Initialization Lists • As another alternative, you can replace the assignment statements within a constructor body for Item with a constructor initialization list • A constructor initialization list provides values for class fields in the same statement that contains the constructor definition 36

9 Using Constructor Initialization Lists Ex 9 -6 • The constructor initialization list shown in Figure 9 -27 initializes item. Num with the value of n and item. Price with the value of p 37

9 Understanding the Difference Between Assignment and Initialization • The difference between assignment and initialization is often very subtle, and programmers sometimes mingle the two terms rather casually • When you declare a simple scalar variable and give it a value, you can declare and assign in two separate statements, for example: int z; z = 100; • Alternatively, you can initialize a variable, declaring it and assigning it a value within a single statement, for example: int z = 100; OR int z(100); 38

9 Understanding the Difference Between Assignment and Initialization • There at least four reasons to understand the use of constructor initialization lists: – Many C++ programmers prefer this method, so it is used in many programs – Technically, a constructor should initialize rather than assign values – Reference variables and constant class members cannot be assigned values; they must be initialized – When you create a derived class and instantiate an object, a parent class object must be constructed first. You add a constructor initialization list to a derived class constructor to construct the parent class. 39

9 Providing for Base Class Construction • When you instantiate an object in a C++ program, you automatically call its constructor function • This pattern holds true whether you write a custom constructor or use a default constructor • When you instantiate a derived class object, a constructor for its base class is called first, followed by the derived class constructor • This format is followed even if the base and derived classes both have only default constructors 40

9 Providing for Base Class Construction 41

9 Providing for Base Class Construction • If Pet. Store. Animal were a simple base class, the Pet. Store. Animal class constructor would require just an integer argument that would be assigned to the pet. Age field • However, because Pet. Store. Animal is derived from Pet. Store. Item, a Pet. Store. Animal object is constructed, and the Pet. Store. Item class constructor also will be called • The prototype for the Pet. Store. Animal class constructor must provide values for all the arguments it needs as well as all the arguments its parent needs 42

9 The Pet. Store. Animal Class 43

9 Providing for Base Class Construction Ex 9 -7 • Figure 9 -31 shows one more option for coding the Pet. Store. Animal class • In this example, the constructor initialization list initializes both Pet. Store. Item and pet. Age 44

9 Overriding Inherited Access • Nine inheritance access specifier combinations are possible: base class members that are private, protected, or public can be inherited with private, protected, or public access • In addition, you can override the class access specifier for any specific class members 45

9 The Demo. Base. Person Class 46

9 Overriding Inherited Access • If a derived class, Somewhat. Shy, uses the protected access specifier when inheriting from Demo. Base. Person, then the following statements hold true: – The field named salary, which is private in Demo. Base. Person, is inaccessible in the derived class – The field named age, which is protected in the base class, remains protected in the derived class – The field named initial, which is public in the base class, becomes protected in the derived class – The function set. Person() and show. Person(), which are public in Demo. Base. Person, become protected in the derived class 47

9 • • • Overriding Inherited Access For the show. Person() function to become public within Somewhat. Shy, the highlighted statement in Figure 9 -33 must appear in the public section of the Somewhat. Shy class Additionally, the Demo. Base. Person class name and scope resolution operator must appear before the show. Person() function name Finally, and most oddly, no parentheses appear after the show. Person() function name within the child class 48

9 Overriding Inherited Access Ex 9 -8 • However, you can override the inherited access to make an individual member’s access more conservative • For most C++ classes, data is private or protected, and most functions are public 49

9 Summary • Inheritance allows you to create classes that derive most of their attributes from existing classes • Programs in which you create classes that are derived from existing classes offer several advantages: you save time because much of the code needed for your class is already written and tested • To create a derived class, you include the keyword class, the derived class name, a colon, a class access specifier, and the base class name in the class header; as with other classes, definition statements within a pair of curly braces follow 50

9 Summary • When a class serves as a base class to others, all of its members can be used within the child class functions, except for any private members • When you define a derived class, you can insert one of the three class access specifiers (public, private, or protected) just prior to the base class name • You can override parent class functions within a child class • As an alternative to separate prototypes and implementation of a constructor, you can use a constructor initialization list 51

9 Summary • When you declare a variable without an initial value, it holds garbage until you make an assignment • If a base class does not contain a default constructor, then you must provide a constructor for use by the derived class • You can override the class access specifier when it does not suit your needs for some of the members of the class 52