CSC 213 Chapter 6, Fall 2007

1  2000 Prentice Hall, Inc. All rights reserved. Chapter 6: Classes and Data Abstraction Outline 6.1 Introduction 6.2 Structure Definitions 6.3 Accessing Members of Structures 6.4 Implementing a User-Defined Type Time with a Struct 6.5 Implementing a Time Abstract Data Type with a Class 6.6 Class Scope and Accessing Class Members 6.7 Separating Interface from Implementation 6.8 Controlling Access to Members 6.9 Access Functions and Utility Functions 6.10 Initializing Class Objects: Constructors 6.11 Using Default Arguments with Constructors 6.12 Using Destructors 6.13 When Constructors and Destructors Are Called 6.14 Using Data Members and Member Functions 6.15 A Subtle Trap: Returning a Reference to a Private Data Member 6.16 Assignment by Default Memberwise Copy 6.17 Software Reusability 2  2000 Prentice Hall, Inc. All rights reserved. 6.1 Introduction • Object-oriented programming (OOP) – Encapsulates data (attributes) and functions (behavior) into packages called classes • Information hiding – Implementation details are hidden within the classes themselves • Classes – Classes are the standard unit of programming – A class is like a blueprint – reusable – Objects are instantiated (created) from the class – For example, a house is an instance of a “blueprint class” 3  2000 Prentice Hall, Inc. All rights reserved. 6.2 Structure Definitions • Structures – Aggregate data types built using elements of other types struct Time { int hour; int minute; int second; }; – Members of the same structure must have unique names – Two different structures may contain members of the same name – Each structure definition must end with a semicolon Structure tag Structure members 4  2000 Prentice Hall, Inc. All rights reserved. 6.2 Structure Definitions • Self-referential structure – Contains a member that is a pointer to the same structure type – Used for linked lists, queues, stacks and trees • struct – Creates a new data type that is used to declare variables – Structure variables are declared like variables of other types – Example: Time timeObject, timeArray[ 10 ], *timePtr, &timeRef = timeObject; 5  2000 Prentice Hall, Inc. All rights reserved. 6.3 Accessing Members of Structures • Member access operators: – Dot operator (.) for structures and objects – Arrow operator (->) for pointers – Print member hour of timeObject: cout << timeObject.hour; OR timePtr = &timeObject; cout << timePtr->hour; – timePtr->hour is the same as ( *timePtr ).hour – Parentheses required: * has lower precedence than .  2000 Prentice Hall, Inc. All rights reserved. Outline 6 Outline 2. Create a struct data type 1. Define the struct 1.1 Define prototypes for the functions 2. Create a struct data type 2.1 Set and print the time 1 // Fig. 6.1: fig06_01.cpp 2 // Create a structure, set its members, and print it. 3 #include <iostream> 4 5 using std::cout; 6 using std::endl; 7 8 struct Time { // structure definition 9 int hour; // 0-23 10 int minute; // 0-59 11 int second; // 0-59 12 }; 13 14 void printMilitary( const Time & ); // prototype 15 void printStandard( const Time & ); // prototype 16 17 int main() 18 { 19 Time dinnerTime; // variable of new type Time 20 21 // set members to valid values 22 dinnerTime.hour = 18; 23 dinnerTime.minute = 30; 24 dinnerTime.second = 0; 25 26 cout << "Dinner will be held at "; 27 printMilitary( dinnerTime ); 28 cout << " military time,\nwhich is "; 29 printStandard( dinnerTime ); 30 cout << " standard time.\n"; 31 Creates the user-defined structure type Time with three integer members: hour, minute and second. Dinner will be held at 18:30 military time, which is 6:30:00 PM standard time.  2000 Prentice Hall, Inc. All rights reserved. Outline 7 Outline 2.2 Set the time to an invalid hour, then print it 3. Define the functions printMilitary and printStandard 32 // set members to invalid values 33 dinnerTime.hour = 29; 34 dinnerTime.minute = 73; 35 36 cout << "\nTime with invalid values: "; 37 printMilitary( dinnerTime ); 38 cout << endl; 39 return 0; 40 } 41 42 // Print the time in military format 43 void printMilitary( const Time &t ) 44 { 45 cout << ( t.hour < 10 ? "0" : "" ) << t.hour << ":" 46 << ( t.minute < 10 ? "0" : "" ) << t.minute; 47 } 48 49 // Print the time in standard format 50 void printStandard( const Time &t ) 51 { 52 cout << ( ( t.hour == 0 || t.hour == 12 ) ? 53 12 : t.hour % 12 ) 54 << ":" << ( t.minute < 10 ? "0" : "" ) << t.minute 55 << ":" << ( t.second < 10 ? "0" : "" ) << t.second 56 << ( t.hour < 12 ? " AM" : " PM" ); 57 } Time with invalid values: 29:73  2000 Prentice Hall, Inc. All rights reserved. Outline 8 Outline Program Output Dinner will be held at 18:30 military time, which is 6:30:00 PM standard time. Time with invalid values: 29:73 9  2000 Prentice Hall, Inc. All rights reserved. 1 class Time { 2 public: 3 Time(); 4 void setTime( int, int, int ); 5 void printMilitary(); 6 void printStandard(); 7 private: 8 int hour; // 0 - 23 9 int minute; // 0 - 59 10 int second; // 0 - 59 11 }; 6.5 Implementing a Time Abstract Data Type with a Class • Classes – Model objects that have attributes (data members) and behaviors (member functions) – Defined using keyword class – Have a body delineated with braces ({ and }) – Class definitions terminate with a semicolon – Example: Public: and Private: are member-access specifiers. setTime, printMilitary, and printStandard are member functions. Time is the constructor. hour, minute, and second are data members. 10  2000 Prentice Hall, Inc. All rights reserved. 6.5 Implementing a Time Abstract Data Type with a Class • Member access specifiers – Classes can limit the access to their member functions and data – The three types of access a class can grant are: • Public — Accessible wherever the program has access to an object of the class • private — Accessible only to member functions of the class • Protected — Similar to private and discussed later • Constructor – Special member function that initializes the data members of a class object – Cannot return values – Have the same name as the class 11  2000 Prentice Hall, Inc. All rights reserved. 6.5 Implementing a Time Abstract Data Type with a Class • Class definition and declaration – Once a class has been defined, it can be used as a type in object, array and pointer declarations – Example: Time sunset, // object of type Time arrayOfTimes[ 5 ], // array of Time objects *pointerToTime, // pointer to a Time object &dinnerTime = sunset; // reference to a Time object Note: The class name becomes the new type specifier.  2000 Prentice Hall, Inc. All rights reserved. Outline 12 Outline 1 // Fig. 6.3: fig06_03.cpp 2 // Time class. 3 #include <iostream> 4 5 using std::cout; 6 using std::endl; 7 8 // Time abstract data type (ADT) definition 9 class Time { 10 public: 11 Time(); // constructor 12 void setTime( int, int, int ); // set hour, minute, second 13 void printMilitary(); // print military time format 14 void printStandard(); // print standard time format 15 private: 16 int hour; // 0 – 23 17 int minute; // 0 – 59 18 int second; // 0 – 59 19 }; 20 21 // Time constructor initializes each data member to zero. 22 // Ensures all Time objects start in a consistent state. 23 Time::Time() { hour = minute = second = 0; } 24 25 // Set a new Time value using military time. Perform validity 26 // checks on the data values. Set invalid values to zero. 27 void Time::setTime( int h, int m, int s ) 28 { 29 hour = ( h >= 0 && h < 24 ) ? h : 0; 30 minute = ( m >= 0 && m < 60 ) ? m : 0; 31 second = ( s >= 0 && s < 60 ) ? s : 0; 32 } Note the :: preceding the function names. 1. Define a Time class 1.1 Define default values for the time  2000 Prentice Hall, Inc. All rights reserved. Outline 13 Outline 33 34 // Print Time in military format 35 void Time::printMilitary() 36 { 37 cout << ( hour < 10 ? "0" : "" ) << hour << ":" 38 << ( minute < 10 ? "0" : "" ) << minute; 39 } 40 41 // Print Time in standard format 42 void Time::printStandard() 43 { 44 cout << ( ( hour == 0 || hour == 12 ) ? 12 : hour % 12 ) 45 << ":" << ( minute < 10 ? "0" : "" ) << minute 46 << ":" << ( second < 10 ? "0" : "" ) << second 47 << ( hour < 12 ? " AM" : " PM" ); 48 } 49 50 // Driver to test simple class Time 51 int main() 52 { 53 Time t; // instantiate object t of class Time 54 55 cout << "The initial military time is "; 56 t.printMilitary(); 57 cout << "\nThe initial standard time is "; 58 t.printStandard(); 59 Notice how functions are called using the dot (.) operator. 1.2 Define the two functions printMilitary and printstandard 2. In main, create an object of class Time 2.1Print the initial (default) time The initial military time is 00:00 The initial standard time is 12:00:00 AM  2000 Prentice Hall, Inc. All rights reserved. Outline 14 Outline 60 t.setTime( 13, 27, 6 ); 61 cout << "\n\nMilitary time after setTime is "; 62 t.printMilitary(); 63 cout << "\nStandard time after setTime is "; 64 t.printStandard(); 65 66 t.setTime( 99, 99, 99 ); // attempt invalid settings 67 cout << "\n\nAfter attempting invalid settings:" 68 << "\nMilitary time: "; 69 t.printMilitary(); 70 cout << "\nStandard time: "; 71 t.printStandard(); 72 cout << endl; 73 return 0; 74 } The initial military time is 00:00 The initial standard time is 12:00:00 AM Military time after setTime is 13:27 Standard time after setTime is 1:27:06 PM After attempting invalid settings: Military time: 00:00 Standard time: 12:00:00 AM 2.2 Set and print the time 2.3 Set the time to an invalid hour 2.4 Print the time Program Output Military time after setTime is 13:27 Standard time after setTime is 1:27:06 PM After attempting invalid settings: Military time: 00:00 Standard time: 12:00:00 AM 15  2000 Prentice Hall, Inc. All rights reserved. 6.5 Implementing a Time Abstract Data Type with a Class • Destructors – Functions with the same name as the class but preceded with a tilde character (~) – Cannot take arguments and cannot be overloaded – Performs “termination housekeeping” • Binary scope resolution operator (::) – Combines the class name with the member function name – Different classes can have member functions with the same name • Format for defining member functions ReturnType ClassName::MemberFunctionName( ){ … } 16  2000 Prentice Hall, Inc. All rights reserved. 6.5 Implementing a Time Abstract Data Type with a Class • If a member function is defined inside the class – Scope resolution operator and class name are not needed – Defining a function outside a class does not change it being public or private • Classes encourage software reuse – Inheritance allows new classes to be derived from old ones 17  2000 Prentice Hall, Inc. All rights reserved. 6.6 Class Scope and Accessing Class Members • Class scope – Data members and member functions • File scope – Nonmember functions • Inside a scope – Members accessible by all member functions • Referenced by name • Outside a scope – Members are referenced through handles • An object name, a reference to an object or a pointer to an object 18  2000 Prentice Hall, Inc. All rights reserved. 6.6 Class Scope and Accessing Class Members • Function scope – Variables only known to function they are defined in – Variables are destroyed after function completion • Accessing class members – Same as structs – Dot (.) for objects and arrow (->) for pointers – Example: • t.hour is the hour element of t • TimePtr->hour is the hour element  2000 Prentice Hall, Inc. All rights reserved. Outline 19 Outline 1. Class definition 2. Create an object of the class 2.1 Assign a value to the object. Print the value using the dot operator 2.2 Set a new value and print it using a reference 1. Class definition 2. Create an object of the class 2.1 Assign a value to the object. Print the value using the dot operator 2.2 Set a new value and print it using a reference 1 // Fig. 6.4: fig06_04.cpp 2 // Demonstrating the class member access operators . and -> 3 // 4 // CAUTION: IN FUTURE EXAMPLES WE AVOID PUBLIC DATA! 5 #include <iostream> 6 7 using std::cout; 8 using std::endl; 9 10 // Simple class Count 11 class Count { 12 public: 13 int x; 14 void print() { cout << x << endl; } 15 }; 16 17 int main() 18 { 19 Count counter, // create counter object 20 *counterPtr = &counter, // pointer to counter 21 &counterRef = counter; // reference to counter 22 23 cout << "Assign 7 to x and print using the object's name: "; 24 counter.x = 7; // assign 7 to data member x 25 counter.print(); // call member function print 26 27 cout << "Assign 8 to x and print using a reference: "; 28 counterRef.x = 8; // assign 8 to data member x 29 counterRef.print(); // call member function print 30 It is rare to have public member variables. Usually only member functions are public; this keeps as much information hidden as possible.  2000 Prentice Hall, Inc. All rights reserved. Outline 20 Outline 2.3 Set a new value and print it using a pointer Program Output 31 cout << "Assign 10 to x and print using a pointer: "; 32 counterPtr->x = 10; // assign 10 to data member x 33 counterPtr->print(); // call member function print 34 return 0; 35 } Assign 7 to x and print using the object's name: 7 Assign 8 to x and print using a reference: 8 Assign 10 to x and print using a pointer: 10 21  2000 Prentice Hall, Inc. All rights reserved. 6.7 Separating Interface from Implementation • Separating interface from implementation – Makes it easier to modify programs – Header files • Contains class definitions and function prototypes – Source-code files • Contains member function definitions  2000 Prentice Hall, Inc. All rights reserved. Outline 22 Outline 1 // Fig. 6.5: time1.h 2 // Declaration of the Time class. 3 // Member functions are defined in time1.cpp 4 5 // prevent multiple inclusions of header file 6 #ifndef TIME1_H 7 #define TIME1_H 8 9 // Time abstract data type definition 10 class Time { 11 public: 12 Time(); // constructor 13 void setTime( int, int, int ); // set hour, minute, second 14 void printMilitary(); // print military time format 15 void printStandard(); // print standard time format 16 private: 17 int hour; // 0 - 23 18 int minute; // 0 - 59 19 int second; // 0 - 59 20 }; 21 22 #endif If time1.h (TIME1_H) is not defined (#ifndef) then it is loaded (#define TIME1_H). If TIME1_H is already defined, then everything up to #endif is ignored. This prevents loading a header file multiple times. 1. Using the same Time class as before, create a header file Dot ( . ) replaced with underscore ( _ ) in file name.  2000 Prentice Hall, Inc. All rights reserved. Outline 23 Outline 2. Create a source code file 2.1 Load the header file to get the class definitions 2.2. Define the member functions of the class 23 // Fig. 6.5: time1.cpp 24 // Member function definitions for Time class. 25 #include <iostream> 26 27 using std::cout; 28 29 #include "time1.h" 30 31 // Time constructor initializes each data member to zero. 32 // Ensures all Time objects start in a consistent state. 33 Time::Time() { hour = minute = second = 0; } 34 35 // Set a new Time value using military time. Perform validity 36 // checks on the data values. Set invalid values to zero. 37 void Time::setTime( int h, int m, int s ) 38 { 39 hour = ( h >= 0 && h < 24 ) ? h : 0; 40 minute = ( m >= 0 && m < 60 ) ? m : 0; 41 second = ( s >= 0 && s < 60 ) ? s : 0; 42 } 43 44 // Print Time in military format 45 void Time::printMilitary() 46 { 47 cout << ( hour < 10 ? "0" : "" ) << hour << ":" 48 << ( minute < 10 ? "0" : "" ) << minute; 49 } 50 51 // Print time in standard format 52 void Time::printStandard() 53 { 54 cout << ( ( hour == 0 || hour == 12 ) ? 12 : hour % 12 ) 55 << ":" << ( minute < 10 ? "0" : "" ) << minute 56 << ":" << ( second < 10 ? "0" : "" ) << second 57 << ( hour < 12 ? " AM" : " PM" ); 58 } Source file uses #include to load the header file Source file contains function definitions 24  2000 Prentice Hall, Inc. All rights reserved. 6.8 Controlling Access to Members • public – Presents clients with a view of the services the class provides (interface) – Data and member functions are accessible • private – Default access mode – Data only accessible to member functions and friends – private members only accessible through the public class interface using public member functions  2000 Prentice Hall, Inc. All rights reserved. Outline 25 Outline 1 // Fig. 6.6: fig06_06.cpp 2 // Demonstrate errors resulting from attempts 3 // to access private class members. 4 #include <iostream> 5 6 using std::cout; 7 8 #include "time1.h" 9 10 int main() 11 { 12 Time t; 13 14 // Error: 'Time::hour' is not accessible 15 t.hour = 7; 16 17 // Error: 'Time::minute' is not accessible 18 cout << "minute = " << t.minute; 19 20 return 0; 21 } Compiling... Fig06_06.cpp D:\Fig06_06.cpp(15) : error C2248: 'hour' : cannot access private member declared in class 'Time' D:\Fig6_06\time1.h(18) : see declaration of 'hour' D:\Fig06_06.cpp(18) : error C2248: 'minute' : cannot access private member declared in class 'Time' D:\time1.h(19) : see declaration of 'minute' Error executing cl.exe. test.exe - 2 error(s), 0 warning(s) Attempt to access private member variable minute. 1. Load header file for Time class 2. Create an object of class Time 2.1 Attempt to set a private variable 2.2 Attempt to access a private variable Program Output Attempt to modify private member variable hour. 26  2000 Prentice Hall, Inc. All rights reserved. 6.9 Access Functions and Utility Functions • Utility functions – private functions that support the operation of public functions – Not intended to be used directly by clients • Access functions – public functions that read/display data or check conditions – Allow public functions to check private data • Following example – Program to take in monthly sales and output the total – Implementation not shown, only access functions  2000 Prentice Hall, Inc. All rights reserved. Outline 27 Outline 87 // Fig. 6.7: fig06_07.cpp 88 // Demonstrating a utility function 89 // Compile with salesp.cpp 90 #include "salesp.h" 91 92 int main() 93 { 94 SalesPerson s; // create SalesPerson object s 95 96 s.getSalesFromUser(); // note simple sequential code 97 s.printAnnualSales(); // no control structures in main 98 return 0; 99 } OUTPUT Enter sales amount for month 1: 5314.76 Enter sales amount for month 2: 4292.38 Enter sales amount for month 3: 4589.83 Enter sales amount for month 4: 5534.03 Enter sales amount for month 5: 4376.34 Enter sales amount for month 6: 5698.45 Enter sales amount for month 7: 4439.22 Enter sales amount for month 8: 5893.57 Enter sales amount for month 9: 4909.67 Enter sales amount for month 10: 5123.45 Enter sales amount for month 11: 4024.97 Enter sales amount for month 12: 5923.92 The total annual sales are: $60120.59 Create object s, an instance of class SalesPerson 1. Load header file and compile with the file that contains the function definitions 2. Create an object 2.1 Use the object’s member functions to get and print sales Program Output Use access functions to gather and print data (getSalesFromUser and printAnnualSales). Utility functions actually calculate the total sales, but the user is not aware of these function calls. Notice how simple main() is – there are no control structures, only function calls. This hides the implementation of the program. 28  2000 Prentice Hall, Inc. All rights reserved. 6.10 Initializing Class Objects: Constructors • Constructors – Initialize class members – Same name as the class – No return type – Member variables can be initialized by the constructor or set afterwards • Passing arguments to a constructor – When an object of a class is declared, initializers can be provided – Format of declaration with initializers: Class-type ObjectName( value1,value2,…); – Default arguments may also be specified in the constructor prototype  2000 Prentice Hall, Inc. All rights reserved. Outline 29 Outline 1 // Fig. 6.8: time2.h 2 // Declaration of the Time class. 3 // Member functions are defined in time2.cpp 4 5 // preprocessor directives that 6 // prevent multiple inclusions of header file 7 #ifndef TIME2_H 8 #define TIME2_H 9 10 // Time abstract data type definition 11 class Time { 12 public: 13 Time( int = 0, int = 0, int = 0 ); // default constructor 14 void setTime( int, int, int ); // set hour, minute, second 15 void printMilitary(); // print military time format 16 void printStandard(); // print standard time format 17 private: 18 int hour; // 0 - 23 19 int minute; // 0 - 59 20 int second; // 0 - 59 21 }; 22 23 #endif Notice that default settings for the three member variables are set in constructor prototype. No names are needed; the defaults are applied in the order the member variables are declared. 1. Define class Time and its default values  2000 Prentice Hall, Inc. All rights reserved. Outline 30 Outline 61 // Fig. 6.8: fig06_08.cpp 62 // Demonstrating a default constructor 63 // function for class Time. 64 #include <iostream> 65 66 using std::cout; 67 using std::endl; 68 69 #include "time2.h" 70 71 int main() 72 { 73 Time t1, // all arguments defaulted 74 t2(2), // minute and second defaulted 75 t3(21, 34), // second defaulted 76 t4(12, 25, 42), // all values specified 77 t5(27, 74, 99); // all bad values specified 78 79 cout << "Constructed with:\n" 80 << "all arguments defaulted:\n "; 81 t1.printMilitary(); 82 cout << "\n "; 83 t1.printStandard(); 84 85 cout << "\nhour specified; minute and second defaulted:" 86 << "\n "; 87 t2.printMilitary(); 88 cout << "\n "; 89 t2.printStandard(); 90 91 cout << "\nhour and minute specified; second defaulted:" 92 << "\n "; 93 t3.printMilitary(); 2. Create objects using default arguments 2.1 Print the objects Notice how objects are initialized: Constructor ObjectName(value1,value2…); If not enough values are specified, the rightmost values are set to their defaults.  2000 Prentice Hall, Inc. All rights reserved. Outline 31 Outline 2.1 (continued) Print the objects. Program Output OUTPUT Constructed with: all arguments defaulted: 00:00 12:00:00 AM hour specified; minute and second defaulted: 02:00 2:00:00 AM hour and minute specified; second defaulted: 21:34 9:34:00 PM hour, minute, and second specified: 12:25 12:25:42 PM all invalid values specified: 00:00 12:00:00 AM When only hour is specified, minute and second are set to their default values of 0. 94 cout << "\n "; 95 t3.printStandard(); 96 97 cout << "\nhour, minute, and second specified:" 98 << "\n "; 99 t4.printMilitary(); 100 cout << "\n "; 101 t4.printStandard(); 102 103 cout << "\nall invalid values specified:" 104 << "\n "; 105 t5.printMilitary(); 106 cout << "\n "; 107 t5.printStandard(); 108 cout << endl; 109 110 return 0; 111} 32  2000 Prentice Hall, Inc. All rights reserved. 6.12 Using Destructors • Destructors – Are member function of class – Perform termination housekeeping before the system reclaims the object’s memory – Complement of the constructor – Name is tilde (~) followed by the class name (i.e., ~Time) • Recall that the constructor’s name is the class name – Receives no parameters, returns no value – One destructor per class • No overloading allowed 33  2000 Prentice Hall, Inc. All rights reserved. 6.13 When Constructors and Destructors Are Called • Constructors and destructors called automatically – Order depends on scope of objects • Global scope objects – Constructors called before any other function (including main) – Destructors called when main terminates (or exit function called) – Destructors not called if program terminates with abort • Automatic local objects – Constructors called when objects are defined – Destructors called when objects leave scope • i.e., when the block in which they are defined is exited – Destructors not called if the program ends with exit or abort 34  2000 Prentice Hall, Inc. All rights reserved. 6.13 When Constructors and Destructors Are Called • Static local objects – Constructors called when execution reaches the point where the objects are defined – Destructors called when main terminates or the exit function is called – Destructors not called if the program ends with abort  2000 Prentice Hall, Inc. All rights reserved. Outline 35 Outline 1 // Fig. 6.9: create.h 2 // Definition of class CreateAndDestroy. 3 // Member functions defined in create.cpp. 4 #ifndef CREATE_H 5 #define CREATE_H 6 7 class CreateAndDestroy { 8 public: 9 CreateAndDestroy( int ); // constructor 10 ~CreateAndDestroy(); // destructor 11 private: 12 int data; 13 }; 14 15 #endif 1. Create a header file 1.1 Include function prototypes for the destructor and constructor  2000 Prentice Hall, Inc. All rights reserved. Outline 36 Outline 16 // Fig. 6.9: create.cpp 17 // Member function definitions for class CreateAndDestroy 18 #include <iostream> 19 20 using std::cout; 21 using std::endl; 22 23 #include "create.h" 24 25 CreateAndDestroy::CreateAndDestroy( int value ) 26 { 27 data = value; 28 cout << "Object " << data << " constructor"; 29 } 30 31 CreateAndDestroy::~CreateAndDestroy() 32 { cout << "Object " << data << " destructor " << endl; } Constructor and Destructor changed to print when they are called. 2. Load the header file 2.1 Modify the constructor and destructor  2000 Prentice Hall, Inc. All rights reserved. Outline 37 Outline 3. Create multiple objects of varying types 33 // Fig. 6.9: fig06_09.cpp 34 // Demonstrating the order in which constructors and 35 // destructors are called. 36 #include <iostream> 37 38 using std::cout; 39 using std::endl; 40 41 #include "create.h" 42 43 void create( void ); // prototype 44 45 CreateAndDestroy first( 1 ); // global object 46 47 int main() 48 { 49 cout << " (global created before main)" << endl; 50 51 CreateAndDestroy second( 2 ); // local object 52 cout << " (local automatic in main)" << endl; 53 54 static CreateAndDestroy third( 3 ); // local object 55 cout << " (local static in main)" << endl; 56 57 create(); // call function to create objects 58 59 CreateAndDestroy fourth( 4 ); // local object 60 cout << " (local automatic in main)" << endl; 61 return 0; 62 }  2000 Prentice Hall, Inc. All rights reserved. Outline 38 Outline 63 64 // Function to create objects 65 void create( void ) 66 { 67 CreateAndDestroy fifth( 5 ); 68 cout << " (local automatic in create)" << endl; 69 70 static CreateAndDestroy sixth( 6 ); 71 cout << " (local static in create)" << endl; 72 73 CreateAndDestroy seventh( 7 ); 74 cout << " (local automatic in create)" << endl; 75 } OUTPUT Object 1 constructor (global created before main) Object 2 constructor (local automatic in main) Object 3 constructor (local static in main) Object 5 constructor (local automatic in create) Object 6 constructor (local static in create) Object 7 constructor (local automatic in create) Object 7 destructor Object 5 destructor Object 4 constructor (local automatic in main) Object 4 destructor Object 2 destructor Object 6 destructor Object 3 destructor Object 1 destructor Notice how the order of the constructor and destructor call depends on the types of variables (automatic, global and static) they are associated with. Program Output 39  2000 Prentice Hall, Inc. All rights reserved. 6.14 Using Data Members and Member Functions • Member functions – Allow clients of the class to set (i.e., write) or get (i.e., read) the values of private data members – Example: Adjusting a customer’s bank balance • private data member balance of a class BankAccount could be modified through the use of member function computeInterest • A member function that sets data member interestRate could be called setInterestRate, and a member function that returns the interestRate could be called getInterestRate – Providing set and get functions does not make private variables public – A set function should ensure that the new value is valid 40  2000 Prentice Hall, Inc. All rights reserved. 6.15 A Subtle Trap: Returning a Reference to a Private Data Member • Reference to an object – Alias for the name of the object – May be used on the left side of an assignment statement – Reference can receive a value, which changes the original object as well • Returning references – public member functions can return non-const references to private data members • Should be avoided, breaks encapsulation  2000 Prentice Hall, Inc. All rights reserved. Outline 41 Outline 1 // Fig. 6.11: time4.h 2 // Declaration of the Time class. 3 // Member functions defined in time4.cpp 4 5 // preprocessor directives that 6 // prevent multiple inclusions of header file 7 #ifndef TIME4_H 8 #define TIME4_H 9 10 class Time { 11 public: 12 Time( int = 0, int = 0, int = 0 ); 13 void setTime( int, int, int ); 14 int getHour(); 15 int &badSetHour( int ); // DANGEROUS reference return 16 private: 17 int hour; 18 int minute; 19 int second; 20 }; 21 22 #endif Notice how member function badSetHour returns a reference (int & is the return type). 1. Define class 1.1 Function prototypes 1.2 Member variables  2000 Prentice Hall, Inc. All rights reserved. Outline 42 Outline 1. Load header 1.1 Function definitions 23 // Fig. 6.11: time4.cpp 24 // Member function definitions for Time class. 25 #include "time4.h" 26 27 // Constructor function to initialize private data. 28 // Calls member function setTime to set variables. 29 // Default values are 0 (see class definition). 30 Time::Time( int hr, int min, int sec ) 31 { setTime( hr, min, sec ); } 32 33 // Set the values of hour, minute, and second. 34 void Time::setTime( int h, int m, int s ) 35 { 36 hour = ( h >= 0 && h < 24 ) ? h : 0; 37 minute = ( m >= 0 && m < 60 ) ? m : 0; 38 second = ( s >= 0 && s < 60 ) ? s : 0; 39 } 40 41 // Get the hour value 42 int Time::getHour() { return hour; } 43 44 // POOR PROGRAMMING PRACTICE: 45 // Returning a reference to a private data member. 46 int &Time::badSetHour( int hh ) 47 { 48 hour = ( hh >= 0 && hh < 24 ) ? hh : 0; 49 50 return hour; // DANGEROUS reference return 51 } badSetHour returns a reference to the pri