_Constructor in Object Oriented Programming

_Constructor in Object Oriented Programming

• 1.
  Constructors and Destructors Departmentof Computer Science & IT, The University of Lahore
• 2.
  Interface vs 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
• 3.
  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. Dot ( . ) replaced with underscore ( _ ) in file name.
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  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
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  Constructors  A constructoris a special function used to create an object. Constructor has exactly the same name as the defining class  Constructors can be overloaded (i.e., multiple constructors with different signatures) [Purpose: making it easy to construct objects with different initial data values).  A class may be declared without constructors. In this case, a no-argument constructor with an empty body is implicitly declared in the class known as default constructor  Note: Default constructor is provided automatically only if no constructors are explicitly declared in the class.
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  Constructors’ Properties  musthave the same name as the class itself.  do not have a return type—not even void.  play the role of initializing objects.
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  Constructors and Destructors –Constructor is a function in every class which is called when class creates its object • Basically it helps in initializing data members of the class • A class may have multiple constructors – Destructors is a function in every class which is called when the object of a class is destroyed • The main purpose of destructor is to remove dynamic memories etc.
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  10 // Timeabstract 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 Default Parameters in Constructor Initializing Objects
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  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(); Same constructor, used in overloaded style
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  OUTPUT Constructed with: all argumentsdefaulted: 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 }
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  Using Destructors • Destructors –Are member function of class – Perform termination housekeeping before the system reclaims the object’s memory – Name is tilde (~) followed by the class name (i.e., ~Time) – Receives no parameters, returns no value – One destructor per class (no overloading) – Destructors cannot be declared const, static – A destructor can be declared virtual or pure virtual
• 12.
  When Constructors andDestructors Are Called Constructors and destructors called automatically – Order depends on scope of objects 1. 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 2. Automatic local objects – Constructors called when objects are defined – Destructors called when objects leave scope – Destructors not called if the program ends with exit or abort
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  7 class CreateAndDestroy{ 8 public: 9 CreateAndDestroy( int ); // constructor 10 ~CreateAndDestroy(); // destructor 11 private: 12 int data; 13 }; 14 15 #endif
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  24 25 CreateAndDestroy::CreateAndDestroy( intvalue ) 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.
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  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 } 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 }
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  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. Static objects are declared with the keyword static. They are initialized only once and stored in the static storage area. The static objects are only destroyed when the program terminates i.e. they live until program termination
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  When do ConstructorsGet Called? • Class object creation time • Dynamically allocated object • Class argument passed by value • Class object returned by value • Object Array element (created)
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  What Constructors Do •Help in initializing: class data members Employee( ) { id = 0; } • Allocate memory for dynamic members Employee() { char* nameptr = new char[20];} • Allocate any needed resources – Such as to open files, etc.
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  Constructing Arrays ofObjects Complex c_arr[10]; Date date_arr[20]; Issue: There is no way to call argument-based constructors (non-default) for array members
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  Arrays of Objectsand Non-Default Constructors • Trick: declare an array of pointer to objects • Allocate and initialize each object in a loop Date *dates[31]; for (int day = 0; day < 31; ++day) { dates[day] = new Date(3, day, 2020); }
• 21.
  Default Member-wise Assignment •Assignment operator (=) can be used to assign an object to another object of the same type. • Member-wise assignment: each data member of the object on the right of the assignment operator is assigned individually to the same data member in the object on the left 