Chapter 6 Constructors and Destructors ?????????

1
Chapter 6 Constructors and Destructors ?????????

• 6.1 Basic Constructors
• 6.2 Dynamic Initialization of Objects
• 6.3 Dynamic Constructors
• 6.4 Copy Constructors
• 6.5 Destructors

2
6.1 Simple Constructors
class integer private int m,n public
integer( )
integer integer( ) m0 n0 int
main() integer i

• A special member function to construct objects
• Plays the role of initializing objects
• Exactly the same name as the class
• Executed automatically by the system, NOT YOU!

3
Constructor is Special

• Should be declared in pubic section
• No return type, not even void
• Make implicit calls to the operators new and
  delete when memory allocation is required.
• Cant refer their addresses
• An object with a constructor (or destructor)
  cant be used as a member of a union

4
Default Constructor

• A special constructor
• No parameters, empty body
• Provided by the system, implicitly
• only if NO explicitly declared constructors

class Circle public double radius
double getArea() return radius
radius 3.14159
5
Parameterized Constructor
class integer int m,n public
integer(int x, int y) integerinteger(int x,
int y) m x n y
integer int1//??????
Call explicitly integer int1
integer(0,100) Call implicitly integer
int1(0,100)
6
Overloading Constructors

• class integer
• int m, n
• public
• integer() //constructor 1
• m0 n0
• integer(int x,int y) //constructor 2
• ma nb
• integer(integer i) //constructor 3
• mi.m ni.n
• //////////////////////////////////////////
• integer I1
• integer I2(20,40)
• integer I3(I2)

7
Overloading Constructors

• class complex
• float x, y
• public
• complex() x y 0
• complex(float a) x y a
• complex(float real, float imag)
• x real
• y imag
• friend complex sum(complex, complex)
• friend void show(complex)

complex sum(complex cl, complex c2) complex
c3 c3.x cl.x c2.x c3.y cl.y
c2.y return(c3) void show(complex c)
coutltltc.xltlt" j"ltltc.yltlt"\n"
8

• int main()
• complex A(2.7, 3.5)// define initialize
• complex B(1.6)// define initialize
• complex C// define
• C sum(A, B)// sum() is a friend
• coutltlt"A " show(A)// show() is also
  friend
• coutltlt"B " show(B)
• coutltlt"C " show(C)
• // Another my to give initial values (second
  method)
• complex P,Q,R// define P, Q and R
• P complex(2.5,3.9)//initialize P
• Q complex(1.6,2.5)// initialize Q
• R sum(P,Q)
• cout ltlt"\n"
• coutltlt"P " show(P)
• coutltlt"Q " show(Q)
• coutltlt"R " show(R)
• return 0

9
Constructors with Default Arguments

• //imag is 0 by default
• complex( float real, float imag0)
• x real y imag
• ////////////////////
• complex C(5.0)
• complex C(2.0,3.0)

default constructor AA() default argument
constructor AA(int 0)
10
Constant Objects
const matrix X(m,n)

• The object is constant
• Attempt to modify the data fields will generate
  compile-time error.
• Constant object can only call const member
  functions.

11
6.2 Dynamic Initialization of Objects

• Initialize the object a runtime
• Constructor match the input will be invoked
• Code sample

class Fixed_deposit long int P_amount//
Principal amount int Years// Period of
investment float Rate// Interest rate
float R_value// Return value of amount public
Fixed_deposit() Fixed_deposit(long int
p, int y, float r0.12) Fixed_deposit(long
int p, int y, int r) void display(void)
12

• Fixed_deposit Fixed_deposit(long int p, int y,
  float r)
• P_amount p Years y Rate r
• R_value P_amount
• for (int i 1 i lt y i)
• R_value R_value (1.0 r)
• Fixed_deposit Fixed_deposit(long int p, int y,
  int r)
• P_amount p Years y Rate r
• R_value P_amount
• for (int i1 ilty i)
• R_value R_value(1.0float(r)/100)
• void Fixed_deposit display(void)
• coutltlt"\n"
• ltlt"Principal Amount "ltlt P_amount ltlt
  "\n"
• ltlt"Return Value " ltlt R_value ltlt"\n"

13

• int main()
• Fixed_deposit FD1, FD2, FD3 // deposits
  created
• long int p int y float r int R
• cout ltlt "Enter amount,period,interest rate(in
  percent)\n"
• cin gtgt p gtgt y gtgt R
• FD1 Fixed_deposit(p,y,R)
• cout ltlt "Enter awount,period,interest
  rate(decimal)\n"
• cin gtgt p gtgt y gtgt r
• FD2 Fixed_deposit(p,y,r)
• cout ltlt "Enter amount and period" ltlt "\n"
• cin gtgt p gtgt y
• FD3 Fixed_deposit(p,y)
• cout ltlt "\nDeposit 1" FD1.display()
• cout ltlt "\nDeposit 2" FD2.display()
• cout ltlt "\nDeposit 3" FD3.display()
• return 0

14
Constructor Initializer
class Time public Time(int hr, int min,
int sec) private int hour int
minute int second
class Action public Action()
id 0 private int
id Time time
Action()time(0, 0, 0) id 0
Action(int hr, int min, int sec)
time(hr, min, sec) id 0
(0, 0, 0) //???
time ?

• To initialize data fields of the class

15
6.3 Dynamic Constructors

• Dynamic constructor
• allocate memory while creating object

class String char name int
length public String()// constructor-l
length 0 name new charlength 1
String(char s) // constructor-2
length strlen(s) name new charlength
1 strcpy(name, s)
void display(void)
cout ltlt name ltlt \n
void join(String a, String b)
16

• void String join(String a, String b)
• length a.length b.length
• delete name
• name new charlength1// dynamic
  allocation
• strcpy(name, a.name)
• strcat(name, b.name)
• int main()
• char first "Joseph "
• String namel(first), name2("Louis")
• String name3("Lagrange"), sl,s2
• sl.join(namel, name2)
• s2.join(sl, name3)
• namel.display() name2.display()
  name3.display()
• sl.display() s2.display()
• return 0

17
6.4 Copy Constructor

• A Special Constructor
• ClassName(ClassName )
• To create an object initialized with another
  objects data
• A default copy constructor is provided by C
• For example

integer(integer i) integer I2(I1) integer I2
I1 Circle circle1(5) Circle circle2(circle1)
equivalent
18
When Is Copy Constructor Invoked

• Explicit object initialization
• Copy of temporary object for parameter
• Copy of temporary object for return value

X a //Common construction X b a //Copy
construction ? X b(a)
X fun2() X t return
t //Copy construction b fun2( ) //Common
copy
void fun(X para)
fun(a) //a-gtcopy construction-gtpara
19
Shallow Copy vs. Deep Copy

• Shallow copy
• The default copy constructor simply copies each
  data field in one object to its counterpart in
  the other object
• If the field is a pointer to some object, the
  address of the pointer is copied rather than its
  contents

Run
ShallowCopyDemo
20
Shallow Copy Demo
PersonPerson(Person person) id person.id
birthDate person.getBirthDate()
21
Deep Copy -- Customized Copy Constructor

• Explicitly define copy constructor
• Allocate new memory for pointers to point

PersonPerson(Person person) id person.id
Date p person.getBirthDate() birthDate
new Date(p)
Person1.cpp
Person1 .h
Run
CustomCopyConstructor
22
6.5 Destructors

• Invoked to destroy an object
• The opposite of constructor
• Like new vs. delete
• Named as ClassName

Circle() numberOfObjects--
TestCircle6.cpp
Circle6.h
Circle6.cpp
Run
23
Notes

• No return value, no parameters
• So, no overloading
• A default destructor is available if
• No destructor is explicitly defined
• A destructor is useful if some resource, e.g.
  memory is allocated dynamically by an object

PersonPerson(int id, int year, int month, int
day) this -gt id id birthDate
new Date(year, month, day)
PersonPerson() delete birthDate
24
Summary

• Constructors to initialize object
• Initialize object and allocate memory
• Special characteristics
• Overloading
• Copy constructor
• Shallow copy vs. deep copy
• Const object for non-modify
• Destructor to destroy objects

25
Review Questions

• Q6.1
• Q6.9