Constructor Destructor.pdf

Rising Technologies, Jalna (MH). + 91 9423156065, http://www.RisingTechnologies.in

1

Introduction A constructor is a special member function whose task is to initialize the data members of an objects of its class. It is special because it has same name as its classname. It invokes automatically whenever a new object of its associated class is created. It is called constructor because it constructs the initial values of data members and build your programmatic object.

Rising Technologies, Jalna (MH). + 91 9423156065, http://www.RisingTechnologies.in

2

Introduction It is very common for some part of an object to require initialization before it can beused. Suppose you are working on 100's of objects and the default value of a particular data member is needed to bezero. Initialising all objects manually will be very tedious job. Instead, you can define a constructor function which initialises that data member to zero. Then all you have to do is declare object and constructor will initialise objectautomatically.

Rising Technologies, Jalna (MH). + 91 9423156065, http://www.RisingTechnologies.in

3

Constructor Example class add { int m, n ; public : add (); };

When a class contains a constructor, it is guaranteed that an object created by the class will be initialized automatically. add a ;

add :: add () { m = 0; n = 0; }

Above declaration not only creates the object a of type add, but also initializes its data members m and n to zero.

Rising Technologies, Jalna (MH). + 91 9423156065, http://www.RisingTechnologies.in

4

Constructors There is no need to write any statement to invoke the constructor function. If a ‘normal’ member function is defined for initialization, we need to invoke that function for each and every objects separately. Aconstructor that accepts noparameters iscalled the default constructor. The default constructor for class A will be A : : A ( )

Rising Technologies, Jalna (MH). + 91 9423156065, http://www.RisingTechnologies.in

5

Characteristics of Constructor They must be declared in the publicscope. They are invoked automatically when the objects are created. They do not have return types, not even void and they cannot return values. They cannot be inherited, though a derived class can call the base class constructor. Like other C++functions, Constructors can have default arguments. Constructors can not be virtual.

Rising Technologies, Jalna (MH). + 91 9423156065, http://www.RisingTechnologies.in

6

Characteristics of Constructor We can not refer to their addresses. An object with a constructor (or destructor) can not be used as a member of a union. They make ‘implicit calls’ to the operators new and delete when memory allocation is required.

Rising Technologies, Jalna (MH). + 91 9423156065, http://www.RisingTechnologies.in

7

Constructor The constructor function is responsible for creation ofobject. But in previous examples, we have not defined any constructor in class, so how come the objects were created of thoseclasses? The answer is, If no constructor is defined in the class in such situation the compiler implicitly provides a constructor, which is called asdefault constructor.

Rising Technologies, Jalna (MH). + 91 9423156065, http://www.RisingTechnologies.in

8

Constructor class sample { int someDataMember; public: void someFunction () { .. .. } };

After Compilation

class sample { int someDataMember; public : sample() { } void someFunction () { .. .. } };

Compiler has implicitly added a constructor to the class, which has empty body, because compiler is not supposed to put any logic in that. Rising Technologies, Jalna (MH). + 91 9423156065, http://www.RisingTechnologies.in

9

Types of Constructor Default Constructor/Non-Arg Constructor Parameterized Constructor Copy Constructor Dynamic Constructor

Rising Technologies, Jalna (MH). + 91 9423156065, http://www.RisingTechnologies.in

10

Default A constructor without Constructor

any parameter is known as non-arg constructor or simply default constructor. If no constructor is defined in a class, then compiler implicitly provides an empty body constructor which is called as default constructor.

Rising Technologies, Jalna (MH). + 91 9423156065, http://www.RisingTechnologies.in

11

Non-Arg Constructor Example class circle { float radius; public: circle() { radius = 0; } };

In example beside, the no constructor function argument, and simply initializes takes radius to zero. Non-arg constructor is also called as default constructor.

Rising Technologies, Jalna (MH). + 91 9423156065, http://www.RisingTechnologies.in

12

Default Constructor Example class circle { float radius; public: };

In example beside, we have not defined any constructor, so compiler will provide an empty body constructor to the class, which is called as default constructor.

class circle { float radius; public: circle() { } }; Rising Technologies, Jalna (MH). + 91 9423156065, http://www.RisingTechnologies.in

13

Parameterised Constructors Sometimes it becomes necessary to initialize the various data elements of an objects with different values when they are created. This is achieved by passing arguments to the constructor function when the objects are created. arguments are called The constructors that can take parameterized constructors.

Rising Technologies, Jalna (MH). + 91 9423156065, http://www.RisingTechnologies.in

14

Parameterised Constructors class circle { float radius; public: circle() { radius = 0; } circle(float r) { radius = r; }

Non-Arg (Default) constructor, which takes no arguments

Parametirised constructor, which takes 1arguments as radius.

};

Rising Technologies, Jalna (MH). + 91 9423156065, http://www.RisingTechnologies.in

15

Parameterised Constructors class circle { float radius; public: circle() { radius = 0; } circle(float r) { radius = r; }

When a constructor is parameterized, we must pass the arguments to the constructor function when an object is declared. Consider following declaration circle circle

firstObject; secondObject(15);

};

Rising Technologies, Jalna (MH). + 91 9423156065, http://www.RisingTechnologies.in

16

Two Ways of calling aConstructor class circle { float radius; public: circle() { radius = 0; }

o Implicit call (shorthand method) circle ob(7.6);

o Explicit call circle ob; ob = circle(7.6);

circle(float r) { radius = r; }

};

Rising Technologies, Jalna (MH). + 91 9423156065, http://www.RisingTechnologies.in

17

Multiple Constructors in aClass C+ + permits to use more than one constructors in a singleclass. Add( ) ; / / No arguments Add (int, int) ; / / Two arguments

Rising Technologies, Jalna (MH). + 91 9423156065, http://www.RisingTechnologies.in

18

Multiple Constructors in aClass class add { int m, n; public : add ( ) {m = 0 ; n = 0;} add (int a, int b) {m = a ; n = b ;} add (add & i) {m = i.m ; n = i.n ;} };

The first constructor receives no arguments. The second constructor receives two integer arguments. The third constructor receives one add object as an argument.

Rising Technologies, Jalna (MH). + 91 9423156065, http://www.RisingTechnologies.in

19

Multiple Constructors in aClass class add { int m, n; public : add ( ) {m = 0 ; n = 0;} add (int a, int b) {m = a ; n = b ;} add (add & i) {m = i.m ; n = i.n ;} };

Add a1; Would automatically invoke the first constructor and set both m and n of a1 to zero.

Add a2(10,20); Would call the second constructor which will initialize the data members m and n of a2 to 10 and 20 respectively.

Rising Technologies, Jalna (MH). + 91 9423156065, http://www.RisingTechnologies.in

20

Multiple Constructors in aClass class add { int m, n; public : add ( ) {m = 0 ; n = 0;} add (int a, int b) {m = a ; n = b ;} add (add & i) {m = i.m ; n = i.n ;} };

Add a3(a2); Would invoke the third constructor which copiesthe values of a2 into a3. This type of constructor is called the “copy constructor”.

Construction Overloading More than one constructor function is defined in a class.

Rising Technologies, Jalna (MH). + 91 9423156065, http://www.RisingTechnologies.in

21

Multiple Constructors in a Class class complex complex ( ) { } {

float x, y; public : complex ( ) { } complex (float a) { x =y =a; } complex (float r, float i) { x =r ; y =i } ------

This contains the emptybody and does not do anything. This is used to create objects without any initial values.

};

Rising Technologies, Jalna (MH). + 91 9423156065, http://www.RisingTechnologies.in

22

Multiple Constructors in aClass C++ compiler has an implicit constructor which creates objects, even though it was not defined in the class. This works well as long as we do not use any other constructor in the class. However, once we define a constructor, we must also define the “do-nothing” implicit constructor.

Rising Technologies, Jalna (MH). + 91 9423156065, http://www.RisingTechnologies.in

23

Constructor Overloading /*.....A program to highlight the concept of constructor overloading.......... */ #include using namespace std; class ABC { private: int x,y; public: ABC () //constructor 1 with no arguments { x = y = 0; } ABC(int a) //constructor 2 with one argument { x = y = a; } Rising Technologies, Jalna (MH). + 91 9423156065, http://www.RisingTechnologies.in

24

Constructor Overloading ABC(int a,int b) //constructor 3 with two argument { x = a; y = b; } void display() { cout << "x = " << x << " and " << "y = " << y << endl; } };

Rising Technologies, Jalna (MH). + 91 9423156065, http://www.RisingTechnologies.in

25

Constructor Overloading int main() { ABC cc1; //constructor 1 ABC cc2(10); //constructor 2 ABC cc3(10,20); //constructor 3 cc1.display(); cc2.display(); cc3.display(); return 0; } //end of program Rising Technologies, Jalna (MH). + 91 9423156065, http://www.RisingTechnologies.in

26

Constructors with DefaultArguments It is possible to define constructors with default arguments. Consider complex (float real, float imag = 0); The default value of the argument imag is zero. complex C1 (5.0) assigns the value 5.0 to the real variable and 0.0 to imag. complex C2(2.0,3.0) assigns the value 2.0 to real and 3.0 to imag.

Rising Technologies, Jalna (MH). + 91 9423156065, http://www.RisingTechnologies.in

27

Dynamic Initialization of Objects Providing initial value to objects at run time. We can provide various initialization Advantage – formats, using overloaded constructors. This provides the flexibility of using different format of data at run time depending upon the situation.

Rising Technologies, Jalna (MH). + 91 9423156065, http://www.RisingTechnologies.in

29

Copy Constructor Acopy constructor is used to declare and initialize an object from another object. integer (integer & i); integer I 2 ( I 1) ; or integer I 2 = I 1 ; The process of initializing through a copy constructor is known as copy initialization.

Rising Technologies, Jalna (MH). + 91 9423156065, http://www.RisingTechnologies.in

30

Copy Constructor The statement I 2 = I 1; will not invoke the copyconstructor. If I 1 and I 2 are objects, thisstatement is legal and assigns the values of I 1 to I2, member-by-member.

Rising Technologies, Jalna (MH). + 91 9423156065, http://www.RisingTechnologies.in

31

Copy Constructor A reference variable has been used as an argument to the copy constructor. We cannot pass the argument by value to a copy constructor.

Rising Technologies, Jalna (MH). + 91 9423156065, http://www.RisingTechnologies.in

32

Example #include using namespace std; class Point { private: int x, y; public: Point(int x1, int y1): x(x1), y(y1) { } // Copy constructor Point(const Point &p2) : x (p2.x), y(p2.y) {}

//

Point(const Point &p2) {x = p2.x; y = p2.y; } int getX() int getY()

};

{ {

return x; } return y; }

Example int main() { Point p1(10, 15); // Normal constructor is called here Point p2 = p1; // Copy constructor is called here // Let us access values assigned by constructors cout << "p1.x = " << p1.getX() << ", p1.y = " << p1.getY(); cout << "\np2.x = " << p2.getX() << ", p2.y = " << p2.getY(); return 0; }

Object as an Argument Example #include using namespace std; class Demo { private: int a; public: void set(int x) { a = x; } void sum(Demo ob1, Demo ob2) { a = ob1.a + ob2.a; } void print() { cout<<"Value of A : "<
35

Example 2 int main() { //object declarations Demo d1; Demo d2; Demo d3; //assigning values to the data member of objects d1.set(10); d2.set(20); //passing object d1 and d2 d3.sum(d1,d2); //printing the values d1.print(); d2.print(); d3.print(); return 0; } Rising Technologies, Jalna (MH). + 91 9423156065, http://www.RisingTechnologies.in

36

Dynamic Constructors The constructors can also be used to allocate memory while creating objects. This will enable the system to allocate the right amount of memory for each object when the objects are not of the same size.

Rising Technologies, Jalna (MH). + 91 9423156065, http://www.RisingTechnologies.in

37

Dynamic Constructors Allocation of memory to objects at the time of their construction is known as dynamic construction of objects. The memory is created with the help of the new operator.

Rising Technologies, Jalna (MH). + 91 9423156065, http://www.RisingTechnologies.in

38

Destructors A destructor is used to destroy the objects that havebeen created by a constructor. Like constructor, the destructor is a member function whose name is the same as the class name but is preceded by atilde. eg: ~ integer ( ) { }

Rising Technologies, Jalna (MH). + 91 9423156065, http://www.RisingTechnologies.in

39

Destructors A destructor never takes any argument nor does it return any value. It will be invoked implicitly by the compiler upon exit from the program – or block or function as the case may be – to clean up storage that is no longer accessible.

Rising Technologies, Jalna (MH). + 91 9423156065, http://www.RisingTechnologies.in

40

Destructors It is a good practice to declare destructors ina program since it releases memory space for further use. Whenever new is used toallocate memory in the constructor, we should use delete to free that memory.

Rising Technologies, Jalna (MH). + 91 9423156065, http://www.RisingTechnologies.in

41