Constructor & Destructor

Constructor & Destructor

Constructors • Def- A constructor is a special member function that is a member of a class and has same name as that class. • Use- It is used to initialize the object of the class type with a legal initial value.

Special Characteristics of Constructors • These are called automatically when the objects are created. • All the objects of the class having a constructor are initialized before some use. • These should be declared in the public section for availability to all the functions. • Constructor has no return type even void. • They can not be inherited. • These cannot be static. • Default and copy constructor are generated by the compiler whenever required. Generated constructors are public. • Constructors can have default argument as other C++ functions.

Declaration and Definition • It can be defined either inside the class definition or outside the class definition class X { int i ; public: int j,k ; X() { i = j = k =0;} };

class X { int i ; public: int j,k ; X() { i = j = k =0;} }; X::X() { I = j = k =0; }

Note • Generally a constructor should be defined under the public section of a class, so that its object can be created in any function

class X { int i ; X() { i = j = k =0;} public: int j,k ; void check(void); // member function }; void X :: check (void) { X obj1; //valid } int main() { X obj2; //invalid }

Default constructor • A constructor that accept no parameter is called the default constructor.

Copy Constructor • A copy Constructor is used to initialize an object from another object. • If you have not defined a copy constructor, the complier automatically creates it and it is public • Copy constructor always takes argument as a reference object. Consider the following class definition

class sample { int i, j; public: sample ( int a, int b) { i = a; j = b; } sample ( sample & s) { i = s.i ; j = s.j ; } void print(void) { cout<<i <<“ “<<j <<“\n”; } Above constructors may be used as follows sample s1 (10,12); //1st const. called sample s2 (s1) // copy const. called sample s3 = s1; copy const. called

Home work • In a class does not define a constructor, what will be the initial value of its object. • What are the significance of default constructor. • How many situations when a copy constructor is automatically called.

Default Arguments • Just like any other function a constructor can also have default arguments • This constructor is equivalent to default constructor. Because its also allows us to create objects without any value provided. • For example

intmain() { A obj1; A obj2(250); A obj3(2,4); getch(); return 0; } class A { int i,j; public: X(int x=10,int y=20); }; A::X(int x,int y) { i=x; j = y; } obj1 obj2 obj3 i=250 j=20 i=10 j=20 i=2 j=4

Order of Constructor Invocation • The objects are constructed in the order they are defined. Consider the following statement • Sample s1,s2,s3; // the order of construction is s1,s2,s3. • But when a class containing objects of another class as its members. In such a case, the constructor for member objects are invoked first and then only, the constructor for containing class is invoked. • For example

class C { private: A objA; B objB; public: C() { cout<<“Constructing C” <<endl; }}; int main() { clrscr(); C objC; getch(); } class A { public: A() { cout<<“Constructing A” <<endl; }}; class B { public: B() { cout<<“Constructing B” <<endl; }};

Constructor & Destructor