Question

constructor and destructors in c++

Answer 1

A classconstructoris a special member function of a class that is executed whenever we create new objects of that class.A constructor will have exact same name as the class and it does not have any return type at all, not even void. Constructors canbe very useful for setting initial values for certain member variables.Following example explainsthe concept of constructor:#include<iostream>usingnamespace std;classLine{public:void setLength(double len );double getLength(void);Line();// This is the constructorprivate:double length;};// Member functions definitions including constructorLine::Line(void){ cout <<"Object is being created"<< endl;}voidLine::setLength(double len ){ length = len;}doubleLine::getLength(void){return length;}// Main function for the programint main(){Line line;// set line length line.setLength(6.0); cout <<"Length of line : "<< line.getLength()<<endl;return0;}When the above code is compiled and executed, it produces the following result:Objectis being createdLength of line :6Parameterized Constructor:A default constructor does not have any parameter, butif you need, a constructor can have parameters. This helps you to assign initial value to an object at the time of its creation as shown in the following example:#include<iostream>usingnamespace std;classLine{public:void setLength(double len );double getLength(void);Line(double len);// This is the constructorprivate:double length;};// Member functions definitions including constructorLine::Line(double len){ cout <<"Object is being created, length = "<< len << endl; length = len;}voidLine::setLength(double len ){ length = len;}doubleLine::getLength(void){return length;}// Main function for the programint main(){Line line(10.0);// get initially set length. cout <<"Length of line : "<< line.getLength()<<endl;// set line length again line.setLength(6.0); cout <<"Length of line : "<< line.getLength()<<endl;return0;}When the above code is compiled and executed, it produces the following result:Objectis being created, length =10Length of line :10Length of line :6Using Initialization Lists to Initialize Fields:In case of parameterized constructor, you can use following syntax to initializethe fields:Line::Line(double len): length(len){ cout <<"Object is being created, length = "<< len << endl;}Above syntax is equal to the following syntax:Line::Line(double len){ cout <<"Object is being created, length = "<< len << endl; length = len;}If for a class C, you have multiple fields X, Y, Z, etc., to be initialized, then use can use same syntax and separate the fields by comma as follows:C::C(double a,double b,double c): X(a), Y(b), Z(c){....}The Class Destructor:Adestructoris a special member function of a class that is executed whenever an object of it's class goes out of scope or whenever the delete expression is applied to a pointer to the object of that class.A destructor will have exactsame name as the class prefixed with a tilde (~) and it can neither return a value nor can it take any parameters. Destructor can be very useful for releasing resources before coming out of the program like closing files, releasing memories etc.Following example explainsthe concept of destructor:#include<iostream>usingnamespace std;classLine{public:void setLength(double len );double getLength(void);Line();// This is the constructor declaration~Line();// This is the destructor: declarationprivate:double length;};// Member functions definitions including constructorLine::Line(void){ cout <<"Object is being created"<< endl;}Line::~Line(void){ cout <<"Object is being deleted"<< endl;}voidLine::setLength(double len ){ length = len;}doubleLine::getLength(void){return length;}// Main function for the programint main(){Line line;// set line length line.setLength(6.0); cout <<"Length of line : "<< line.getLength()<<endl;return0;}When the above code is compiled and executed, it produces the following result:Objectis being createdLength of line :6Objectis being deleted