C++ default constructor

A class constructor is 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 can be very useful for setting initial values for certain member variables.

Following example explains the concept of constructor −


 Live Demo

#include <iostream>
using namespace std;
class Line {
      void setLength( double len );
      double getLength( void );
      Line(); // This is the constructor
      double length;
// Member functions definitions including constructor
Line::Line(void) {
   cout << "Object is being created" << endl;
void Line::setLength( double len ) {
   length = len;
double Line::getLength( void ) {
   return length;
// Main function for the program
int main() {
   Line line;
   // set line length
   cout << "Length of line : " << line.getLength() <<endl;
   return 0;


Object is being created
Length of line : 6