C++ program to explain default, parameterized and copy constructor

#include<iostream.h>
class vector
{
 public:
  int *a;
  int b;
//default constructor
 vector()
  {
   a = new int;
   *a=10;
  }
//parameterized constructor
 vector(int k)
  {
   a = new int;
   *a=k;
  }
 void getvalue()
  {
   cout<<"a="<<*a<<"\n";
  }
//copy constructor
  vector(vector &v)
  {
  a = new int;
  *a=*v.a;
  }
};
void main()
{
 vector vector1; //invoke default constructor
 vector1.getvalue();
 vector vector2(25); //invoke parameterized constructor
 vector.getvalue();
 vector vector3;
 vector vector4(vector3); //invoke copy constructor
 vector4.getvalue();
}
output
a=10
a=25
a=10