WAP to understand the constructor

#include<iostream.h>

#include<conio.h>

class code

{

 public:

int id;

code()

{

id=0;

}

code (int a)

{

id=a;

}

code (code &x)

{

id=x.id;

}

void display()

{

cout<<"

 id="<<id;

}

};

void main()

{

       // int a;

       // cout<<"

 enter number :";

       // cin>>a;

code c;

c.display();

code d(50);

d.display();

code c1);

c1.display();

getch();

clrscr();

}