Library Management System in C++

#include<iostream.h>
#include<conio.h>

int a=2,b=3,c=4,i=0;
class lib
{
int n;
public:

void issue();
void show();
void ret();
};

 void lib::issue()
{                                                               
cout<<"enter code:";
cin>>n;


switch(n)
{
case 101:
if(a!=i)
{
a--;
cout<<"morish mano is issued\nno. of books="<<a<<"\n\n";

}
break;
case 102:
if(b!=i)
{
b--;
cout<<"peter syckes is issued\n no. of book="<<b<<"\n\n";

}
break;
case 103:
if(c!=i)
{
c--;
cout<<"ncert maths is issued\n no. of book="<<c<<"\n\n";

}
break;
}
}
void lib::ret()
{
cout<<"enter code to return the book:";
cin>>n;


switch(n)
{
case 101:
if(a!=2)
{
a++;
cout<<"morish mano is returned\nno. of books="<<a;

}
break;
case 102:
if(b!=3)
{
b++;
cout<<"peter syckes is returned\n no. of book="<<b;

}
break;
case 103:
if(c!=4)
{
c++;
cout<<"ncert maths is returned\n no. of book="<<c;

}
break;
}
}

void lib::show()
{
cout<<"book code:101\n";
cout<<"name: morish mano\n";
cout<<"availability:"<<a<<"\n\n";
cout<<"book code:102\n";
cout<<"name: peter sykes\n";
cout<<"availability:"<<b<<"\n\n";
cout<<"book code:103\n";
cout<<"name: ncert maths\n";
cout<<"availability:"<<c<<"\n\n";
}
void main()
{
class lib lb;
int k,m;
clrscr();
x:
cout<<"enter 1 to see the list of books\n";
cout<<"enter 2 to issue the book\n";
cout<<"enter 3 to return the book\n";
cout<<"number:";
cin>>k;
switch(k)
{
case 1:
lb.show();
cout<<"\nenter 0 to go to previous:";
cin>>m;
if(m==0)
goto x;
else
break;
case 2:
lb.issue();
cout<<"\nenter 0 to go to previous:";
cin>>m;
if(m==0)
goto x;
else
break;
case 3:
lb.ret();
cout<<"\nenter 0 to go to previous:";
cin>>m;
if(m==0)
goto x;
else
break;
}
getch();
}