Queue implementing array

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


class queue
{
private:
    int a[10],f,r,max;
public:
    queue()
    {
     f=r=0;
     cout<<"Queue is created"<<endl;
     cout<<"Enter limit : ";
     cin>>max;
     }
    void insertion();
    void deletion();
    void display();
    void menu();
};

void queue::insertion()
{
    if(r<max)
    {
       cout<<"Enter a value :";
       cin>>a[r++];
    }
    else
       cout<<"Queue is full"<<endl;
}

void queue::deletion()
{
    if(f==max)
       cout<<"Queue is empty"<<endl;
    else
    {
       cout<<a[f]<<" is deleted from the queue"<<endl;
       a[f++]=NULL;
    }
}

void queue::display()
{
    cout<<"Queue elements are : ";
    for(int i=f;i<r;i++)
       cout<<a[i]<<" ";
}

void queue::menu()
{
    int ch;
    while(1)
    {
    cout<<endl;
    cout<<"1.Insert"<<endl
        <<"2.Delete"<<endl
        <<"3.Display"<<endl
        <<"4.Exit"<<endl
        <<"Enter your choice : ";
    cin>>ch;
    switch(ch)
    {
      case 1: insertion();
          break;
      case 2: deletion();
          break;
      case 3: display();
          break;
      case 4: exit(0);

      default: cout<<"sorry! wrong entry "<<endl;
           break;
    }
    }
}

void main()
{
clrscr();
queue q;
q.menu();
getch();
}