Functions in C++

Hi guys!the program below is of a small standard calculator.
It is a program by passing a reference to a function..
If this program give errors please let me know,i am ready to help any time..
May this program helps you alot!!


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

using namespace std;
char op;
void add(double, double,char );
void sub(double, double,char);
void mul(double, double,char);
void div(double, double,char);

main()
{
double num1,num2;
char op;
cout<<"Enter 1st number:";
cin>>num1;
cout<<"Enter 2nd number:";
cin>>num2;
cout<<"Tell the operation:";
cin>>op;
add(&num1,&num2,&op);
getch();
return 0;
}

void add(double *a, double *b)
{
      
    switch(op)
    {
     case 1:
     cout<<"Sum is:"<<*a+*b;
     break;
     case 2:
     sub(&a,&b);
     break;
     case 3:
     mul(&a,&b);
     break;
     case 4:
    div(&a,&b);
    break;
     default:
    cout<<"operation not available"
     }
     }
void sub(double *c, double *d)
{
   
     cout<<"Difference is:"<<*c-*d;
     }
void mul(double *e, double *f)
{
     cout<<"Product is:"<<*e**f;

     }
void div(double *g, double *h)
{

     cout<<"Quotient is:"<<*g / *h;
     }