Program:

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

    class sumdigit
    {
    public:
    int add(int);
    };
    int sumdigit::add(int(num)
    {
    if(num<=9)
    return(num);
    else
    return(num%10+add(num%10));
    }
    void main()
    {
    sumdigit ob;
    int num,sum;
    clrscr();
    cout<<"Enter the Positive integer \n";
    cin>>num;
    sum=ob.add(num);
    cout<<"\nSum of Digits of "<<num<<" = "<<sum;
    getch();
    }