private怎么用？

既然不能输入，要来干什么？好象也无法修改吧？
#include <iostream.h>
class yunsuan
{
public:
    int add(int a,int b);
    int cha(int a,int b);
    int ji(int a,int b);
    int shang(int a,int b);
private:
    int a,b;
};
yunsuan::add(int a,int b)
{
    return a+b;
}
yunsuan::cha(int a,int b)
{
    return a-b;
}
yunsuan::ji(int a,int b)
{
    return a*b;
}
yunsuan::shang(int a,int b)
{
    return a/b;
}




int main()
{
    yunsuan temp;
    int a,b;
    int c;
    cout<<"a=";
    cin>>a;
    cout<<"b=";
    cin>>b;
    cout<<"select 1:+;2:-;3:*;4:/;";
    cin>>c;
    switch (c){
    case 1:
        {
            cout<<temp.add(a,b)<<endl;
            break;
        }
    case 2:
        {
            cout<<temp.cha(a,b)<<endl;
            break;
        }
    case 3:
        {
            cout<<temp.ji(a,b)<<endl;
            break;
        }
    case 4:
        {
            if(b==0)cout<<"erreo"<<endl;
        else
            cout<<temp.shang(a,b)<<endl;
        break;
        }
    default:
        cout<<"erreo"<<endl;
    }

return 0;
}