新手求进!!!(谢谢)

#include<iostream.h>
class xxx
{
    private:
        int a;
        int b;
    public:
        xxx(){a=5;b=45;}
        xxx(int x,int y){a=x;b=y;}
        int setk(){return a;}
        void print();
};
void xxx::print()
{
    cout<<"a="<<a<<",b="<<b<<endl;
}
int add(const xxx &s1,const xxx &s2)
{
    int sum;
    sum=s1.setk()+s2.setk();
    return sum;
}
int main()
{
    xxx oa,ob(99,77),*p1,*p2;
    p1=&oa;
    p2=&ob;
    p1->print();
    p2->print();
    cout<<"sum=";
    cout<<add(oa,ob)<<endl;
    return 0;
}                如果把类中的私有数据成员改为const int a,那么其它的地方怎么改,我照课本的程序但是出错了,帮忙改下,不胜感激呀!