未解之谜

class D {
 int a,b;
 public:
     D(int i = 0, int j=0)：a(i),b(j){
     } 
    D(D&p){a = p.a;b=p.a;}
    void show(){cout<<a<<""<<b<<endl;    }
};
D f(D p){
    D d1(p);
    return d1;
}
int main()
{
    D o1,o2(2,3);//？？
    o2 = f(o1);
    o2.show();
    return 0;
}