[問題] operator overloading

[popo4231 (小泰)]

為什麼operator overloading函式一定要回傳ostream&
回傳ostream會compile error
可是在cascading call的時候就不會有這種情形發生
請問問題是出在哪邊?
code:
#include<iostream>
#include<iomanip>

using namespace std;

class student
{
friend istream operator>>(istream&,student&);
friend ostream operator<<(ostream&,student&);
private:
int num;
char ch;
string str;

};

istream operator>>(istream& input,student& stu)
{
   input>>setw(2)>>stu.num;
   input.ignore(1);
   input>>setw(1)>>stu.ch;
   input.ignore(3);
   input>>setw(10)>>stu.str;
   return input;

}
ostream operator<<(ostream& output,student& stu)
{
   output<<stu.num<<' '<<stu.ch<<"   "<<stu.str<<endl;
   return output;
}


int main()
{
   student S;

   cin>>S;
   cout<<S;


   system("pause");
   return 0;


}
只要把回傳型態改成reference就可以過了

但是cascading就可以回傳object而不用回傳reference
code:
#include<iostream>
using namespace std;
class A
{
public:
   A xxx(void)
   {
      cout<<"xd囧"<<endl;
      return (*this);
   }
};
int main()
{
   A a;
   a.xxx().xxx();
   system("pause");
}

為什麼會這樣呢?

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