重载逗号运算符,有什么参数和类型的限制?编译异常

重载逗号运算符,有什么参数和类型的限制?编译错误啊
我在VC2010下面尝试重载逗号运算符:

C/C++ code

class x{    int i;public:    x():i(23){}    void operator,(){cout<<i<<endl;}};

C/C++ code

# include <iostream>using namespace std;class foo {    int i;public:    foo(): i(23) {}    int operator ,(int n)    {        cout << i << endl;        return i;    }};int main(){    foo x;    int y = (x, 100);    cout << y << endl;    return 0;}