c++操作符重载问题

#include <iostream>
using namespace std;
class stu
{
    public:
        int operator()(int value) 
        {
            return value*value;
        }
    private:
        int a;
};
int operator()(stu &another,int value)//这行报错
{
    return value*value;
}
int main()
{
    
    return 0;
}

报错[Error] 'int operator()(stu&, int)' must be a nonstatic member function

看看呢