C++ 线程和仿函数的问题？？

#include<iostream>
#include<thread>
using namespace std;
void function_1()
{
    std::cout << "Hello,world!" << std::endl;
}
class Fctor {                                      //仿函数
public:
    void operator()() {                    //对（）进行重载
        for (int i = 0; i > -100; i--) {
            cout << "from t1 " << i << endl;
        }
    }
};
int main() {
    //Fctor fct;
    std::thread t1((Fctor()));         //t1  starts  running. 实例化一个线程对象t1
    try {
        for (int i = 0; i < 100; i++)
            cout << "from main: " << i << endl;
    }
    catch (...) {
        t1.join();
        throw;
    }
    t1.join();
    //t1.detach();        
    return 0;
}

其中这句 std::thread t1((Fctor())); 为什么要再“Fctor()”两边还要加括号？否则“ t1.join();”这句会提示错误？？
还有能解释下执行结果吗？