std::async

https://www.cnblogs.com/qicosmos/p/3534211.html

https://bobjin.com/blog/c_cpp_docs/reference/en/cpp/thread/future/wait_for.html

https://www.cnblogs.com/chengyuanchun/p/5394843.html

使用下面的方法，回避成员函数问题。

std::future<int> f1 = std::async(std::launch::async, [](){ 
        return 8;  
    });