宏定义函数https://gcc.gnu.org/onlinedocs/gcc-7.3.0/gcc.pdf搜索__VA_ARGS__
#define debug(format, ...) fprintf (stderr, format, __VA_ARGS__)
Here ... is a variable argument. In the invocation of such a mac...
4.2 使用futurestd::future
`头文件
std::future<> 可移动不可拷贝(构造函数和赋值重载操作符)
An asynchronous operation (created via std::async, std::packaged_task, or std::promise) can provide a std::future object ...