最接近的是 std::thread::joinable().从参考中引用:

The closest you can get is std::thread::joinable(). To cite from the reference:

检查线程对象是否标识了一个活动的执行线程.具体来说，如果 get_id() != std::thread::id()，则返回 true.所以默认构造的 thread 是不可连接的.
已完成代码执行但尚未加入的线程仍被视为活动的执行线程，因此是可加入的.

Checks if the thread object identifies an active thread of execution. Specifically, returns true if get_id() != std::thread::id(). So a default constructed thread is not joinable.
A thread that has finished executing code, but has not yet been joined is still considered an active thread of execution and is therefore joinable.