c++ - 异常时 call_once 的行为

Question

如果我用 call_once 调用一个函数，如果它在完成执行前抛出异常，那么 once_flag 的值是多少？

Answer 1

once_flag状态未修改，因此下一次调用 call_once与相同once_flag将调用可调用对象。

来自 [thread.once.callonce]/2 (强调)

Effects: An execution of call_­once that does not call its func is a passive execution. An execution of call_­once that calls its func is an active execution. An active execution shall call INVOKE(​std​::​forward<Callable>(func), std​::​forward<Args>(args)...). If such a call to func throws an exception the execution is exceptional, otherwise it is returning. An exceptional execution shall propagate the exception to the caller of call_­once. Among all executions of call_­once for any given once_­flag: at most one shall be a returning execution; if there is a returning execution, it shall be the last active execution; and there are passive executions only if there is a returning execution.