c++ - 什么时候应该使用 std::thread::joinable？

Question

关于 std::thread::joinable 的网站 cppreference 上有说明:

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.

接下来是 std::thread::join文档:

Error Conditions

resource_deadlock_would_occur if this->get_id() == std::this_thread::get_id() (deadlock detected)

这种方法的唯一目的是检测这种情况吗？我们目前厚颜无耻地只调用 thread->join 而没有可连接的包装器，这种方法有什么危险？

Answer 1

根据您引用的信息:

So a default constructed thread is not joinable.

这就是你的答案。如果您不知道您的线程是否是默认构造的，您就不知道它是否可连接。

因此，在您的程序/函数/例程即将结束时，当您想加入线程时(并且您确实需要这样做，在 std::thread 超出范围之前) ，您必须有条件地这样做。这就是您如何做到这一点。

#include <thread>

void bar(const unsigned int);

/**
 * This class may be default-constructed (in which case it does nothing),
 * or constructed with an `unsigned int` argument which shall be passed
 * to `bar()` in a worker thread.
 */
struct Foo
{
   Foo() {}

   Foo(const unsigned int x)
      : doThingsThread([&]() { bar(x); })
   {}

   ~Foo()
   {
      // The thread *MUST* be joined before destruction, if it
      // is joinable. Otherwise, it *CAN'T* be joined.
      if (doThingsThread.joinable())
         doThingsThread.join();
   }
private:
   std::thread doThingsThread;
};