join后线程上的共享指针计数为1？

发布于 2024-12-06 01:27:31 字数 583 浏览 0 评论 0原文

有一个等待线程完成的 boost::condition_variable ：

boost::condition_variable mContd;
boost::shared_ptr<boost::thread> mThread;

想象一下，该线程之前已启动一段时间，现在等待：

if(!mContd.timed_wait(tLock, boost::posix_time::seconds(1))) {
  // cancel thread if deadline is reached
  mThread.interrupt();
  mThread.join();

  std::cout 
    << "Thread count = " 
    << mThread.use_count() // still prints '1'
    << std::endl;

} else {
  // continue
}

那么，这个计数何时设置为零？我假设，在join之后，一个线程就完成了。但什么时候呢？

原文

Having a boost::condition_variable which waits for a thread to complete:

boost::condition_variable mContd;
boost::shared_ptr<boost::thread> mThread;

Imagine, the Thread was started some time before, and now wait:

if(!mContd.timed_wait(tLock, boost::posix_time::seconds(1))) {
  // cancel thread if deadline is reached
  mThread.interrupt();
  mThread.join();

  std::cout 
    << "Thread count = " 
    << mThread.use_count() // still prints '1'
    << std::endl;

} else {
  // continue
}

So, when is this count set to a zero? I assumed, after a join a thread is finished. But when it is?

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大海や 2024-12-13 01:27:31

use_count() 只是告诉您有多少 shared_ptr 对象指向同一个 boost::thread 对象。它与线程是否终止无关。

当 mThread 超出范围时，计数器将自动递减。

如果您不再需要thread对象，您可以调用mThread.reset()。这也会导致 mThread.use_count() 降至零。

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阪姬 2024-12-13 01:27:31

对象无法像这样正确地删除自己。

当线程终止时，表示它的 boost::thread 对象进入“完成”状态，但它必须仍然“存在”，因为 shared_ptr 正在控制它。您仍然拥有之前的那个现在“已完成”的 boost::thread 对象，因此计数仍然为 1。

事实上，一般来说，boost::shared_ptr:: use_count()仅在表示“空指针”而不是实际存在的对象时返回0。

一个直接的类比如下：

boost::thread mThread(&f); // Create thread object

mThread.interrupt();
mThread.join();            // Thread is now "finished"

cout << (mThread.get_id() == boost::thread::id());
// ^ Outputs `true`, because the object mThread is now in the
//   not-a-thread state, but of course it still must exist.

Objects can't properly delete themselves like that.

When the thread terminates, the boost::thread object representing it goes into a "finished" state, but it must still "exist" because shared_ptr is controlling it. You still have the one, now-"finished" boost::thread object that you have before, so the count is still 1.

In fact, in general, boost::shared_ptr::use_count() will only return 0 when it's representing a "null pointer" rather than an actual object that exists.

A direct analogy follows:

boost::thread mThread(&f); // Create thread object

mThread.interrupt();
mThread.join();            // Thread is now "finished"

cout << (mThread.get_id() == boost::thread::id());
// ^ Outputs `true`, because the object mThread is now in the
//   not-a-thread state, but of course it still must exist.

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