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According to http://doc.qt.io/qt-5/qpointer.html, QPointer is very useful. But I found it could be inefficient in the following context:

If I want to show label for three times or do something else, I have to use

if(label) label->show1(); if(label) label->show2(); if(label) label->show3();

instead of if(label) { label->show1();label->show2();label->show3(); }

just because label might be destroyed in another thread after label->show1(); or label->show2();.

Is there a beautiful way other than three ifs to get the same functionality?

Another question is, when label is destroyed after if(label), is if(label) label->show1(); still wrong?

I don't have experience in multi-threaded programs. Any help is appreciated. ;)

Christophe Weis
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echo
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2 Answers2

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I think the only safe way to do it is to make sure you only access your QWidgets from within the main/GUI thread (that is, the thread that is running Qt's event loop, inside QApplication::exec()).

If you have code that is running within a different thread, and that code wants the QLabels to be shown/hidden/whatever, then that code needs to create a QEvent object (or a subclass thereof) and call qApp->postEvent() to send that object to the main thread. Then when the Qt event loop picks up and handles that QEvent in the main thread, that is the point at which your code can safely do things to the QLabels.

Alternatively (and perhaps more simply), your thread's code could emit a cross-thread signal (as described here) and let Qt handle the event-posting internally. That might be better for your purpose.

Christophe Weis
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Jeremy Friesner
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1

Neither of your approaches is thread-safe. It's possible that your first thread will execute the if statement, then the other thread will delete your label, and then you will be inside of your if statement and crash.

Qt provides a number of thread synchronization constructs, you'll probably want to start with QMutex and learn more about thread-safety before you continue working on this program.

Using a mutex would make your function would look something like this:

mutex.lock();
label1->show();
label2->show();
label3->show();
mutex.unlock()

As long as your other thread is using locking that same mutex object then it will prevented from deleting your labels while you're showing them.

Christophe Weis
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Chris
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    AFAIK Qt's rule is "don't touch GUI objects from threads other than the main thread". In particular, I'm not sure event putting a mutex lock around the above calls would be sufficient, since there is no guarantee that Qt's main thread isn't accessing those QLabels (via their parent widget) e.g. for a repaint, and Qt won't know to lock the mutex. – Jeremy Friesner Sep 28 '11 at 23:11