Why do some QWidgets create own threads?

[wysota]

QFileDialog might be using a separate thread for populating QFileSystemModel. QLineEdit shouldn't be using separate threads unless you are using a completer with QFileSystemModel.

[donglebob]

Thanks,

you are right

QFileDialog *aaa = new QFileDialog(this);
 
somewhere in the code:
    model = new QFileSystemModel(q);

QFileDialog *aaa = new QFileDialog(this);

somewhere in the code:
    model = new QFileSystemModel(q);

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But QLineEdit creates 2 Threads in my simple case.

QLineEdit *bbb = new QLineEdit(this);

QLineEdit *bbb = new QLineEdit(this);

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Somewhere in the code in its init

q->setAcceptDrops(true);  //here it creates 2 threads

q->setAcceptDrops(true);  //here it creates 2 threads

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[donglebob]

Am I the only one where QLineEdit creates threads?

[wysota]

QLineEdit is not using any threads, at least not by itself. Why did you think it was using threads?

[Strix Code]

donglebob is right. Instantiating QLineEdit creates threads of its own. On my system (Windows 7, Qt 4.6.2) it creates 4 threads! Actually, the threads are created inside QWidget::setAcceptDrops(), because QLineEdit calls this method inside the constructor. After a bit more debugging I've found out that the threads are created by a WinAPI function CoLockObjectExternal

Edit: I've found a way to prevent this behavior. One must call

QLineEdit::setAcceptDrops(false);

QLineEdit::setAcceptDrops(false);

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before the widget is shown.