Q_OBJECT memory allocation in a thread
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Hello, I have a question about memory management when a thread is deleted. For example, if I create a thread using the class below, will the memory for image and mutex be freed automatically or does this need to be done explicitly in a destructor?
class aThread :: public QThread
{
Q_OBJECTpublic: aThread (QObject *parent); ~aThread() override; private: QImage image; QMutex mutex;}
wrote on 15 Dec 2020, 18:51 last edited by JonB@chessking5544
And what would you do in a destructor? :)BTW, what are you intending to do with the
QImagein this non-UI thread? -
@JonB said in Q_OBJECT memory allocation in a thread:
BTW, what are you intending to do with the QImage in this non-UI thread?
QImage can be used in a non GUI thread, it's QPixmap which is tied to GUI backend and thus cannot be. See the mandelbrot example for such a use case.
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wrote on 15 Dec 2020, 19:32 last edited by
Actually, I am using the mandelbrot example as a starting point for my app. I would like to allocate and use a few QImages in the run function, and the question is whether they will be deleted when the thread exits?
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That depends on how you allocate them.
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@SGaist said in Q_OBJECT memory allocation in a thread:
That depends on how you allocate them.
And is nothing which is specific to Qt btw.
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wrote on 15 Dec 2020, 19:46 last edited by
how should I allocate QImage so that the memory is freed when the thread is deleted.
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how should I allocate QImage so that the memory is freed when the thread is deleted.
@chessking5544 said in Q_OBJECT memory allocation in a thread:
how should I allocate QImage so that the memory is freed when the thread is deleted.
Please don't start with Qt when you don't have basic c++ knowledge.
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how should I allocate QImage so that the memory is freed when the thread is deleted.
wrote on 15 Dec 2020, 20:05 last edited by JonB@chessking5544
Sorry, what I meant earlier byAnd what would you do in a destructor? :)
is that this is a not a
new/allocated (pointer) variable. You don't have to destroy it, and what would you write to do so anyway even if you needed to? When the object (youraThreadinstance) goes out of scope/is destroyed, theQImage/QMutexmembers will be destroyed without you doing anything. -
wrote on 15 Dec 2020, 20:06 last edited by
well, it seems that regardless of whether I allocate QImage off the stack or the heap, the memory is not freed when the thread exits. that is, an explicit delete must be called. I mention this only because you mentioned that it depends on how you allocate them.
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@chessking5544 said in Q_OBJECT memory allocation in a thread:
well, it seems that regardless of whether I allocate QImage off the stack or the heap, the memory is not freed when the thread exits.
This has nothing to do with the thread. Since you add it as member of your class the memory is freed when your class gets destroyed. No thread or Qt magic here. Plain c++
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wrote on 15 Dec 2020, 20:21 last edited byThis post is deleted!
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wrote on 17 Dec 2020, 08:16 last edited by
As @Christian-Ehrlicher already said: the image gets deleted when the thread object is deleted. Do you delete the thread object?
Furthermore, I am not sure how you are trying to check if the memory is actually freed. The operating system might not be able to show you this. Your software will request memory from the OS when necessary, but can choose to keep it for later allocations when memory is freed. The only way you can test this is by repeatedly allocating and deallocating an image. If the memory usage does not grow then memory is correctly freed even if the OS does not show you this.
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