Q_OBJECT memory allocation in a thread

chessking5544 · 15 Dec 2020, 18:51

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_OBJECT

 public:
      aThread (QObject *parent);
      ~aThread() override;

 private:
      QImage image;
      QMutex mutex;

}

JonB · wrote on 15 Dec 2020, 18:51

@chessking5544
And what would you do in a destructor? :)

BTW, what are you intending to do with the QImage in this non-UI thread?

SGaist · wrote on 15 Dec 2020, 19:15

@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.

chessking5544 · wrote on 15 Dec 2020, 19:32

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?

SGaist · 15 Dec 2020, 19:37

That depends on how you allocate them.

Christian Ehrlicher · S SGaist 15 Dec 2020, 19:36

@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.

chessking5544 · 15 Dec 2020, 19:57

how should I allocate QImage so that the memory is freed when the thread is deleted.

Christian Ehrlicher · wrote on 15 Dec 2020, 19:57

@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.

JonB · wrote on 15 Dec 2020, 20:05

@chessking5544
Sorry, what I meant earlier by

And 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 (your aThread instance) goes out of scope/is destroyed, the QImage/QMutex members will be destroyed without you doing anything.

chessking5544 · wrote on 15 Dec 2020, 20:06

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.

Christian Ehrlicher · wrote on 15 Dec 2020, 20:08

@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++

chessking5544 · wrote on 15 Dec 2020, 20:21

This post is deleted!

SimonSchroeder · wrote on 17 Dec 2020, 08:16

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.