Doubt related to Implicit Sharing



  • Greetings.

    I'm working on a project using Qt and OpenCV.
    I'm using QStack in one of my classes and I read in the documentation about 'Implicit Sharing'.

    Well, although I think I understand almost everything, however there is an issue that I'm not entirely sure, and since I have been having some problems with the amount of memory available to an application or process, thought I'd clarify my doubt.

    Suppose you have a stack QStack<T> where 'T' is a type not belonging to Qt (specifically it is an image cv::Mat... although I think it is irrelevant to my problem).

    Assume also that the stack several images have been added, what happens if you run a statement like this:

    imgsStack = QStack cv::Mat ();

    What happens with the memory space occupied by the old stack of images? ... Is it free?

    I understand by 'Implicit Sharing', the space must be released, although I'm not entirely sure.

    Is the space occupied by each image cv::Mat is also released automatically or, on the contrary, I release myself manually?

    Thanks in advance for any responses and/or comments.



  • If the memory of imgsStack is not referenced by ("shared with") another QStackcv::Mat variable, then yes, the cv::Mat is freed, assuming cv::Mat frees its memory properly in the destructor. And of course assuming, you're not using a QStackcv::Mat*. In that case, the memory of the pointer addresses (n*4 bytes) is freed, but not the memory the pointers point to (the actual cv::Mat data).

    With implicit sharing you don't have to take special care not to use more memory than necessary. It actually helps you to use less memory.


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