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Thread: Question about the undo framework example

  1. #1
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    Default Question about the undo framework example

    Hello,

    My question is about the undoframework http://doc.qt.io/qt-5/qtwidgets-tool...k-example.html and http://doc.qt.io/qt-5/qtwidgets-tool...mands-cpp.html

    The AddCommand class has a destructor :

    Qt Code:
    1. AddCommand::~AddCommand()
    2. {
    3. if (!myDiagramItem->scene())
    4. delete myDiagramItem;
    5. }
    To copy to clipboard, switch view to plain text mode 

    Let's say that the stored item, myDiagramItem, is inside a scene. The scene has the responsability to delete it. So I wonder how this destructor can work because if the destructor of the scene is called before the destructor of AddCommand the pointer myDiagramItem will be invalid, so the first line (if (!myDiagramItem->scene()) ) should lead to a crash. How can we be sure that it will not happened ?


    Second question, it seems that diagramScene stored in MainWindow doesn't have any parent, so how the memory of diagramScene is released ?
    (See : http://doc.qt.io/qt-5/qtwidgets-tool...indow-cpp.html)

    Thank you very much for your help.

  2. #2
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    Default Re: Question about the undo framework example

    Quote Originally Posted by nilot View Post
    How can we be sure that it will not happened ?
    One of your options is to track the undo/redo state. I.e. if your command knows that it is in state after undo then it owns the pointer, if it is in the state after redo, then the scene owns the pointer.

    Or you pass a pointer/reference of the command to the item and the item "notifies" the command when it is being deleted.

    Quote Originally Posted by nilot View Post
    Second question, it seems that diagramScene stored in MainWindow doesn't have any parent, so how the memory of diagramScene is released ?
    If you don't pass a parent, you need are in basic C++ land: you delete it yourself or store the raw pointer in a smart pointer that deletes its data on destruction.

    Cheers,
    _

  3. The following user says thank you to anda_skoa for this useful post:

    nilot (10th February 2017)

  4. #3
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    Default Re: Question about the undo framework example

    Thank you, I agree with everything you said, I am just wondering why this isn't done in the example.

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