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Thread: Allocation of object within a function with pointer local on stack

  1. #1
    Join Date
    Oct 2015
    Qt products
    Unix/X11 Windows

    Default Allocation of object within a function with pointer local on stack

    I'm still an amateur at Qt and C++ so I'm sure I'm missing something obvious here.
    I am allocating an object, call it A, on the heap within a function (or method) using a locally-defined pointer. My function passes in A's parent, PA so that A can be deleted automatically when PA is deleted as shown in the example below:

    void f(PA,datatoplot)
    AType *A = new AType(PA,datatoplot); // note that PA is the parent of A, passed to A's constructor and PA is a pointer
    ..... // more code, but A is not explicitly deleted or deallocated

    A->show() // say A is some kind of plot or other widget and we're displaying it here. Repeated calls to f show different plots

    My question: Will this lead to memory leaks and/or dangling pointers? The pointer A gets deleted from the local stack of f() each time we exit f() but a copy of A is created in RAM every time f() is called. I'm presuming that it doesn't matter if the local pointer to A is lost due to the disposal of the stack of f() when exiting f() because PA retains a copy of each pointer of each copy of A created (one for each call to f()) and when PA is deleted, all the copies of A will also be deleted? If only PA needs to handle the memory management of all the copies of A, does the above lead to problems?
    Thanks so much,

  2. #2
    Join Date
    Jul 2008
    Thanked 61 Times in 59 Posts
    Qt products
    Qt4 Qt5
    Unix/X11 Windows

    Default Re: Allocation of object within a function with pointer local on stack

    Hi, the parent widget will delete its children, so PA will delete all the new A objects, but only when PA itself gets deleted. So while PA exists you will accumulate AType objects which you cannot access because you did not store a pointer (you could get a pointer by using methods like QObject::findChild, but I try to avoid that if I can).

    It would be nicer if you would clean unused AType objects, or (probably better because it avoids the new allocations) you can create it once on the first call, then re-use the AType object from the first call and only use show() and hide().


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

    montanaviking (3rd November 2020)

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