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POPorting threads from QT3 to QT4
 

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  1. POPorting threads from QT3 to QT4
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    <p>I am porting an application from QT3 to QT4, and keep running into problems when a thread is updating a QProgressDialog. The original code was roughly designed like so:</p> <pre><code>class ScanProcess : QObject{ Q_OBJECT public: QProgressDialog* progress; private: ScanProcessThread* thread; }; class ScanProcessThread : QThread { Q_OBJECT public: void run(); }; </code></pre> <p>This is after running qt3to4 and making the appropriate changes from the QT Porting guide.</p> <p>In the original design, inside the ScanProcessThread:</p> <pre><code>void ScanProcessThread::run(){ //... ProgressInfo *prog = new ProgressInfo(); //then fill it in QCustomEvent* progEvent = new QCustomEvent(QEvent::User+1, (void*)prog); QCoreApplication::postEvent(parent, progEvent);//Parent is pointer to the ScanProcess //... } void ScanProcess::customEvent(QCustomEvent *e){ if(e-&gt;type() == QEvent::User+1){ //update QProgressDialog progres-&gt;setValue(prog.index);//Value from the ProgressInfo passed above //This line crashes deep in ntdll, but I have traced it to the QProgressDialog::repaint() } } struct ProgressInfo {int count; int index; QString text;}; </code></pre> <p>So, that's the important code. The call to QProgressDialog::Repaint seems to always crash somewhere deep in ntdll. I have tried two other methods: 1) Use QCoreApplication::postEvent() 2) Pass the QProgressDialog* and let ScanProcessThread update it directly. In every case, it's the QProgressDialog::repaint() that fails. Any ideas?</p> <p>(qt4.4, windows xp sp3, Visual Studio 2008/)</p>
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    1. USDavid Souther
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    1. COJust a note, but I would make my own custom event class to convey the progress information, instead of casting to void and back. You have a potential memory leak if the object you are posting to doesn't delete the pointer, which the code you posted doesn't. Also, there is nothing to free the memory if the class posted to goes away before processing the events, or if a derived class overrides it and forgets to do so, or...
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      1. POPorting threads from QT3 to QT4
      1. USCaleb Huitt - cjhuitt
    2. CO@cjhuitt I would do many things differently if I had written the app myself :) That said, this code does take care of cleaning its own pointers up in the customEvent (I forgot to add that line).
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      1. USDavid Souther
 

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