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Thread: QList question

  1. #1
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    Default QList question

    Does the insertion operator make a copy of the value it is inserting?

    For example... Lets say I have the following code...

    Qt Code:
    1. struct someStruct {
    2. int x;
    3. double y;
    4. };
    5.  
    6. QList<someStruct> testList;
    7.  
    8. void className::methodName()
    9. {
    10. someStruct tempStruct;
    11. for (int x = 0; x < 10; x++) {
    12. tempStruct.x = x;
    13. tempStruct.y = 0;
    14. testList <<tempStruct;
    15. }
    16. }
    To copy to clipboard, switch view to plain text mode 

    tempStruct is local to the "methodName" method.
    After this function returns, is testList valid?
    In my test code it is. All works ok. But I'm trying to track down a particularly elusive bug and this is one of my concerns.

    Here's a snippet of my actual code. I'll try to explain the error.

    Qt Code:
    1. LayerDef layer; // LayerDef is the struct definition
    2. if (line.startsWith("Original")) {
    3. while (!line.startsWith(',')) {
    4. line = stream.readLine();
    5. if (line.startsWith(',')){
    6. continue;
    7. }
    8. tokens = line.split(',');
    9. layer.t = tokens[1].toDouble(); // layer is a struct.
    10. layer.dc = tokens[2].toDouble();
    11. layer.dlt = tokens[3].toDouble();
    12. layer.mlt = tokens[5].toDouble();
    13. layer.dcc = tokens[6].toDouble();
    14. layer.rp = tokens[7].toDouble();
    15. layer.name = tokens[8];
    16. layersOriginal <<layer; // layersOriginal is a QList (QList<LayerDef> layersOriginal;)
    17. if (firstPass) {
    18. firstPass = false;
    19. // Update slider values on first pass only.
    20. ui->sldT->setValue(layer.t); // Note this line.
    21. ui->sldDC->setValue(layer.dc);
    22. ui->sldDLT->setValue(layer.dlt);
    23. ui->sldMLT->setValue(layer.mlt);
    24. ui->sldDCC->setValue(layer.dcc);
    25. ui->sldRP->setValue(layer.rp);
    26. }
    27. ui->lstLayers->addItem(layer.name);
    28. }
    To copy to clipboard, switch view to plain text mode 

    This code runs ok. However, after it runs, any attempt to access sldT (a QSlider) causes an assertion failure.
    The failure is "ASSERT failure in QVector<T>:perator[]: "index out of range", file ..\..\Qt\Qt5.3.2\5.3\mingw482_32\include/QtCore/qvector.h, line 385".
    I've went through the code for QSlider and QAbstractSlider. I can't find any reference to a QVector in either of them.

    This is my final step before I start banging my head on my desk.

  2. #2
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    Default Re: QList question

    A list keeps a copy of your original object.

    As for your error, what is the type of tokens? Isn't it by any chance QVector? What does tokens.size() return?
    Your biological and technological distinctiveness will be added to our own. Resistance is futile.

    Please ask Qt related questions on the forum and not using private messages or visitor messages.


  3. #3
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    Default Re: QList question

    Sorry... I should've mentioned that.

    line is a QString and tokens is a QStringList.

    I'm reading a spreadsheet and splitting the line by the commas.

  4. #4
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    Default Re: QList question

    Why do you say the problem is with some slider? You are not running this code from a worker thread, are you?
    Your biological and technological distinctiveness will be added to our own. Resistance is futile.

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  5. #5
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    Default Re: QList question

    No. This code is in the UI thread.

    That's what is confusing me. I can't figure out why the slider is crashing. It works fine before this code runs.
    I've worked on this for two days now. My best guess is that some code or memory is getting corrupted somehow.

  6. #6
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    Default Re: QList question

    So if you comment out lines #17-26 then some other code stops crashing?
    Your biological and technological distinctiveness will be added to our own. Resistance is futile.

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  7. #7
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    Default Re: QList question

    I tried that. It still crashes.
    If I comment out the lines dealing with the QList, then the other code stops crashing.
    The line that crashes is...

    Qt Code:
    1. ui->sldT->setValue(10);
    To copy to clipboard, switch view to plain text mode 


    Added after 4 minutes:


    Also... it's only sldT. The other sliders work fine.
    Last edited by DoomerDan; 29th January 2015 at 15:36.

  8. #8
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    Default Re: QList question

    What is the size of the list (check, don't guess)?
    Your biological and technological distinctiveness will be added to our own. Resistance is futile.

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  9. The following user says thank you to wysota for this useful post:

    DoomerDan (29th January 2015)

  10. #9
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    Default Re: QList question

    I found it. This is a slap myself in the face moment. In the event handler for the slider value change I was attempting to assign a value to a non-existent QVector element.
    The debugger stopped on the setValue line. If it had stopped in the event handler instead I wouldn't have wasted hours of work.

    Thanks for your help wysota.

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