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Thread: Qlist Serialization by qdatastream error

  1. #1
    Join Date
    Dec 2013
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    Default Qlist Serialization by qdatastream error

    Hi,
    I try to serialize my own class, which inherits from QGraphicsItem and QObject, but i have a problem when i want to read it from file:

    Qt Code:
    1. #include "zarzadcaodczytu.h"
    2.  
    3. #include <QMetaProperty>
    4. #include <QFile>
    5. #include <QMetaType>
    6. #include <qdebug.h>
    7.  
    8. #include "../editorwindow.h"
    9. #include "ui_editorwindow.h"
    10.  
    11.  
    12.  
    13. ZarzadcaOdczytu::ZarzadcaOdczytu()
    14. {
    15. }
    16.  
    17. bool ZarzadcaOdczytu::wczytaj(QString nazwaPliku, EditorWindow *edytor)
    18. {
    19. //HBMalyMis and HBDuzyMis inherits from HoneyBearsMapItem
    20. qRegisterMetaType<HBMalyMis>("HBMalyMis");
    21. qRegisterMetaType<HBDuzyMis>("HBDuzyMis");
    22.  
    23. QFile file(nazwaPliku);
    24. file.open(QIODevice::ReadOnly);
    25. QDataStream in(&file);
    26. QList< HoneyBearsMapItem * > items;
    27.  
    28. //in debugger i can see that every objects exists and values are set properly
    29. in >> items;
    30. file.close();
    31.  
    32. delete edytor->ui->graphicsView->scene();
    33.  
    34. //PlanszaScene inherits from QGraphicsScene
    35. PlanszaScene * nowa = new PlanszaScene(1336, 640, edytor);
    36. edytor->ui->graphicsView->setScene(nowa);
    37.  
    38.  
    39. qDebug() << items.count();
    40.  
    41. for(HoneyBearsMapItem *item : items){
    42. nowa->addItem(item); // <- in this line app crash, when I try to call metaObject() app crash too
    43. }
    44.  
    45.  
    46. return true;
    47. }
    48.  
    49. HoneyBearsMapItem *ZarzadcaOdczytu::create(const char *classname)
    50. {
    51. QString name(classname);
    52.  
    53. if(name == "HBDuzyMis")
    54. return new HBDuzyMis();
    55. if(name == "HBMalyMis")
    56. return new HBMalyMis();
    57.  
    58. return NULL;
    59. }
    60.  
    61. QDataStream &operator>>(QDataStream &in, HoneyBearsMapItem *item)
    62. {
    63. char * className;
    64. in >> className;
    65.  
    66. item = ZarzadcaOdczytu().create(className);
    67.  
    68. const QMetaObject *meta = item->metaObject();
    69. int count = meta->propertyCount();
    70. for (int i=0; i<count; ++i) {
    71. QMetaProperty metaproperty = meta->property(i);
    72. if (metaproperty.isWritable()){
    73. const char *name = metaproperty.name();
    74. QVariant value;
    75. in >> value;
    76. item->setProperty(name, value);
    77.  
    78. //every values are read properly
    79. qDebug() << "Property " << name << " " << value;
    80.  
    81. }
    82.  
    83. }
    84.  
    85.  
    86. return in;
    87.  
    88. }
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    What I'm doing wrong :/ ? In the operator >> function I can call metaObject(), but when I try do this outside this function after it, the application crash. Other objects methods works fine. The main problem is when I try add object to scene :/

  2. #2
    Join Date
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    Default Re: Qlist Serialization by qdatastream error

    There are all kinds of fishy things. For example who frees className or why does the operator ignore the passed in item?

    My recommendation would be to separate serialization from the item.

    For example like this

    Qt Code:
    1. class MyItem : public QGraphicsItem
    2. {
    3. public:
    4. QVariantMap serialize() const;
    5.  
    6. static MyItem* deserizalize(const QVariantMap &data);
    7. }
    To copy to clipboard, switch view to plain text mode 

    Used like this
    Qt Code:
    1. QDataStream stream(...);
    2. foreach (MyItem* item, items) {
    3. stream << item->serialize();
    4. }
    To copy to clipboard, switch view to plain text mode 
    Qt Code:
    1. QDataStream stream(...);
    2. while (!stream.atEnd()) {
    3. QVariantMap data;
    4. stream >> data;
    5.  
    6. items << MyItem::deserialize(data);
    7. }
    To copy to clipboard, switch view to plain text mode 

    For the classname I would suggest to either use QByteArray or to have an entry for that in the QVariantMap (but also a QByteArray to avoid manual memory management)

    Cheers,
    _

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

    WezSieTato (4th January 2014)

  4. #3
    Join Date
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    Qt products
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    Default Re: Qlist Serialization by qdatastream error

    I found a way to handle this. This may be non standard and need template specialization.
    I created a separate header file with the template and included it wherever required. Here it is. In case you need other data structures to be serialized, you may follow the similar pattern.

    Cheers !

    #ifndef MODSER_H
    #define MODSER_H
    #include <QDataStream>

    template <typename T>
    QDataStream& operator>>(QDataStream& s, QList<T*>& l)
    {
    l.clear();
    quint32 c;
    s >> c;
    l.reserve(c);
    for(quint32 i = 0; i < c; ++i)
    {
    T * t = new T;
    s >> t;
    l.append(t);
    if (s.atEnd())
    break;
    }
    return s;
    }

    #endif // MODSER_H

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