Problems with deserialization using QDataStream.

JoeJoe_000

Hello I'm having a lot of problems reading a file back into my QWidget application. It appears that writing/serialization is not an issue as I can verify data via text editor to a degree and my test strings are visible next to the binary data.

When I attempt to load, it appears I freeze at a point where i begin to stream into a QList using a for loop. Using debug I've pinpointed where the program shuts down unexpectedly. I have marked it in the comments of the code I've provided.

Here is my overloaded << data output operator in one class that will be called in another widget class that is set to the central widget. This seems to work great.

QDataStream &operator<<(QDataStream &out, const aCustomWidget *ac)
{


    out << ac->aInt << ac->bInt << ac->AqRect << ac->AqString  << ac->qListofInt;
    qDebug() << "touching ac out << operator";
    return out;
}

QDataStream &operator>>(QDataStream &in ,aCustomWidget *ac)
{
     //all int are static casted to qint32

    in >> ac->aInt >> ac->bInt >> ac->AqRect >> ac->AqString >> ac->qListofInt;
    qDebug() << "touching ac in << opeators";
    ac->prepareDataIn();   //A class that cast items of qint32 back to int
    return in;
}

In my central widget class CenWid I've overloaded << >> operators as such...
I have a QList<aCustomWidget> which i iterate through to store each element...


QDataStream &operator<<(QDataStream &out, const CenWid &cw)
{
    int CustomWidget_List_Size =cw.aCustomWidgetList.size();
    out << CustomWidget_List_Size;

    for(int i = 0; i < CustomWidget_List_Size; ++i)
    {
        cw.aCustomWidget[i]->prepareDataOutput(); //this is function that static casts  int to qint32
        aCustomWidget *aCustWid = cw.aCustomWidgetList.at(i);
        out << aCustWid;
    }
    int aMap_size= cw.aMap.size(); //This Map item works great.
    out << aMap_size;
    out << cw.aMap;

   qDebug() << "touching cw out << opeators";
    return out;
}

Here is my in >> operation code that fails.

QDataStream &operator>>(QDataStream &in, CenWid &cw)
{
    int CustomWidget_List_Size;
    in >> CustomWidget_List_Size;
    aMap<int, QLine> aMap;
    


    qDebug() << ">> "CustomWidget_List_Size" << IDAList_Size;

    for(int i = 0; I < CustomWidget_List_Size; i  ; ++i)
   {
        qDebug() << "entered in for loop";
        aCustomWidget *aCustWid = cw.CustomWidgetList.at(i);
        qDebug() << "1";
        in >> aCustWid;                   //here is where my program fails unexpectedly.
        qDebug() << "2";
        cw.aCustomWidgetList.at(i)->prepareDataIn();
        cw.addCustWid(aCustWid);          
        qDebug() << "iterating into CustomwidgetList" << i << "times";
    }


   int aMap_size;
   in >> aMap_size;
   in >> aMap;

   int a = 0;

   QMapIterator<int, QLine> j(aMap);
   while(j.hasNext()){
       j.next();
       cw.aMap.insert(j.key(), j.value());
        qDebug() << "iterating into aMap" << a << "times";
        ++a;
   }
    qDebug() << "touching cw in << opeators";
    return in;
}

I open a file in MainWindow() as such: The Central Widget class is called aCenWid


void MainWindow::openFile()
{


       QString fileName = QFileDialog::getOpenFileName(this,
                     tr("Open a Tree File"), "", tr("test Files (*.te);; All Files(*)" ));

        if(fileName.isEmpty()) return;
        else{
            QFile file(fileName);

            if(!file.open(QIODevice::ReadOnly)){
                QMessageBox::information(this, tr("Unable to open file"), file.errorString());
                return;
            }


            QDataStream in(&file);
            in.setVersion(QDataStream::Qt_5_9);
            qDebug() << "in has been set";


            aCenWid->clear();  //This is a function that clears all my QList and QMap in my aCenWid class
            qDebug() << "size of aCustomWidgetList  after clear is: " <<  aCenWid->aCustomWidgetList.size();
            in >> *aCenWid;     //here I set the input data stream to aCenWid

            qDebug() <<"size of aCustomWidgetlist is: " << aCenWid.aCustomWidgetList->size();
            if( aCenWid->ACustomWidgetList.size() == 0)
            {
              QMessageBox::information(this, tr("File is empty"), tr("The File lacks data"));
            }  else{

                      //we currently dont reach this point. 
                         //  qDebug() << "aCenWid will now be constructed...";
                       //aCenWid->reIntCustomWidget(cw);
                        
           }


    }
 }

In addition to addressing my issue I also anticipate needing info on a good method to re-construct once you have reloaded your data if you can shine some light on some of these methods that would be much appreciated. Thanks for looking at my problem.

mrjj

@JoeJoe_000

Hi
So when you stop at break point at
in >> aCustWid
is the aCustWid a valid object ?

To get access violation entering the >> the operator
suggests the object instance is invalid/unallocated.

This is one of those bugs that are hard to guess at and must be found using the debugger.

if I change to *aCustWid; I imediately get a semantic error.

Well what error ?

I checked my code and im using *objectptr all the time and if i remove * in from it stops saving the data members.
So i think this is might be important.

mrjj

Hi
From quick read of your code i notice
out << aCustWid;

aCustWid being a pointer so that will just stream the address of the object and not as expected trigger its << operator.

This is due to the signature being
QDataStream &operator>>(QDataStream &in, CenWid &cw)
(it wants a ref, not a pointer )
so i think you want
out << *aCustWid;
To let the object stream its members.

JoeJoe_000

Hi I don't think that's the issue if I change to *aCustWid; I imediately get a semantic error. Also I believe streaming out is okay.

mrjj

@JoeJoe_000
Ok as that would really crash hard when you read it in again. :)

Looking at your read in code

you read in the list count
in >> CustomWidget_List_Size;
Then you take out a widget from the list
aCustomWidget *aCustWid = cw.CustomWidgetList.at(i);

Did you create the widgets before calling the read in function ?

Also did you place break point at
in >> aCustWid; //here is where my program fails unexpectedly.

and see what happens. ?

JoeJoe_000

Hello MrJJ thanks for your assistance again.

I get exceptions error of read access violations when I set a break point at the in>> operation.

The code I provided you is for cw.CustomWidget object. I've tried to get around this by using a secondary instance but that seems
to give me the same issues.

mrjj

@JoeJoe_000

Hi
So when you stop at break point at
in >> aCustWid
is the aCustWid a valid object ?

To get access violation entering the >> the operator
suggests the object instance is invalid/unallocated.

This is one of those bugs that are hard to guess at and must be found using the debugger.

if I change to *aCustWid; I imediately get a semantic error.

Well what error ?

I checked my code and im using *objectptr all the time and if i remove * in from it stops saving the data members.
So i think this is might be important.

JoeJoe_000

Sorry about my lack of information to be honest I don't know a lot about debuging and should proably spend some time learning how to debug. I had to install the CBD installer just to reply to your post.

In anycase what I have found is if I append new items to the list before calling in >> I actually end up loading correctly. so I must I guess I've mistaken how objects are created and need to reassess the process.

To better answer your questions

is the aCustWid a valid object ? - No in my case I had not yet appended and thought thats what was happening in the overloaded operators I still have to append new objects externally for everything to load correctly.

if I change to *aCustWid; I immediately get a semantic error.
Well what error ?

I get invalid binary operator errors I believe I can change this around by changing the function prototype parameters.

Thanks again for your assistance.

mrjj

Hi
Ok super
so it was that the list did not contain allocated objects.

Well I know using the debugger is not particularly exciting but it does offer help to
find bugs that is hard to catch without it.
So it really is a "must use time on" tool. :)

JoeJoe_000

No it was good you pointed me in this direction I was sort of ignoring the fact that it was not setup. So yeah thanks again!

JoeJoe_000

Just as an FYI for other people who may have the same problem in the future.

My final solution was to add a function to my CenWid &cw object that takes CustomWidget_List_Size; as an argument
and appends this value to the list. I had to do this before inputing my aCusWid object.

Qt6 topics merged with General

Problems with deserialization using QDataStream.