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.