Can't find linker symbol for virtual table

ZapB

Does the variable printPreview point to a valid QWidget? I don't see that you are setting it anywhere. So your connect statement could be making the emit calls try to do things with invalid memory leading to a crash.

poporacer

Yes it was a typo when I posted the code. The variable is declared. I changed the above posted code to correct how it should have read.

ZapB

But does it also point to a valid object? If it wasn't declared it woudl not compile. If it is declared but pointing at random memory you could get a crash like you are seeing.

giesbert

initialise the variable at top of constructor to 0. The you will see if it's initialised properly

goetz

There is no variable printPreview in your code, if you modify it, please leave a note, so that readers can follow the discussion.

Can you show us the code for your slot in Dialog2, please. Also, a stack trace of the crash would be helpful.

poporacer

I assume that the variable is initialized properly as the dialog2 does open when the emit signal is commented out. The dialog opens without a problem but obviously the data is not sent.

goetz

We know nothing about what is sent or not, as we do not have a stack trace.

ZapB

You have completely changed the code. You are now connecting the signal up to a slot on dialog2 now instead of printPreview.

Can you please either:

Provide a complete backtrace as Volker asked or

upload a tarball of a small compileable example that reproduces the problem

You will most likely spot the problem yourself in preparing the example.

poporacer

How do I get a stack trace? I am not at a computer with a complier. I will provide the info when I find out how and get back to my computer.
Here is the code for the slot:
dialog2.h
@private slots:
void createReportTable(QStringList);
@
dialog2.cpp
@void Dialog2::createReportTable(QStringList stringList)
{
QStringList reportOptions=stringList;
printModel= new QSqlRelationalTableModel (this);
printModel-> setEditStrategy(QSqlTableModel::OnRowChange);
printModel-> setTable (mTableName);
printModel-> setRelation (2, QSqlRelation("rider", "id", "LName"));
printModel-> setRelation (3, QSqlRelation("track", "id", "TrackName"));
printModel-> setRelation (4, QSqlRelation("bike", "id", "BikeName"));

//set up printview
ui->printView->setModel(printModel);
ui->printView->setItemDelegate(new QSqlRelationalDelegate(this));
ui->printView->setSelectionMode(QAbstractItemView::SingleSelection);
ui->printView->setSelectionBehavior(QAbstractItemView::SelectRows);
ui->printView->setColumnHidden(0,true);
ui->printView->setColumnHidden(1, true);    
ui->printView->setColumnHidden(2, true); 
ui->printView->setColumnHidden(3, true); 
ui->printView->setColumnHidden(4, true);

if (reportOptions.at(0)=="FSChecked") {
printModel->setHeaderData (5, Qt::Horizontal, "Front \n Spring");
printModel->setHeaderData (6, Qt::Horizontal, "Spring \n Preload");
printModel->setHeaderData (7, Qt::Horizontal, "Oil \n Weight");
printModel->setHeaderData (8, Qt::Horizontal, "Oil \n Height");
printModel->setHeaderData (9, Qt::Horizontal, "Front \n Compression");
printModel->setHeaderData (10, Qt::Horizontal, "Front \n Rebound");
printModel->setHeaderData (11, Qt::Horizontal, "Fork \n Tube Height");
}

else //hide column and place in labels
{
        ui->printView->setColumnHidden(5,true); //Front Spring Rate
  ui->printView->setColumnHidden(6,true); //Front Spring Preload
  ui->printView->setColumnHidden(7,true); //Front Oil Weight
  ui->printView->setColumnHidden(8,true); //Oil Height
  ui->printView->setColumnHidden(9,true); //Front Compression
  ui->printView->setColumnHidden(10,true); //Front rebound
  ui->printView->setColumnHidden(11, true); //Fork Tube Height

  //Need to place data in lables

 
}

if (reportOptions.at(1)== "FTireChecked") //Front tire changes
{
printModel->setHeaderData (12, Qt::Horizontal, "Tire Type");
printModel->setHeaderData (13, Qt::Horizontal, "Tire \n Pressure");
printModel->setHeaderData (14, Qt::Horizontal, "Tire \n Modifications");
}
else
{
//itemColumn +=;
ui->printView->setColumnHidden(12,true);//Tire Type
ui->printView->setColumnHidden(13,true);//Tire Pressure
ui->printView->setColumnHidden(14,true);//Tire Modifications
}

printModel->setFilter(mFilterString); //mFilterString is initialized earlier
printModel->select();

//Set lables and static data

}@
I will get a small compileable example when I get to my computer.

goetz

If you run your program in an development environment (Qt Creator, Visual Studio or the like), you will get it almost automatically, once the program crashes.

As you do not give us information about your environment, we cannot help you further on this.

poporacer

I am using QT Creator. I am not getting anything backtrace info. I posted the error messages but I am sure there is more than that in a backtrace. After digging deeper and stepping through the program a million times, I think the problem actually lies in dialog2 when it is creating the table. in particular this line:
@printModel->setFilter(mFilterString); //mFilterString is initialized earlier @
I am stepping through it...I think I might get it...if not I will keep you posted
But if you could give me the info on getting the backtrace, it would be greatly appreciated. Another tool to help me resolve problems.

Thanks

poporacer

That was the problem! In the string manipulation, there were two single quotes when there should have been a single quote! I would still like the info on getting a backtrace.

Thanks again

ZapB

When you pause the app in the debugger (either manually or hitting a breakpoint or getting a crash) you should be able to see the call stack in one of the debug panels in qt-creator. From there you can copy it to the clipboard via the context menu.

Beka

Hello, i have the same problem, it wrote me:
can't find linker symbol for virtual table for QStandardItem' value found qEmptyModel()::cleanup' instead

debugger stopped on:
@void MyClass::currentChanged ( const QModelIndex & indexCurrent, const QModelIndex & indexPrevious ){
QStandardItem *current = comparisonModel->item(indexCurrent.row(), 0);
}@

i know, im accesing something what didnt exist, because of the message,
because I have model set on QTreeView, and indexes have children... but how to acces that item, when i have only index?

osirisgothra

Its not always an issue. When debugging, you need the proper symbols so you can step through code otherwise you will be stepping through assembler language instructions instead (the compiled binary instructions) - There are cases when parts of the API, especially when dealing with core parts that change a lot between versions, are mismatched. In those cases the correct vtable signature matches are not going to happen. And yeah, you will get a message about it if you step into a function that allocates one of such objects, and/or if one or more global static objects are allocated when begin stepping.

To resolve this, be sure your linker has access to the libraries with the correct debug symbols. If you often compile software (like in Linux for example) you may have dependencies for other versions mixed in, and possibly symlinked as the "default" library. The compiler just walks through it all hoping to find a match, and when it fails -- thats where you come in (and that message).

You can ignore these if they are generated because of symbol lookup during step debugging. If you get them during runtime without debugging, then that's something else altogether. Be advised these messages can happen for more than one root cause. Be sure you keep all your debug and compiler options set in harmony with the version of g++, lib, and other tools that are in use and be mindful of debugging libraries you install - you likely dont need to install -dev versions of libraries unless you want to step into them while debugging or need to statically link with them.

All this means is the symbols couldn't be found, your end users will rarely need that, unless your tool is a debugging tool but if it was I would suspect you'd already know all this and more anyway. Just be sure.

Christian Ehrlicher

@osirisgothra Can you tell me why you revive such old topics? What's the point of this? Please stop it.