Re: undefined reference to vtable

[Axel Freyn <axel-freyn at gmx dot de>]

Hi Alfkil,
On Fri, Nov 20, 2009 at 08:22:12AM -0800, alfkil wrote:
> 
> After compiling and linking libQtGui.a I try building an example which
> returns this error:
> 
> libQtGui.a(qprintdialog_unix.o):(.got2+0x94): undefined reference to `vtable
> for QUnixPrintWidget'
> 
> The class in question looks like this:
> 
> class Q_GUI_EXPORT QUnixPrintWidget : public QWidget
> {
> Q_OBJECT
> 
> public:
> QUnixPrintWidget(QPrinter *printer, QWidget *parent = 0);
> ~QUnixPrintWidget();
> void updatePrinter();
> 
> private:
> friend class QPrintDialogPrivate;
> friend class QUnixPrintWidgetPrivate;
> QUnixPrintWidgetPrivate *d;
> Q_PRIVATE_SLOT(d, void _q_printerChanged(int))
> Q_PRIVATE_SLOT(d, void _q_btnBrowseClicked())
> Q_PRIVATE_SLOT(d, void _q_btnPropertiesClicked())
> };
> 
> GCC version 4.2.4 (amigaos4). Tested the same code on mac os x 10.4 w/gcc
> 4.0.1 and it runs fine.
> 
> As you can see, it doesn't declare any methods virtual. So why would I get
> the error here and not with the parent class? I also looked for "#pragma"
> calls (inspired by a previous post) and didn't find any. I've read the gcc
> faq section on vtable error, and read a lot of posts here and there, and I'm
> still not sure, that I fully understand the issue at hand. Any
> help/explanation/magic would be strongly appreciated!!

Even if I don't know the solution to your problem: try the minimal (?)
example:
struct Base{
    virtual void f(){}
};
struct Derived : Base{
  void f();
};
int main(){
    Derived d;
}

This reproduces your error: I obtain " undefined reference to `vtable
for Derived'"
The reason is: As the Base-class has a virtual function (the Destructor
in my example) and the derived class overloads ist, this is
automatically also a virtual function - even if it is not explicitely
declared virtual. The vtable for Derived is created as soon as I add the
definition of Derived::f. After adding
void Derived::f(){}
to the code, it compiles fine.

So I would subspect that you are missing the definitions of the
functions of QUnixPrintWidget. 

Hth,

Axel