Qt Linker Error: "Undefined Reference to Vtable"

Qt undefined reference to vtable

This is a subtle bug (and probably partly at least a compiler bug) that I've seen before. Since QWidget has a virtual destructor, the compiler needs a vtable for your class. But your class doesn't have any virtual functions, so it didn't build one for your Communicate class.

Add a virtual ~Communicate() {}; to your class, and all will be well.

Yes, it took me some time to figure this out too!

How resolve undefined reference to vtable in c++ on QT

Typically, errors like that can be resolved by running QMake. Anytime you create a new class that derives from QObject, Qt's model meta-object compiler (MOC) needs to auto-generate the code for the meta-class of the new class -- QMake ensures that this happens.

If you are using Qt Creator, just select Run qmake from the Build menu.

You may also have to run Clean project X or Clean all, also found in the Build menu.

The MOC can be temperamental, so you need to do the following:

• Move your QObject-derived class into a separate source and header file (In your case, create bot.h and bot.cpp, and move the class code there)
• Separate declaration and definition of your slot code (define txt as bot::txt in bot.cpp)

The MOC generates a corresponding meta-class file (moc_bot.cpp, in your case), and sometimes gets confused when there are multiple QObject-derived classes in one file. Best practice is to use one header and one source file for each QObject-derived class.

If all else fails, you may need to delete the .pro.user file for your project, and exit and restart Qt Creator. Then from Build menu, choose Clean all, Run qmake, Rebuild All.

Undefined reference to vtable

So, I've figured out the issue and it was a combination of bad logic and not being totally familiar with the automake/autotools world. I was adding the correct files to my Makefile.am template, but I wasn't sure which step in our build process actually created the makefile itself. So, I was compiling with an old makefile that had no idea about my new files whatsoever.

Thanks for the responses and the link to the GCC FAQ. I will be sure to read that to avoid this problem occurring for a real reason.

Undefined reference to vtable. Trying to compile a Qt project

Warning: Do not do this if you already have a .pro file - you'll lose it!

In order to automatically ensure that all moc cpp files are generated, you can get qmake to automatically generate a .pro file for you instead of writing one yourself.

Run

qmake -project

in the project directory, and qmake will scan your directory for all C++ headers and source files to generate moc cpp files for.

Nothing helped to solve Undefined reference to vtable in Qt

Your problem is caused by the fact that you have the declaration of a class subclassing QObject and having Q_OBJECT macro inside it within a .cpp file; That macro contains, among other things, the declaration of a virtual method:

virtual const QMetaObject *metaObject() const;

qmake tool only processes header files to locate QObject subclasses containing Q_OBJECT macro to automatically generate the actual code corresponding to the declarations produced by Q_OBJECT macro. In your case, qmake does not process main.cpp file and hence it is left containing unimplemented virtual function - hence the linker error you are receiving.

The proper solution is to move the declaration of the class to a separate header file and add the header to your project. There's, however, another solution which is kinda hacky but it can be used if you for some reason absolutely need to leave the declaration of your class inside the .cpp file:

class CustomUndoStack : public QObject
{
    Q_OBJECT
public:
    CustomUndoStack(QObject * parent = 0) : QObject(parent) {}
};

#include "main.moc"

int main(int argc, char *argv[])
{
    QCoreApplication qCoreApplication(argc, argv);

    CustomUndoStack uStack;

    return qCoreApplication.exec();
}

Notice the #include "main.moc" line. The presence of this line forces qmake to actually process the declarations from your .cpp file and generate the required .moc file which contains the autogenerated code corresponding to the declarations produced by Q_OBJECT macro.

Qt: Undefined reference to 'vtable'

Absence of a vtable is usually a symptom of failing to include the output of moc in the linker arguments. Make sure that you ran moc on your header, and that you linked the result.

Note that if you're using qmake, you may need to re-run qmake to generate new makefiles if you change a class that was not Q_OBJECT so that is now Q_OBJECT - it will not otherwise know that moc should be run.

In passing, it's a good idea to add a constructor that takes an optional parent QObject, to get some of the benefits of Qt's memory management (freeing of child objects) where the user wants it.

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