Question

Possible Duplicate:
Qt: Signals and slots Error: undefined reference to `vtable for

Here we have test.cpp:

#include <QApplication>
#include <QMainWindow>

class MainWindow : public QMainWindow
{
    Q_OBJECT
};

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MainWindow w;
    w.show();

    return a.exec();
}

Place alone in a new directory and:

$ qmake -project
$ qmake
$ make

It doesn’t work:

test.o: In function `MainWindow::~MainWindow()':
test.cpp:(.text._ZN10MainWindowD2Ev[_ZN10MainWindowD5Ev]+0x3): undefined reference to `vtable for MainWindow'
test.cpp:(.text._ZN10MainWindowD2Ev[_ZN10MainWindowD5Ev]+0xb): undefined reference to `vtable for MainWindow'
test.o: In function `main':
test.cpp:(.text.startup+0x48): undefined reference to `vtable for MainWindow'
test.cpp:(.text.startup+0x51): undefined reference to `vtable for MainWindow'
test.o: In function `MainWindow::~MainWindow()':
test.cpp:(.text._ZN10MainWindowD0Ev[_ZN10MainWindowD0Ev]+0x7): undefined reference to `vtable for MainWindow'
test.o:test.cpp:(.text._ZN10MainWindowD0Ev[_ZN10MainWindowD0Ev]+0xf): more undefined references to `vtable for MainWindow' follow
collect2: error: ld returned 1 exit status
make: *** [tmp] Error 1

In general such errors are either because moc is not invoked or because of unimplemented virtual methods.

moc should be invoked automatically by qmake, and afaik QMainWindow doesn’t have any pure virtual methods – so what is the problem here?

When I remove Q_OBJECT it works. Why is this? What is going on?

I see that Q_OBJECT tells moc to generate the signals/slots meta-data for that object, why can’t it do that in this case?

Update:

Solution is to add #include "test.moc" just below class:

#include <QApplication>
#include <QMainWindow>

class MainWindow : public QMainWindow
{
    Q_OBJECT
};

#include "test.moc"  // <----------- HERE

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MainWindow w;
    w.show();

    return a.exec();
}

Answer 1

It is because you declared the class inside the .cpp file. Put it into a separate .h header.

By default, the MOC is not invoked for source files, just for headers (edit: according to @Kamil, MOC is ran against .cpp source files, except that the generated .moc files should be #include-d manually in the .cpp files — see my edit below).

Then, add the respective header file to the HEADERS variable inside the .pro file.

Of course, #include the same header into test.cpp.

Also, see this answer here:

https://stackoverflow.com/a/5854884/1284631

edit:

According to this thread: http://www.qtcentre.org/threads/28580-Why-does-qmake-moc-only-process-header-files

to force MOC running for .cpp files, you should add:

#include "test.moc"

right below the definition of your class. And re-run qmake. However, I did not test this.

edit2: See this reference for a discussion on the line on what OP is looking for:

http://harmattan-dev.nokia.com/docs/library/html/qt4/moc.html