A missing vtable while implementing MainWindow in main.cpp

Question

The Qt projects on QtCreator usually keep main.cpp separated from QtMainWindow derived classes.

I tried to move it to main.cpp but got stuck:

myapp.pro

QT       += core gui

greaterThan(QT_MAJOR_VERSION, 4): QT += widgets

TEMPLATE = app

CONFIG += c++11

SOURCES += main.cpp

main.cpp

#include <QApplication>
#include <QMainWindow>

class MainWindow : public QMainWindow {
    Q_OBJECT

public:
    MainWindow(QWidget *parent=0) : QMainWindow(parent) { }
    virtual ~MainWindow() override {}
};

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

Then and got:

Undefined symbols for architecture x86_64:
  "vtable for MainWindow", referenced from:
      _main in main.o
  NOTE: a missing vtable usually means the first non-inline virtual member function has no definition.
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
make: *** [demo1.app/Contents/MacOS/demo1] Error 1

Having a difficult time trying to find the mistake I made. Any comments?

maybe this? https://stackoverflow.com/questions/33633056/undefined-reference-to-vtable-for-mainwindow — Richard Critten, Aug 21 '19 at 16:21
I tried make distclean, also rm all other files and qmake then make.. still the same error — KcFnMi, Aug 21 '19 at 16:29

score 3 · Accepted Answer · answered Aug 21 '19 at 16:32

3

It is a moc problem. When subclassing a QObject that uses the Q_OBJECT macro, you need to put it in its own file. You can't compile it with your main.cpp.

answered Aug 21 '19 at 16:32

JimmyG

131
1
9

score 1 · Answer 2 · edited Aug 22 '19 at 03:16

1

The meta-object compiler (moc) only processes header files (.h). The moc documentation mentions this briefly. I'm guessing it's because the declaration of a QObject subclass alone is sufficient to generate the signals/slots code; i.e. the implementation that would normally be in the .cpp file is unnecessary for moc to do its thing.

edited Aug 22 '19 at 03:16

Alexis Wilke

19,179
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84
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answered Aug 21 '19 at 18:10

Carlton

4,217
2
24
40

score 0 · Answer 3 · answered Aug 21 '19 at 15:54

0

In most cases this is because you did not declare the destructor:

virtual ~MainWindow() override {}

That's the one function that will trigger the compiler code necessary to create the vtable.

answered Aug 21 '19 at 15:54

Alexis Wilke

19,179
10
84
156

Good to know. Altought still having the same error. – KcFnMi Aug 21 '19 at 16:03
It may be a `moc` problem, which would add a class you have to derive from to complete the `Q_OBJECT`. – Alexis Wilke Aug 21 '19 at 16:13

A missing vtable while implementing MainWindow in main.cpp

3 Answers3