未定义对“ vtable for MyCalss”的引用

Question

I've created new project with QTCreator using Empty Qt Project

MocTest.pro

QT += core
QT -= gui

TARGET = MocTest
CONFIG   += console

TEMPLATE = app


SOURCES += \
    main.cpp

main.cpp

#include <QCoreApplication>
#include <iostream>

class A : public QObject
{
    Q_OBJECT

public:
    A() {}

public slots:
    void slotA() {std::cout<<"Hi!";}
};

class B : public QObject
{
    Q_OBJECT

public:
    B () {}

    void doB() {emit ss();}

signals:
    void ss();
};

int main(int argc, char *argv[])
{
    QCoreApplication app (argc, argv);
    A a;
    B b;
    return app.exec();
}

I got errors

D:\\Test\\MocTest\\debug\\main.o:-1: In function `ZN1AC1Ev':

D:\\Test\\MocTest\\main.cpp:9: error: undefined reference to `vtable for A'

D:\\Test\\MocTest\\main.cpp:20: error: undefined reference to `vtable for B'

D:\\Test\\MocTest\\main.cpp:4: error: undefined reference to `vtable for A'

D:\\Test\\MocTest\\main.cpp:15: error: undefined reference to `vtable for B'

collect2.exe:-1: error: error: ld returned 1 exit status

Answer 1

Qmake ensures Moc is run on:

• Files listed in the HEADERS variable. Qmake arranges to compile and link the resulting C++ implementation file by adding it to SOURCES.
• Files listed in the SOURCES or OBJECTIVE_SOURCES variables are processed by moc. Qmake does not link any potential output, so the resulting moc output must be #included.

Looks like you need to put this:

#include "main.moc"

somewhere at main.cpp (probably at the end). Details here: https://qt-project.org/forums/viewthread/21420

Answer 2

vtable references are generated by moc for class heriting QObject . These are generated with files from HEADERS variable. Then you have to separate headers and implementations. This would gives:

myclass.h

class A : public QObject
{
    Q_OBJECT

public:
    A();
public slots:
    void slotA();
};

class B : public QObject
{
    Q_OBJECT

public:
    B();
    void doB();
signals:
    void ss();
};

and myclass.cpp

#include "myclass.h"

A::A()
{}
void A::slotA()
{ std::coud << "Hi!; }

B::B()
{}
void B::doB
{ emit ss(); }

Also then TestMoc.pro

QT = core # default is QT = core gui, so -= gui is enough or just =core
HEADERS = myclass.h
SOURCES = myclass.cpp main.cpp

Keep in mind that it generally a bad idea to call signals in inline methods. Also if you call doB in your main, before the app.exec(), it might not be handle because signals/slots are managed once the event-loop is run