未定义引用`vtable for MainWindow'

Question

I'm following Qt's guide for using a QPushButton .

I did exactly as the guide suggests, but I'm getting a compilation error:

./src/mainwindow.o: In function `MainWindow::MainWindow(QWidget*)':
mainwindow.cpp:(.text+0x1d): undefined reference to `vtable for MainWindow'
./src/mainwindow.o:mainwindow.cpp:(.text+0x25): more undefined references to `vtable for MainWindow' follow
collect2: error: ld returned 1 exit status
make: *** [HelloWorldProj] Error 1

I tried adding a destructor:

~MainWindow(){};

but the problem persisted.

I have no virtual functions declared, except one function inside QMainWindow (the class I'm inheriting from):

virtual QMenu *createPopupMenu();

Should this be defined in my class?

Answer 1

You need to set CMAKE_AUTOMOC to ON in our CMake file.

# Instruct CMake to run moc automatically when needed.
set(CMAKE_AUTOMOC ON)

"The AUTOMOC target property controls whether cmake inspects the C++ files in the target to determine if they require moc to be run, and to create rules to execute moc at the appropriate time." - Reference

Answer 2

Try running qmake on the project and then do a rebuild.

Your example is broken (I will update it shortly if possible)

Create an empty file and name it PushButtonExample.pro and add the following:

QT       += core gui
greaterThan(QT_MAJOR_VERSION, 4): QT += widgets

TARGET = PushButtonExample
TEMPLATE = app

SOURCES += main.cpp \
    mainwindow.cpp

HEADERS  += mainwindow.h

Then run qmake on that file, then make .

I also recommend that you download Qt Creator and use that as your IDE when building Qt project. Most Qt installs also install the Qt Creator IDE and it has some nice examples and wizards to create new projects.

Make sure all 4 files are in the same folder

main.cpp
mainwindow.cpp
mainwindow.h
PushButtonExample.pro

In command line, navigate into that folder and run qmake

qmake PushButtonExample.pro

This should create the following file in the same folder

Makefile

Then run make

This should build the example, and finally run the application:

./PushButtonExample

(I am also updating the wiki page)