Why won't this compile (link) with the Q_OBJECT macro in place?

Question

I made a prototype of a project with PyQt and made it work there, now I'm trying to convert it to C++ and am having some problems.

If I don't put the Q_OBJECT macro in, it compiles and works, but if I comment it out, I get the following errors:

Undefined symbols:
  "vtable for MapView", referenced from:
      MapView::~MapView()in mapview.o
      MapView::~MapView()in mapview.o
      MapView::MapView(QObject*)in mapview.o
      MapView::MapView()in mapview.o
  "MapView::staticMetaObject", referenced from:
      MapView::MapView(QObject*)in mapview.o
      MapView::MapView()in mapview.o

Here's the header:

#ifndef MAPVIEW_H
#define MAPVIEW_H

#include <QtGui>
#include <QObject>

class MapView : public QGraphicsScene
{
    //Q_OBJECT

public:
    MapView();
    explicit MapView(QObject *parent = 0);
    QGraphicsPixmapItem *mappixmap;
    ~MapView();

private:
    bool dragging;
    float offsetX, offsetY, downoffsetX, downoffsetY;

protected:
    void mousePressEvent(QGraphicsSceneMouseEvent *event);
    void mouseMoveEvent(QGraphicsSceneMouseEvent *event);
    void mouseReleaseEvent(QGraphicsSceneMouseEvent *event);

};

#endif // MAPVIEW_H

Secondary question is will Bad Things happen if I just omit the Q_OBJECT macro here?

And yes, I'm aware of that it's stupid to call a QGraphicsScene a "view".

Answer 1

This kind of errors usually happen when you add the Q_OBJECT macro and forget to rerun moc. If you use qmake, just run make qmake after you added the macro.

As for your second question: you won't be able to use signals/slots (among other things) without the Q_OBJECT macro. See the docs for more information about this.

Answer 2

Recently I tried to compile QDeviceWatcher on Linux and I get the same error WRT QDeviceWatcherPrivate class, which declaration can be found in qdevicewatcher_p.h and definition in qdevicewatcher_linux.cpp (on Linux).

I use cmake as build system and my CMakeLists.txt looks like:

cmake_minimum_required(VERSION 3.8)

project("QDeviceWatcher" LANGUAGES CXX)

set(CMAKE_INCLUDE_CURRENT_DIR ON)

set(CMAKE_AUTOMOC ON)

find_package(Qt5 REQUIRED COMPONENTS Core Network)

set(SOURCES)
list(APPEND SOURCES "qdevicewatcher.cpp")
if(WINCE)
    list(APPEND SOURCES "qdevicewatcher_wince.cpp")
elseif(WIN32)
    list(APPEND SOURCES "qdevicewatcher_win32.cpp")
elseif(APPLE)
    list(APPEND SOURCES "qdevicewatcher_mac.cpp")
elseif(UNIX)
    list(APPEND SOURCES "qdevicewatcher_linux.cpp")
else()
    message(FATAL_ERROR "no supported platform detected")
endif()

add_library(${PROJECT_NAME} STATIC ${SOURCES})
target_include_directories(${PROJECT_NAME} PUBLIC ".")

target_link_libraries(${PROJECT_NAME} PUBLIC Qt5::Core Qt5::Network)

set_target_properties(${PROJECT_NAME} PROPERTIES
    CXX_STANDARD 17
    CXX_STANDARD_REQUIRED YES
    CXX_EXTENSIONS YES
    )

As you can see in add_library I only provide *.cpp files. I suspect, that cmake runs Meta object compiler and looks through all the sources, maybe except #include dependencies, which filenames (w/o extension) not match *.cpp's filenames (just assumption), for the QObject/QWidget/... bases and Q_OBJECT macro. And I think cmake missed qdevicewatcher_p.h to look through and run MOC against it.

After I added the "qdevicewatcher_p.h" to the list of sources the error is ceased to exist.