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 macr

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.