Qt What is Q_OBJECT

I encountered a link error after adding Q_OBJECT macro to the class definition. The error message was similar to vtable for XXX is missing, NOTE: a missing vtable usually means the first non-inline virtual member function has no definition.. However, the error disappeared if I removed the Q_OBJECT macro.

The Q_OBJECT macro, used in Qt, enables features like slot and signal. This macro requires the Meta Object Compiler(moc) to process the class definition. I presume that the moc generates source code that must be compiled and linked into the final target.

Even though I have enabled the AUTOMOC in my CMakeLists.txt file, I still encountered this issue. After searching online, I realized that the header file(hpp) containing the class with Q_OBJECT macro needed to be explicitly listed as a source file for the target in CMakeLists.txt. For instance,

set(GUI_SOURCES
  MainWindow.cpp
  MainWindow.hpp # <--- here!!!
)

set(GUI_UIS
  MainWindow.ui
)

set(GUI_RESOURCES
    # Add resource files here as they are created
)

add_library(gui STATIC
    ${GUI_SOURCES}
    ${GUI_UIS}
    ${GUI_RESOURCES}
    )

After adding the MainWindow.hpp (which contains the Q_OBJECT macro) to the source list for gui target in CMakeLists.txt, the project compiled and linked successfully. I hope this explanation helps others who encounter similar vtable missing errors when use Q_OBJECT with CMake, even when AUTOMOC is enabled.