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“Undefined reference to vtable” for QObject-derived classes in the implementation (.cpp) file.

Filed under: Programming,Qt — Tags: , — Davor @ 18:14

“undefined reference to vtable” seems to be a common problem with Qt’s meta-object system in combination with GCC.

The main cause due to GCC is described here: http://gcc.gnu.org/faq.html#vtables

But with QObject it gets a bit more complex. When you use the macro Q_OBJECT, you define some virtual methods which might trigger the above GCC error under certain circumstances. So make sure your use of the Q_OBJECT macro fulfills the following requirements:

  1. Make sure the Q_OBJECT macro is present in the definition of all QObject-derived classes
  2. Make sure you define your QObject-derived classes in your header files ONLY
  3. Make sure all of your header files are listed in your .pro file in the HEADERS-list
  4. Run qmake every time you add Q_OBJECT to one of your classes or modify your .pro file

(Source: http://www.theirishpenguin.com/2007/07/01/qobject-qmake-and-sadness-undefined-reference-to-vtable/)

Not fulfilling point 2:
You get the “undefined reference to vtable” error if you put your QObject-derived class in the implementation file because moc will not “add” the implementation of the virtual functions to the cpp file. Apparently you can enforce this by adding “#include cpp_file_name.moc” to your “cpp_file_name.cpp” file. Moc will detect this and will generate a “cpp_file_name.moc” file.

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