Build error in bgsegm::BackgroundSubtractors

[limann77]

I'm trying to build git master OpenCV+contrib with gcc 15.2.1. My build with

    cmake -B build -S $pkgname \
        -DWITH_VTK=OFF \
        -DWITH_OPENCL=OFF \
        -DWITH_OPENGL=ON \
        -DWITH_TBB=ON \
        -DBUILD_WITH_DEBUG_INFO=OFF \
        -DBUILD_TESTS=OFF \
        -DBUILD_PERF_TESTS=OFF \
        -DBUILD_EXAMPLES=OFF \
        -DINSTALL_C_EXAMPLES=OFF \
        -DINSTALL_PYTHON_EXAMPLES=OFF \
        -DCMAKE_INSTALL_PREFIX=/usr \
        -DOPENCV_EXTRA_MODULES_PATH="$srcdir/opencv_contrib/modules" \
        -DOPENCV_SKIP_PYTHON_LOADER=ON \
        -DOPENCV_PYTHON3_INSTALL_PATH=$_pythonpath \
        -DLAPACK_LIBRARIES="/usr/lib/liblapack.so;/usr/lib/libblas.so;/usr/lib/libcblas.so" \
        -DLAPACK_CBLAS_H="/usr/include/cblas.h" \
        -DLAPACK_LAPACKE_H="/usr/include/lapacke.h" \
        -DOPENCV_GENERATE_PKGCONFIG=ON \
        -DOPENCV_ENABLE_NONFREE=ON \
        -DOPENCV_GENERATE_SETUPVARS=OFF \
        -DWITH_1394=OFF \
        -DEIGEN_INCLUDE_PATH=/usr/include/eigen3 \
        -DCPU_BASELINE_DISABLE=SSE3,AVX512_SKX \
        -DCPU_DISPATCH= \
        -DBUILD_opencv_java=OFF \
        -DBUILD_opencv_cvv=OFF \
        -DBUILD_opencv_dnn=ON \
        -DBUILD_highgui=OFF \
        -DWITH_FFMPEG=ON \
        -DWITH_QT=OFF \
        -DWITH_PROTOBUF=OFF \
        -DWITH_IPP=OFF \
        -DWITH_ZLIB_NG=ON

    LC_ALL=C cmake --build build

leads to a compilation failure in opencv2/bgsegm.hpp:

In file included from ./opencv_contrib/modules/bgsegm/src/precomp.hpp:45,
                 from ./opencv_contrib/modules/bgsegm/src/bgfg_gaussmix.cpp:43:
./opencv_contrib/modules/bgsegm/include/opencv2/bgsegm.hpp:89:26: error: 'virtual void cv::bgsegm::BackgroundSubtractorMOG::apply(cv::InputArray, cv::InputArray, cv::OutputArray, double)' marked 'override', but does not override
   89 |     CV_WRAP virtual void apply(InputArray image, InputArray knownForegroundMask, OutputArray fgmask, double learningRate=-1) CV_OVERRIDE = 0;
      |                          ^~~~~
./opencv_contrib/modules/bgsegm/include/opencv2/bgsegm.hpp:153:26: error: 'virtual void cv::bgsegm::BackgroundSubtractorGMG::apply(cv::InputArray, cv::InputArray, cv::OutputArray, double)' marked 'override', but does not override
  153 |     CV_WRAP virtual void apply(InputArray image, InputArray knownForegroundMask, OutputArray fgmask, double learningRate=-1) CV_OVERRIDE = 0;
      |                          ^~~~~
./opencv_contrib/modules/bgsegm/include/opencv2/bgsegm.hpp:269:26: error: 'virtual void cv::bgsegm::BackgroundSubtractorCNT::apply(cv::InputArray, cv::InputArray, cv::OutputArray, double)' marked 'override', but does not override
  269 |     CV_WRAP virtual void apply(InputArray image, InputArray knownForegroundMask, OutputArray fgmask, double learningRate=-1) CV_OVERRIDE = 0;
      |                          ^~~~~
./opencv_contrib/modules/bgsegm/include/opencv2/bgsegm.hpp:330:26: error: 'virtual void cv::bgsegm::BackgroundSubtractorGSOC::apply(cv::InputArray, cv::InputArray, cv::OutputArray, double)' marked 'override', but does not override
  330 |     CV_WRAP virtual void apply(InputArray image, InputArray knownForegroundMask, OutputArray fgmask, double learningRate=-1) CV_OVERRIDE = 0;
      |                          ^~~~~
./opencv_contrib/modules/bgsegm/include/opencv2/bgsegm.hpp:342:26: error: 'virtual void cv::bgsegm::BackgroundSubtractorLSBP::apply(cv::InputArray, cv::InputArray, cv::OutputArray, double)' marked 'override', but does not override
  342 |     CV_WRAP virtual void apply(InputArray image, InputArray knownForegroundMask, OutputArray fgmask, double learningRate=-1) CV_OVERRIDE = 0;
      |                          ^~~~~

Just removing the CV_OVERRIDE=0 leads to

/usr/bin/ld: CMakeFiles/opencv_bgsegm.dir/src/bgfg_gaussmix.cpp.o:(.data.rel.ro._ZTIN2cv6bgsegm27BackgroundSubtractorMOGImplE[_ZTIN2cv6bgsegm27BackgroundSubtractorMOGImplE]+0x10): undefined reference to `typeinfo for cv::bgsegm::BackgroundSubtractorMOG'
/usr/bin/ld: CMakeFiles/opencv_bgsegm.dir/src/bgfg_gmg.cpp.o:(.data.rel.ro._ZTIN2cv6bgsegm27BackgroundSubtractorGMGImplE[_ZTIN2cv6bgsegm27BackgroundSubtractorGMGImplE]+0x10): undefined reference to `typeinfo for cv::bgsegm::BackgroundSubtractorGMG'
/usr/bin/ld: CMakeFiles/opencv_bgsegm.dir/src/bgfg_gsoc.cpp.o:(.data.rel.ro._ZTIN2cv6bgsegm28BackgroundSubtractorGSOCImplE[_ZTIN2cv6bgsegm28BackgroundSubtractorGSOCImplE]+0x10): undefined reference to `typeinfo for cv::bgsegm::BackgroundSubtractorGSOC'
/usr/bin/ld: CMakeFiles/opencv_bgsegm.dir/src/bgfg_gsoc.cpp.o:(.data.rel.ro._ZTIN2cv6bgsegm28BackgroundSubtractorLSBPImplE[_ZTIN2cv6bgsegm28BackgroundSubtractorLSBPImplE]+0x10): undefined reference to `typeinfo for cv::bgsegm::BackgroundSubtractorLSBP'
/usr/bin/ld: CMakeFiles/opencv_bgsegm.dir/src/bgfg_subcnt.cpp.o:(.data.rel.ro._ZTIN2cv6bgsegm27BackgroundSubtractorCNTImplE[_ZTIN2cv6bgsegm27BackgroundSubtractorCNTImplE]+0x10): undefined reference to `typeinfo for cv::bgsegm::BackgroundSubtractorCNT'

[Kumataro]

Thank you for your report !
I cannot reproduce with Ubutn 25.10 + gcc 15.2.0 .

Maybe the OpenCV repository you are using is older than OpenCV Contrib.
Could you share your CMake output and confirm it ?

CMake output

General configuration for OpenCV 4.13.0-dev =====================================
  Version control:               4.12.0-272-gd7f6201219

  Extra modules:
    Location (extra):            /home/kmtr/work/opencv_contrib/modules
    Version control (extra):     4.12.0-50-g945c8a52

For example ...

kmtr@kmtr-VMware-Virtual-Platform:~/work/opencv$ git log --oneline | head -5
d7f6201219 Merge pull request #27939 from Kumataro:fix27938
f836f67203 openexr: suppress -Wnontrivial-memcall warnings
52393156c8 Merge pull request #27524 from DDmytro:multichannel_ecc
12b64b0e2a Merge pull request #27927 from tailsu:tailsu/tiff-error-handler
09c893477e Merge pull request #27443 from ritamelo06:gapi-feature

kmtr@kmtr-VMware-Virtual-Platform:~/work/opencv_contrib$ git log --oneline | head -5
945c8a52 Merge pull request #4025 from MaximSmolskiy:fix_findEllipses
b23b7653 Merge pull request #4026 from cudawarped:fix_cudacodec_colorcvt_for-streams
68fd5415 Merge pull request #4029 from asmorkalov:as/cvv_demo_build_fix
61ccd6e1 cvv_demo.cpp build fix.
48e4b384 [cudadcodec] remove dependance on global constant memory for color conversion coefficients

g + d7f6201219 = gd7f6201219 for OpenCV
g + 945c8a52 = g945c8a52 for OpenCV Contrib

They are recent 4.x branch on opencv and opencv_contrib.

Background

BackgroundSubtractor(MOG|GMG|CNT|GSOC|LSBP) defined in opencv_contrib/bgsegm refer BackgroundSubtractor defined in opencv/video.

So in this module, opencv and opencv_contrib version maching is very important.

kmtr@kmtr-VMware-Virtual-Platform:~/work/opencv$ git grep "CV_EXPORTS_W BackgroundSubtractor" modules/video/include/
modules/video/include/opencv2/video/background_segm.hpp:class CV_EXPORTS_W BackgroundSubtractor : public Algorithm
modules/video/include/opencv2/video/background_segm.hpp:class CV_EXPORTS_W BackgroundSubtractorMOG2 : public BackgroundSubtractor
modules/video/include/opencv2/video/background_segm.hpp:class CV_EXPORTS_W BackgroundSubtractorKNN : public BackgroundSubtractor

kmtr@kmtr-VMware-Virtual-Platform:~/work/opencv_contrib$ git grep "public BackgroundSubtractor" modules/bgsegm/include/opencv2/bgsegm.hpp
modules/bgsegm/include/opencv2/bgsegm.hpp:class CV_EXPORTS_W BackgroundSubtractorMOG : public BackgroundSubtractor
modules/bgsegm/include/opencv2/bgsegm.hpp:class CV_EXPORTS_W BackgroundSubtractorGMG : public BackgroundSubtractor
modules/bgsegm/include/opencv2/bgsegm.hpp:class CV_EXPORTS_W BackgroundSubtractorCNT  : public BackgroundSubtractor
modules/bgsegm/include/opencv2/bgsegm.hpp:class CV_EXPORTS_W BackgroundSubtractorGSOC : public BackgroundSubtractor
modules/bgsegm/include/opencv2/bgsegm.hpp:class CV_EXPORTS_W BackgroundSubtractorLSBP : public BackgroundSubtractor