Why include OpenCV's DNN module gives linker error on macOS? - c++

Question

I try to compile C++ code with OpenCV's DNN module included, but I got linker errors. The project uses a lot of other OpenCV module without any problem so I guess the headers and sources are added fine and it has a problem with the DNN module only. Linker flags are added for sure it has -lopencv_dnn_objdetect and -lopencv_dnn too. OpenCV installed with Homebrew not complied from source if that's matter. I use Xcode 11 beta, other members of my team use Visual Studio with Windows and they don't have any problem with the same code.
The errors I got:
Ld /Users/hordon/Library/Developer/Xcode/DerivedData/project-fmllzddvpyajkjbwntzsyqvmunjx/Build/Products/Debug/OpenCV normal x86_64 (in target: OpenCV)
cd /Users/hordon/Desktop/GreenFox/OpenCV
/Applications/Xcode-beta.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang++ -target x86_64-apple-macos10.14 -isysroot /Applications/Xcode-beta.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.15.sdk -L/Users/hordon/Library/Developer/Xcode/DerivedData/project-fmllzddvpyajkjbwntzsyqvmunjx/Build/Products/Debug -L/usr/local/Cellar/opencv/4.1.0_2/lib -L/usr/local/Cellar/sqlite/3.28.0/lib -L/usr/local/Cellar/tesseract/4.0.0_1/lib -L/usr/local/Cellar/leptonica/1.78.0/lib -F/Users/hordon/Library/Developer/Xcode/DerivedData/project-fmllzddvpyajkjbwntzsyqvmunjx/Build/Products/Debug -filelist /Users/hordon/Library/Developer/Xcode/DerivedData/project-fmllzddvpyajkjbwntzsyqvmunjx/Build/Intermediates.noindex/project.build/Debug/OpenCV.build/Objects-normal/x86_64/OpenCV.LinkFileList -Xlinker -object_path_lto -Xlinker /Users/hordon/Library/Developer/Xcode/DerivedData/project-fmllzddvpyajkjbwntzsyqvmunjx/Build/Intermediates.noindex/project.build/Debug/OpenCV.build/Objects-normal/x86_64/OpenCV_lto.o -Xlinker -export_dynamic -Xlinker -no_deduplicate -stdlib=libc++ -I/usr/local/Cellar/opencv/4.1.0_2/include/opencv4/opencv -I/usr/local/Cellar/opencv/4.1.0_2/include/opencv4 -L/usr/local/Cellar/opencv/4.1.0_2/lib -lopencv_gapi -lopencv_stitching -lopencv_aruco -lopencv_bgsegm -lopencv_bioinspired -lopencv_ccalib -lopencv_dnn_objdetect -lopencv_dpm -lopencv_face -lopencv_freetype -lopencv_fuzzy -lopencv_hfs -lopencv_img_hash -lopencv_line_descriptor -lopencv_quality -lopencv_reg -lopencv_rgbd -lopencv_saliency -lopencv_sfm -lopencv_stereo -lopencv_structured_light -lopencv_phase_unwrapping -lopencv_superres -lopencv_optflow -lopencv_surface_matching -lopencv_tracking -lopencv_datasets -lopencv_text -lopencv_dnn -lopencv_plot -lopencv_videostab -lopencv_video -lopencv_xfeatures2d -lopencv_shape -lopencv_ml -lopencv_ximgproc -lopencv_xobjdetect -lopencv_objdetect -lopencv_calib3d -lopencv_features2d -lopencv_highgui -lopencv_videoio -lopencv_imgcodecs -lopencv_flann -lopencv_xphoto -lopencv_photo -lopencv_imgproc -lopencv_core -ltesseract -lsqlite3 -Xlinker -dependency_info -Xlinker /Users/hordon/Library/Developer/Xcode/DerivedData/project-fmllzddvpyajkjbwntzsyqvmunjx/Build/Intermediates.noindex/project.build/Debug/OpenCV.build/Objects-normal/x86_64/OpenCV_dependency_info.dat -o /Users/hordon/Library/Developer/Xcode/DerivedData/project-fmllzddvpyajkjbwntzsyqvmunjx/Build/Products/Debug/OpenCV
Undefined symbols for architecture x86_64:
"cv::dnn::dnn4_v20180917::blobFromImage(cv::_InputArray const&, cv::_OutputArray const&, double, cv::Size_<int> const&, cv::Scalar_<double> const&, bool, bool, int)", referenced from:
detectText(cv::Mat, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >) in detectText.o
"cv::dnn::dnn4_v20180917::Net::forward(cv::_OutputArray const&, std::__1::vector<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >, std::__1::allocator<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > > > const&)", referenced from:
detectText(cv::Mat, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >) in detectText.o
"cv::dnn::dnn4_v20180917::Net::setInput(cv::_InputArray const&, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, double, cv::Scalar_<double> const&)", referenced from:
detectText(cv::Mat, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >) in detectText.o
"cv::dnn::dnn4_v20180917::Net::~Net()", referenced from:
detectText(cv::Mat, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >) in detectText.o
"cv::dnn::dnn4_v20180917::readNet(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&)", referenced from:
detectText(cv::Mat, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >) in detectText.o
"cv::dnn::dnn4_v20180917::NMSBoxes(std::__1::vector<cv::RotatedRect, std::__1::allocator<cv::RotatedRect> > const&, std::__1::vector<float, std::__1::allocator<float> > const&, float, float, std::__1::vector<int, std::__1::allocator<int> >&, float, int)", referenced from:
detectText(cv::Mat, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >) in detectText.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
Every code I tried throw this linker error if it has the DNN module included, for example, this sample code from OpenCV.

Answer 1

Meanwhile, I found a solution, so I leave it here in case anybody struggles with the same issue. If you installed OpenCV with Homebrew's brew install opencv command the DNN module won't be compiled. You have to rebuild it from source which you can achieve with the
brew reinstall opencv --build-from-source
command. Since at this point my search paths were added correctly I just had to build my project again, which was done without any error.