✅ A C++ project using OpenCV that can be compiled with Clang but not with GCC

main.cpp

//
//  main.cpp
//
//  Created by treastrain / Tanaka Ryoga on 2021/01/25.
//

#include "main.hpp"

int main(int, char**) {
    std::cout << "Hello, OpenCV World!" << std::endl;

    std::cout << "OpenCV version: " << CV_VERSION << std::endl;

    cv::Mat sourceImage = cv::imread("./image.jpg");
    if (!sourceImage.data) {
        std::cerr << "Data cannot be read." << std::endl;
        exit(-1);
    }

    return 0;
}

main.hpp

//
//  main.hpp
//
//  Created by treastrain / Tanaka Ryoga on 2021/01/25.
//

#include <iostream>

#include <opencv2/opencv.hpp>

Makefile

OpenCV-Playground:
	echo OpenCV-Playground

buildclang:
	clang++ -Wall -std=c++17 -O0 \
	-I../Libraries/opencv/include/opencv4 -L../Libraries/opencv/lib \
	-lopencv_core -lopencv_videoio -lopencv_imgproc -lopencv_imgcodecs \
	-o OpenCV-Playground \
	main.cpp

buildgcc:
	g++ -Wall -std=c++17 -O0 \
	-I../Libraries/opencv/include/opencv4 -L../Libraries/opencv/lib \
	-lopencv_core -lopencv_videoio -lopencv_imgproc -lopencv_imgcodecs \
	-o OpenCV-Playground \
	main.cpp

• Ubuntu 20.04.01 LTS
• OpenCV 4.5.1
• Clang 10.0.0
• g++ (Ubuntu 9.3.0-17ubuntu1~20.04) 9.3.0

$ make buildclang
clang++ -Wall -std=c++17 -O0 \
-I../Libraries/opencv/include/opencv4 -L../Libraries/opencv/lib \
-lopencv_core -lopencv_videoio -lopencv_imgproc -lopencv_imgcodecs \
-o OpenCV-Playground \
main.cpp

$ make buildgcc
g++ -Wall -std=c++17 -O0 \
-I../Libraries/opencv/include/opencv4 -L../Libraries/opencv/lib \
-lopencv_core -lopencv_videoio -lopencv_imgproc -lopencv_imgcodecs \
-o OpenCV-Playground \
main.cpp
/usr/bin/ld: /tmp/cc2mRTwF.o: in function `main':
main.cpp:(.text+0xc0): undefined reference to `cv::imread(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, int)'
/usr/bin/ld: main.cpp:(.text+0x118): undefined reference to `cv::Mat::~Mat()'
/usr/bin/ld: main.cpp:(.text+0x16c): undefined reference to `cv::Mat::~Mat()'
collect2: error: ld returned 1 exit status
make: *** [Makefile:12: buildgcc] エラー 1

エラーのイメージ
https://twitter.com/treastrain/status/1353375033188470785

Clang でも同じエラーを発生させるためには、
-lopencv_core -lopencv_videoio (…省略)
を消す
https://twitter.com/treastrain/status/1353379531227205632

ref: https://stackoverflow.com/questions/62098479/opencv-undefined-reference-to-cvimreadcvstring-const-int
参考: https://stackoverflow.com/questions/62098479/opencv-undefined-reference-to-cvimreadcvstring-const-int

pkg-config needed to be after the source files.

pkg-config はソースファイルの後ろに指定する必要があります (訳: @Qs-F )

So put main.cpp before -I../Libraries/opencv/include/op....
The whole script will be like

なので main.cpp を -I../Libraries/opencv/include/op.... の前に書いてください。
全体像は次のようになります

g++ -Wall -std=c++17 -O0 \
main.cpp \
-I../Libraries/opencv/include/opencv4 -L../Libraries/opencv/lib \
-lopencv_core -lopencv_videoio -lopencv_imgproc -lopencv_imgcodecs \
-o OpenCV-Playground \

@Qs-F You're my sunshine!

🦀🦀🦀🦀🦀🦀