即使在链接库时，也会获得对 OpenCV 元素的未定义引用。 使用 CMake

Question

I'm trying to compile this code:

#include <opencv2/opencv.hpp>

using namespace cv;

int main()
{
  Mat image = imread ("tree.jpg", IMREAD_GRAYSCALE);
  imshow ("Grayscale Image", image);
  waitKey (0);
  return 0;
}

But I get undefined references to all the OpenCV elements. From reading on the site it seems this happens because of problems with linking the OpenCV libraries.

Is there a problem with my CMake file?

cmake_minimum_required(VERSION 3.17)
project(Programming)

set(CMAKE_CXX_STANDARD 17)
find_package(OpenCV REQUIRED PATHS C:\\opencv\\build\\x64\\vc15\\lib)
include_directories(${OpenCV_INCLUDE_DIRS})
add_executable(project main.cpp)
target_link_libraries(project ${OpenCV_LIBS})

MESSAGE("OpenCV_LIBS: " ${OpenCV_LIBS})

The OpenCV libraries seem to be found:

OpenCV_LIBS: opencv_calib3dopencv_coreopencv_dnnopencv_features2dopencv_flannopencv_gapiopencv_highguiopencv_imgcodecsopencv_imgprocopencv_mlopencv_objdetectopencv_photoopencv_stitchingopencv_videoopencv_videoioopencv_world

So I'm not sure the problem is.

EDIT: This is the complete error message:

/usr/lib/gcc/x86_64-pc-cygwin/10/../../../../x86_64-pc-cygwin/bin/ld: CMakeFiles/project.dir/main.cpp.o:/cygdrive/c/Users/adibo/Programming/CLionProjects/Programming/main.cpp:9: undefined reference to `cv::imread(std::string const&, int)'
/usr/lib/gcc/x86_64-pc-cygwin/10/../../../../x86_64-pc-cygwin/bin/ld: CMakeFiles/project.dir/main.cpp.o: in function `main':
/cygdrive/c/Users/adibo/Programming/CLionProjects/Programming/main.cpp:11: undefined reference to `cv::imshow(std::string const&, cv::_InputArray const&)'
/usr/lib/gcc/x86_64-pc-cygwin/10/../../../../x86_64-pc-cygwin/bin/ld: /cygdrive/c/Users/adibo/Programming/CLionProjects/Programming/main.cpp:12: undefined reference to `cv::waitKey(int)'
/usr/lib/gcc/x86_64-pc-cygwin/10/../../../../x86_64-pc-cygwin/bin/ld: /cygdrive/c/Users/adibo/Programming/CLionProjects/Programming/main.cpp:9: undefined reference to `cv::Mat::~Mat()'
/usr/lib/gcc/x86_64-pc-cygwin/10/../../../../x86_64-pc-cygwin/bin/ld: /cygdrive/c/Users/adibo/Programming/CLionProjects/Programming/main.cpp:9: undefined reference to `cv::Mat::~Mat()'
collect2: error: ld returned 1 exit status
make[3]: *** [CMakeFiles/project.dir/build.make:120: project.exe] Error 1
make[2]: *** [CMakeFiles/Makefile2:96: CMakeFiles/project.dir/all] Error 2
make[1]: *** [CMakeFiles/Makefile2:103: CMakeFiles/project.dir/rule] Error 2
make: *** [Makefile:138: project] Error 2

Answer 1

The problem is, your openCV libraries are installed in a non standard path, and the names in OpenCV_LIBS do not give you an absolute path. You need to indicate to the linker where to find the libraries.

To do that, you can use the target_link_directories() cmake command. https://cmake.org/cmake/help/latest/command/target_link_directories.html

I guess the path you want to pass it is "C:\opencv\build\x64\vc15\lib".