Can't compile OpenCV header files in CLion - c++

Question

I recently installed OpenCV on my Ubuntu system. I used instructions from here and here to install OpenCV c++.
When I include the opencv header in my file:
#include <opencv2/opencv.hpp>
It gives me the following error while building:
note: ‘cv::Range’ declared here
589 | class CV_EXPORTS Range
| ^~~~~
In file included from /usr/local/include/opencv4/opencv2/core.hpp:59,
from /usr/local/include/opencv4/opencv2/core/core.hpp:48,
from /home/mo/CLionProjects/DisjointForest/main.cpp:4:
/usr/local/include/opencv4/opencv2/core/mat.hpp:1640:43: error: template argument 1 is invalid
1640 | Mat operator()(const std::vector<Range>& ranges) const;
| ^
/usr/local/include/opencv4/opencv2/core/mat.hpp:1640:43: error: template argument 2 is invalid
/usr/local/include/opencv4/opencv2/core/mat.hpp:1640:9: error: ‘Mat Mat::operator()(const int&) const’ cannot be overloaded with ‘Mat Mat::operator()(const int&) const’
1640 | Mat operator()(const std::vector<Range>& ranges) const;
| ^~~~~~~~
/usr/local/include/opencv4/opencv2/core/mat.hpp:1630:9: note: previous declaration ‘Mat Mat::operator()(const int&) const’
1630 | Mat operator()( const Rect& roi ) const;
| ^~~~~~~~
/usr/local/include/opencv4/opencv2/core/mat.hpp:1643:44: error: expected type-specifier
1643 | template<typename _Tp, int n> operator Vec<_Tp, n>() const;
| ^~~~~~~~~~~
/usr/local/include/opencv4/opencv2/core/mat.hpp:1644:51: error: expected type-specifier
1644 | template<typename _Tp, int m, int n> operator Matx<_Tp, m, n>() const;
| ^~~~~~~~~~~~~~~
/usr/local/include/opencv4/opencv2/core/mat.hpp:1850:38: error: ‘Vec’ does not name a type
1850 | template<int n> uchar* ptr(const Vec<int, n>& idx);
| ^~~
/usr/local/include/opencv4/opencv2/core/mat.hpp:1850:41: error: expected ‘,’ or ‘...’ before ‘<’ token
1850 | template<int n> uchar* ptr(const Vec<int, n>& idx);
| ^
/usr/local/include/opencv4/opencv2/core/mat.hpp:1852:44: error: ‘Vec’ does not name a type
1852 | template<int n> const uchar* ptr(const Vec<int, n>& idx) const;
| ^~~
/usr/local/include/opencv4/opencv2/core/mat.hpp:1852:47: error: expected ‘,’ or ‘...’ before ‘<’ token
1852 | template<int n> const uchar* ptr(const Vec<int, n>& idx) const;
| ^
/usr/local/include/opencv4/opencv2/core/mat.hpp:1877:50: error: ‘Vec’ does not name a type
1877 | template<typename _Tp, int n> _Tp* ptr(const Vec<int, n>& idx);
| ^~~
/usr/local/include/opencv4/opencv2/core/mat.hpp:1877:53: error: expected ‘,’ or ‘...’ before ‘<’ token
1877 | template<typename _Tp, int n> _Tp* ptr(const Vec<int, n>& idx);
| ^
/usr/local/include/opencv4/opencv2/core/mat.hpp:1879:56: error: ‘Vec’ does not name a type
1879 | template<typename _Tp, int n> const _Tp* ptr(const Vec<int, n>& idx) const;
| ^~~
/usr/local/include/opencv4/opencv2/core/mat.hpp:1879:59: error: expected ‘,’ or ‘...’ before ‘<’ token
1879 | template<typename _Tp, int n> const _Tp* ptr(const Vec<int, n>& idx) const;
| ^
/usr/local/include/opencv4/opencv2/core/mat.hpp:1950:49: error: ‘Vec’ does not name a type
1950 | template<typename _Tp, int n> _Tp& at(const Vec<int, n>& idx);
| ^~~
/usr/local/include/opencv4/opencv2/core/mat.hpp:1950:52: error: expected ‘,’ or ‘...’ before ‘<’ token
1950 | template<typename _Tp, int n> _Tp& at(const Vec<int, n>& idx);
| ^
/usr/local/include/opencv4/opencv2/core/mat.hpp:1952:55: error: ‘Vec’ does not name a type
1952 | template<typename _Tp, int n> const _Tp& at(const Vec<int, n>& idx) const;
| ^~~
/usr/local/include/opencv4/opencv2/core/mat.hpp:1952:58: error: expected ‘,’ or ‘...’ before ‘<’ token
1952 | template<typename _Tp, int n> const _Tp& at(const Vec<int, n>& idx) const;
| ^
/usr/local/include/opencv4/opencv2/core/mat.hpp:1958:36: error: ‘Point’ was not declared in this scope; did you mean ‘cv::Point’?
1958 | template<typename _Tp> _Tp& at(Point pt);
| ^~~~~
| cv::Point
In file included from /usr/local/include/opencv4/opencv2/core.hpp:58,
from /usr/local/include/opencv4/opencv2/core/core.hpp:48,
from /home/mo/CLionProjects/DisjointForest/main.cpp:4:
/usr/local/include/opencv4/opencv2/core/types.hpp:194:17: note: ‘cv::Point’ declared here
194 | typedef Point2i Point;
| ^~~~~
In file included from /usr/local/include/opencv4/opencv2/core.hpp:59,
from /usr/local/include/opencv4/opencv2/core/core.hpp:48,
from /home/mo/CLionProjects/DisjointForest/main.cpp:4:
/usr/local/include/opencv4/opencv2/core/mat.hpp:1958:45: error: data member ‘at’ cannot be a member template
1958 | template<typename _Tp> _Tp& at(Point pt);
| ^
/usr/local/include/opencv4/opencv2/core/mat.hpp:1963:42: error: ‘Point’ was not declared in this scope; did you mean ‘cv::Point’?
1963 | template<typename _Tp> const _Tp& at(Point pt) const;
| ^~~~~
| cv::Point
In file included from /usr/local/include/opencv4/opencv2/core.hpp:58,
from /usr/local/include/opencv4/opencv2/core/core.hpp:48,
from /home/mo/CLionProjects/DisjointForest/main.cpp:4:
/usr/local/include/opencv4/opencv2/core/types.hpp:194:17: note: ‘cv::Point’ declared here
194 | typedef Point2i Point;
| ^~~~~
In file included from /usr/local/include/opencv4/opencv2/core.hpp:59,
from /usr/local/include/opencv4/opencv2/core/core.hpp:48,
from /home/mo/CLionProjects/DisjointForest/main.cpp:4:
/usr/local/include/opencv4/opencv2/core/mat.hpp:1963:51: error: expected ‘;’ before ‘const’
1963 | template<typename _Tp> const _Tp& at(Point pt) const;
| ^~~~~~
| ;
/usr/local/include/opencv4/opencv2/core/mat.hpp:1963:57: error: data member ‘at’ cannot be a member template
1963 | template<typename _Tp> const _Tp& at(Point pt) const;
| ^
/usr/local/include/opencv4/opencv2/core/mat.hpp:2001:28: error: ‘MatIterator_’ does not name a type
2001 | template<typename _Tp> MatIterator_<_Tp> begin();
| ^~~~~~~~~~~~
/usr/local/include/opencv4/opencv2/core/mat.hpp:2002:28: error: ‘MatConstIterator_’ does not name a type
2002 | template<typename _Tp> MatConstIterator_<_Tp> begin() const;
| ^~~~~~~~~~~~~~~~~
/usr/local/include/opencv4/opencv2/core/mat.hpp:2009:28: error: ‘MatIterator_’ does not name a type
2009 | template<typename _Tp> MatIterator_<_Tp> end();
| ^~~~~~~~~~~~
/usr/local/include/opencv4/opencv2/core/mat.hpp:2010:28: error: ‘MatConstIterator_’ does not name a type
2010 | template<typename _Tp> MatConstIterator_<_Tp> end() const;
| ^~~~~~~~~~~~~~~~~
/usr/local/include/opencv4/opencv2/core/mat.hpp:957:68: error: ‘Range’ has not been declared
957 | Mat(const Mat& m, const Range& rowRange, const Range& colRange=Range::all());
| ^~~~~
/usr/local/include/opencv4/opencv2/core/mat.hpp:2181:22: error: expected nested-name-specifier before ‘DataType’
2181 | typedef typename DataType<_Tp>::channel_type channel_type;
| ^~~~~~~~
/usr/local/include/opencv4/opencv2/core/mat.hpp:2181:22: error: expected ‘;’ at end of member declaration
2181 | typedef typename DataType<_Tp>::channel_type channel_type;
| ^~~~~~~~
| ;
/usr/local/include/opencv4/opencv2/core/mat.hpp:2181:30: error: expected unqualified-id before ‘<’ token
2181 | typedef typename DataType<_Tp>::channel_type channel_type;
| ^
/usr/local/include/opencv4/opencv2/core/mat.hpp:2182:13: error: ‘MatIterator_’ does not name a type
2182 | typedef MatIterator_<_Tp> iterator;
| ^~~~~~~~~~~~
/usr/local/include/opencv4/opencv2/core/mat.hpp:2183:13: error: ‘MatConstIterator_’ does not name a type
2183 | typedef MatConstIterator_<_Tp> const_iterator;
| ^~~~~~~~~~~~~~~~~
/usr/local/include/opencv4/opencv2/core/mat.hpp:2192:23: error: expected ‘)’ before ‘_size’
2192 | explicit Mat_(Size _size);
| ~ ^~~~~~
| )
/usr/local/include/opencv4/opencv2/core/mat.hpp:2194:14: error: expected ‘)’ before ‘_size’
2194 | Mat_(Size _size, const _Tp& value);
| ~ ^~~~~~
| )
/usr/local/include/opencv4/opencv2/core/mat.hpp:2208:31: error: ‘Range’ does not name a type
2208 | Mat_(const Mat_& m, const Range& rowRange, const Range& colRange=Range::all());
| ^~~~~
/usr/local/include/opencv4/opencv2/core/mat.hpp:2208:54: error: ‘Range’ does not name a type
2208 | Mat_(const Mat_& m, const Range& rowRange, const Range& colRange=Range::all());
| ^~~~~
.....................
.....................
In file included from /usr/local/include/opencv4/opencv2/core.hpp:3294,
from /usr/local/include/opencv4/opencv2/core/core.hpp:48,
from /home/mo/CLionProjects/DisjointForest/main.cpp:4:
/usr/local/include/opencv4/opencv2/core/optim.hpp:296:26: error: ‘InputArray’ was not declared in this scope; did you mean ‘OutputArray’?
296 | CV_EXPORTS_W int solveLP(InputArray Func, InputArray Constr, OutputArray z);
| ^~~~~~~~~~
| OutputArray
/usr/local/include/opencv4/opencv2/core/optim.hpp:296:43: error: ‘InputArray’ was not declared in this scope; did you mean ‘OutputArray’?
296 | CV_EXPORTS_W int solveLP(InputArray Func, InputArray Constr, OutputArray z);
| ^~~~~~~~~~
| OutputArray
/usr/local/include/opencv4/opencv2/core/optim.hpp:296:74: error: expected primary-expression before ‘z’
296 | CV_EXPORTS_W int solveLP(InputArray Func, InputArray Constr, OutputArray z);
| ^
/usr/local/include/opencv4/opencv2/core/optim.hpp:296:75: error: expression list treated as compound expression in initializer [-fpermissive]
296 | CV_EXPORTS_W int solveLP(InputArray Func, InputArray Constr, OutputArray z);
| ^
make[3]: *** [CMakeFiles/DisjointForest.dir/build.make:63: CMakeFiles/DisjointForest.dir/main.cpp.o] Error 1
make[2]: *** [CMakeFiles/Makefile2:76: CMakeFiles/DisjointForest.dir/all] Error 2
make[1]: *** [CMakeFiles/Makefile2:83: CMakeFiles/DisjointForest.dir/rule] Error 2
make: *** [Makefile:118: DisjointForest] Error 2
I have no idea what this error means. Could someone please guide me how I can fix this error and starting working with openCV?
If it helps, here's my cmake file:
cmake_minimum_required(VERSION 3.16)
project(DisjointForest)
set(CMAKE_CXX_STANDARD 20)
find_package(OpenCV 4.3.0 REQUIRED)
include_directories(${OpenCV_INCLUDE_DIRS})
add_executable(DisjointForest main.cpp LinkedListDisjointSet.cpp DisjointForest.cpp DisjointForest.h)
target_link_directories(DisjointForest PUBLIC ${OpenCV_LIBS})

Answer 1

Okay so after a lot of research about linkers and linking libraries, I've realized the two problems:
1) Changing c++ 20 to c++ 17 compiled the header files. Perhaps the library is not compatible with c++20 yet.
2) Changing the above won't suffice as we have still not correctly linked the libraries. It will still give unresolved error. In my cmake file I need to replace the target_link_directories() with target_link_libraries(). This was a silly mistake I made while following the instructions from the links in the question.