Opencv4.1链接so错误（android sdk) error: undefined reference to 'cv::error(int, std::string const...
如果遇到链接错误，一般是lib的路径不对，但是显然这次不是，错误如下：
native/jni/include\opencv2/core/mat.inl.hpp:548: error: undefined reference to 'cv::error(int, std::string const&, char const*, char const*, int)'
native/jni/include\opencv2/core/mat.inl.hpp:561: error: undefined reference to 'cv::error(int, std::string const&, char const*, char const*, int)'
xxx_std.h:355: error: undefined reference to 'cv::putText(cv::_InputOutputArray const&, std::string const&, cv::Point_, int, double, cv::Scalar_, int, int, bool)'
显然是链上了，但是找不到特定函数的实现，比如error() ,imread(),imwrite()等等
幸而stackover上opencv4.0.1已经有过这个问题了，见链接
根本原因是NDK从r16开始就切换到了llvm的libC++，所以opencv4从libstdC++切换到了libc++。
导致链接错误的根本原因是设置了-DANDRID_STL=gnustl_shared，因为opencv利用的libc++编的，而非gnustl
所以build.gradle设置成-DANDROID_STL=c++_shared即可，参照：
externalNativeBuild {
cmake {
//sample cpp flag parameters
cppFlags "-std=c++14 -Ofast -Rpass-analysis=loop-vectorize -fsave-optimization-record -fdiagnostics-show-hotness"
//sample abi filter parameters
abiFilters 'x86', 'x86_64', 'armeabi-v7a', 'arm64-v8a'
//set -DANDROID_STL to c++_shared
arguments "-DANDROID_STL=c++_shared"
}
}done!
如果有需求从opencv4降到opencv3，很多废弃的宏定义可能也够喝一壶，添加一个头文件：
/*
* OpenCV 4.0.1 backward compatibility header.
* */
#ifndef ANDROID_DOXCAPTUREUI_OPENCV4_COMPAT_H
#define ANDROID_DOXCAPTUREUI_OPENCV4_COMPAT_H
#include
#include
#if CV_MAJOR_VERSION > 3
// namespace cv;
//{
#ifndef CV_TERMCRIT_ITER
#define CV_TERMCRIT_ITER cv::TermCriteria::MAX_ITER
#endif
#ifndef CV_BGR2GRAY
#define CV_BGR2GRAY cv::COLOR_BGR2GRAY
#endif
#ifndef CV_BGR2RGB
#define CV_BGR2RGB cv::COLOR_BGR2RGB
#endif
#ifndef CV_RGB2GRAY
#define CV_RGB2GRAY cv::COLOR_RGB2GRAY
#endif
#ifndef CV_BGRA2GRAY
#define CV_BGRA2GRAY cv::COLOR_BGRA2GRAY
#endif
#ifndef CV_GRAY2RGBA
#define CV_GRAY2RGBA cv::COLOR_GRAY2RGBA
#endif
#ifndef CV_GRAY2RGB
#define CV_GRAY2RGB cv::COLOR_GRAY2RGB
#endif
#ifndef CV_GRAY2BGRA
#define CV_GRAY2BGRA cv::COLOR_GRAY2BGRA
#endif
#ifndef CV_GRAY2BGR565
#define CV_GRAY2BGR565 cv::COLOR_GRAY2BGR565
#endif
#ifndef CV_RGBA2BGR565
#define CV_RGBA2BGR565 cv::COLOR_RGBA2BGR565
#endif
#ifndef CV_RGBA2RGB
#define CV_RGBA2RGB cv::COLOR_RGBA2RGB
#endif
#ifndef CV_RGB2RGBA
#define CV_RGB2RGBA cv::COLOR_RGB2RGBA
#endif
#ifndef CV_RGB2BGR565
#define CV_RGB2BGR565 cv::COLOR_RGB2BGR565
#endif
#ifndef CV_RGBA2BGR
#define CV_RGBA2BGR cv::COLOR_RGBA2BGR
#endif
#ifndef CV_RGBA2GRAY
#define CV_RGBA2GRAY cv::COLOR_RGBA2GRAY
#endif
#ifndef CV_ADAPTIVE_THRESH_GAUSSIAN_C
#define CV_ADAPTIVE_THRESH_GAUSSIAN_C cv::ADAPTIVE_THRESH_GAUSSIAN_C
#endif
#ifndef CV_THRESH_BINARY
#define CV_THRESH_BINARY cv::THRESH_BINARY
#endif
#ifndef CV_BGR2HSV
#define CV_BGR2HSV cv::COLOR_BGR2HSV
#endif
#ifndef CV_BGR2Lab
#define CV_BGR2Lab cv::COLOR_BGR2Lab
#endif
#ifndef CV_RGB2HSV
#define CV_RGB2HSV cv::COLOR_RGB2HSV
#endif
#ifndef CV_HSV2RGB
#define CV_HSV2RGB cv::COLOR_HSV2RGB
#endif
#ifndef CV_THRESH_OTSU
#define CV_THRESH_OTSU cv::THRESH_OTSU
#endif
#ifndef CV_THRESH_BINARY
#define CV_THRESH_BINARY THRESH_BINARY
#endif
#ifndef CV_MOP_CLOSE
#define CV_MOP_CLOSE cv::MORPH_CLOSE
#endif
#ifndef CV_RETR_CCOMP
#define CV_RETR_CCOMP cv::RETR_CCOMP
#endif
#ifndef CV_RETR_LIST
#define CV_RETR_LIST cv::RETR_LIST
#endif
#ifndef CV_RETR_TREE
#define CV_RETR_TREE cv::RETR_TREE
#endif
#ifndef CV_RETR_FLOODFILL
#define CV_RETR_FLOODFILL cv::RETR_FLOODFILL
#endif
#ifndef CV_RETR_EXTERNAL
#define CV_RETR_EXTERNAL cv::RETR_EXTERNAL
#endif
#ifndef CV_CHAIN_APPROX_SIMPLE
#define CV_CHAIN_APPROX_SIMPLE cv::CHAIN_APPROX_SIMPLE
#endif
#ifndef CV_CHAIN_APPROX_TC89_KCOS
#define CV_CHAIN_APPROX_TC89_KCOS cv::CHAIN_APPROX_TC89_KCOS
#endif
#ifndef CV_FILLED
#define CV_FILLED cv::FILLED
#endif
#ifndef CV_REDUCE_SUM
#define CV_REDUCE_SUM cv::REDUCE_SUM
#endif
#ifndef CV_StsBadArg
#define CV_StsBadArg cv::Error::StsBadArg
#endif
//}
#ifndef cvFastArctan
#define cvFastArctan( Y, X ) ( std::atan2( (Y), (X) ) )
#endif
#endif //endif CV_MAJOR_VERSION
#endif //ANDROID_DOXCAPTUREUI_OPENCV4_COMPAT_H
参考： stackover