一、环境介绍
window系统：win10 X64
QT版本: 5.12
QT5.12自带的MinGW编译器版本：mingw730_32 与mingw730_64
在QT的安装目录下，可以查看MinGW编译器的版本:
二、使用OpenCV出现的问题
在QT框架代码里使用老版本的分类器(cvLoad、cvHaarDetectObjects)处理图像时，正常编译没有问题，当使用新版本级联分类器(CascadeClassifier)时，程序编译就会出现一堆没有定义的错误：
release/widget.o:widget.cpp:(.text+0x2a1): undefined reference to `cv::fastFree(void*)' release/widget.o:widget.cpp:(.text+0x465): undefined reference to `cv::fastFree(void*)' release/widget.o:widget.cpp:(.text+0x631): undefined reference to `cv::fastFree(void*)' release/widget.o:widget.cpp:(.text+0x63b): undefined reference to `cv::_OutputArray::_OutputArray(cv::Mat&)' release/widget.o:widget.cpp:(.text+0x64b): undefined reference to `cv::_InputArray::_InputArray(cv::Mat const&)' release/widget.o:widget.cpp:(.text+0x66a): undefined reference to `cv::cvtColor(cv::_InputArray const&, cv::_OutputArray const&, int, int)' release/widget.o:widget.cpp:(.text+0x6c9): undefined reference to `cv::Mat::copySize(cv::Mat const&)' release/widget.o:widget.cpp:(.text+0x6de): undefined reference to `cv::Mat::copySize(cv::Mat const&)' release/widget.o:widget.cpp:(.text+0x6f9): undefined reference to `cv::Mat::copySize(cv::Mat const&)' release/widget.o:widget.cpp:(.text+0x786): undefined reference to `cv::Mat::deallocate()' release/widget.o:widget.cpp:(.text+0x796): undefined reference to `cv::Mat::deallocate()' release/widget.o:widget.cpp:(.text+0x7a4): undefined reference to `cv::Mat::deallocate()' release/widget.o:widget.cpp:(.text+0x7b3): undefined reference to `cv::Mat::deallocate()' release/widget.o:widget.cpp:(.text+0x7c6): undefined reference to `cv::Mat::deallocate()' release/widget.o:widget.cpp:(.text+0x7d4): more undefined references to `cv::Mat::deallocate()' follow release/widget.o:widget.cpp:(.text+0x150b): undefined reference to `cv::_OutputArray::_OutputArray(cv::Mat&)' release/widget.o:widget.cpp:(.text+0x151e): undefined reference to `cv::_InputArray::_InputArray(cv::Mat const&)' release/widget.o:widget.cpp:(.text+0x1540): undefined reference to `cv::cvtColor(cv::_InputArray const&, cv::_OutputArray const&, int, int)' release/widget.o:widget.cpp:(.text+0x15e1): undefined reference to `cv::fastFree(void*)' release/widget.o:widget.cpp:(.text+0x15f2): undefined reference to `cv::_OutputArray::_OutputArray(cv::Mat&)' release/widget.o:widget.cpp:(.text+0x1605): undefined reference to `cv::_InputArray::_InputArray(cv::Mat const&)' release/widget.o:widget.cpp:(.text+0x1627): undefined reference to `cv::cvtColor(cv::_InputArray const&, cv::_OutputArray const&, int, int)' release/widget.o:widget.cpp:(.text+0x1634): undefined reference to `cv::Mat::deallocate()' release/widget.o:widget.cpp:(.text+0x23a9): undefined reference to `cv::CascadeClassifier::CascadeClassifier()' release/widget.o:widget.cpp:(.text+0x23b4): undefined reference to `cv::CascadeClassifier::CascadeClassifier()' release/widget.o:widget.cpp:(.text+0x23bf): undefined reference to `cv::CascadeClassifier::CascadeClassifier()' release/widget.o:widget.cpp:(.text+0x2458): undefined reference to `cv::CascadeClassifier::load(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)' release/widget.o:widget.cpp:(.text+0x24f5): undefined reference to `cv::CascadeClassifier::~CascadeClassifier()' release/widget.o:widget.cpp:(.text+0x2500): undefined reference to `cv::CascadeClassifier::~CascadeClassifier()' release/widget.o:widget.cpp:(.text+0x250b): undefined reference to `cv::CascadeClassifier::~CascadeClassifier()' release/widget.o:widget.cpp:(.text+0x25af): undefined reference to `cv::CascadeClassifier::load(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)' release/widget.o:widget.cpp:(.text+0x268f): undefined reference to `cv::CascadeClassifier::load(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)' release/widget.o:widget.cpp:(.text+0x280a): undefined reference to `cv::Mat::create(int, int const*, int)' release/widget.o:widget.cpp:(.text+0x28b1): undefined reference to `cv::_OutputArray::_OutputArray(cv::Mat&)' release/widget.o:widget.cpp:(.text+0x28c8): undefined reference to `cv::_InputArray::_InputArray(cv::Mat const&)' release/widget.o:widget.cpp:(.text+0x28f3): undefined reference to `cv::cvtColor(cv::_InputArray const&, cv::_OutputArray const&, int, int)' release/widget.o:widget.cpp:(.text+0x2907): undefined reference to `cv::_OutputArray::_OutputArray(cv::Mat&)' release/widget.o:widget.cpp:(.text+0x291e): undefined reference to `cv::_InputArray::_InputArray(cv::Mat const&)' release/widget.o:widget.cpp:(.text+0x2939): undefined reference to `cv::equalizeHist(cv::_InputArray const&, cv::_OutputArray const&)' release/widget.o:widget.cpp:(.text+0x29cb): undefined reference to `cv::CascadeClassifier::detectMultiScale(cv::Mat const&, std::vector<cv::Rect_<int>, std::allocator<cv::Rect_<int> > >&, double, int, int, cv::Size_<int>, cv::Size_<int>)' release/widget.o:widget.cpp:(.text+0x2aac): undefined reference to `cv::ellipse(cv::Mat&, cv::Point_<int>, cv::Size_<int>, double, double, double, cv::Scalar_<double> const&, int, int, int)' release/widget.o:widget.cpp:(.text+0x2acc): undefined reference to `cv::Mat::Mat(cv::Mat const&, cv::Rect_<int> const&)' release/widget.o:widget.cpp:(.text+0x2b61): undefined reference to `cv::CascadeClassifier::detectMultiScale(cv::Mat const&, std::vector<cv::Rect_<int>, std::allocator<cv::Rect_<int> > >&, double, int, int, cv::Size_<int>, cv::Size_<int>)' release/widget.o:widget.cpp:(.text+0x2bf6): undefined reference to `cv::CascadeClassifier::detectMultiScale(cv::Mat const&, std::vector<cv::Rect_<int>, std::allocator<cv::Rect_<int> > >&, double, int, int, cv::Size_<int>, cv::Size_<int>)' release/widget.o:widget.cpp:(.text+0x2cce): undefined reference to `cv::circle(cv::Mat&, cv::Point_<int>, int, cv::Scalar_<double> const&, int, int, int)' release/widget.o:widget.cpp:(.text+0x2d90): undefined reference to `cv::fastFree(void*)' release/widget.o:widget.cpp:(.text+0x2ec0): undefined reference to `cv::fastFree(void*)' release/widget.o:widget.cpp:(.text+0x2f42): undefined reference to `cv::fastFree(void*)' release/widget.o:widget.cpp:(.text+0x2fc8): undefined reference to `cv::fastFree(void*)' release/widget.o:widget.cpp:(.text+0x3047): undefined reference to `cv::Mat::deallocate()' release/widget.o:widget.cpp:(.text+0x3057): undefined reference to `cv::Mat::deallocate()' release/widget.o:widget.cpp:(.text+0x3067): undefined reference to `cv::Mat::deallocate()' release/widget.o:widget.cpp:(.text+0x3077): undefined reference to `cv::Mat::deallocate()' release/widget.o:widget.cpp:(.text+0x3094): undefined reference to `cv::CascadeClassifier::~CascadeClassifier()' release/widget.o:widget.cpp:(.text+0x309f): undefined reference to `cv::CascadeClassifier::~CascadeClassifier()' release/widget.o:widget.cpp:(.text+0x30aa): undefined reference to `cv::CascadeClassifier::~CascadeClassifier()' release/widget.o:widget.cpp:(.text$_ZN2cv3MatD1Ev[__ZN2cv3MatD1Ev]+0x64): undefined reference to `cv::fastFree(void*)' release/widget.o:widget.cpp:(.text$_ZN2cv3MatD1Ev[__ZN2cv3MatD1Ev]+0x71): undefined reference to `cv::Mat::deallocate()' collect2.exe: error: ld returned 1 exit status mingw32-make[1]: *** [Makefile.Release:71: release/qt_opencv.exe] Error 1 mingw32-make: *** [Makefile:38: release] Error 2 mingw32-make[1]: Leaving directory 'D:/linux-share-dir/QT/build-qt_opencv-Desktop_Qt_5_12_6_MinGW_32_bit-Release' 21:11:46: 进程"C:\Qt\Qt5.12.6\Tools\mingw730_32\bin\mingw32-make.exe"退出，退出代码 2 。 Error while building/deploying project qt_opencv (kit: Desktop Qt 5.12.6 MinGW 32-bit) When executing step "Make"
三、解决办法
方法1: 使用当前QT使用的MinGW编译器，重新编译一份OpenCV的源码再使用。
方法2: 使用已经编译好的MinGW版本OpenCV库，直接调用。
既然已经有编译好的MinGW库，就不用麻烦自己再去编译了，各种OpenCV对应的MinGW编译器编译的库下载地址: https://github.com/huihut/OpenCV-MinGW-Build 。
由于我的源代码里使用了OpenCV3.x和2.X版本的很多函数，在OpenCV4.X里已经不存在，为了兼容代码，我这里下载OpenCV3.X版本的库。
下载之后，将bin目录加到系统的环境变量PATH里，方便程序在运行时能够找到对应的动态库。
在QT的工程文件里加入OpenCV头文件的路径和库路径即可：
QT += core gui QT += multimediawidgets QT += xml QT += multimedia QT += network QT += widgets QT += serialport greaterThan(QT_MAJOR_VERSION, 4): QT += widgets CONFIG += c++11 # The following define makes your compiler emit warnings if you use # any Qt feature that has been marked deprecated (the exact warnings # depend on your compiler). Please consult the documentation of the # deprecated API in order to know how to port your code away from it. DEFINES += QT_DEPRECATED_WARNINGS # You can also make your code fail to compile if it uses deprecated APIs. # In order to do so, uncomment the following line. # You can also select to disable deprecated APIs only up to a certain version of Qt. #DEFINES += QT_DISABLE_DEPRECATED_BEFORE=0x060000 # disables all the APIs deprecated before Qt 6.0.0 SOURCES += \ main.cpp \ widget.cpp HEADERS += \ widget.h FORMS += \ widget.ui # Default rules for deployment. qnx: target.path = /tmp/$${TARGET}/bin else: unix:!android: target.path = /opt/$${TARGET}/bin !isEmpty(target.path): INSTALLS += target #linu平台的路径设置 linux { message('运行linu版本') #添加opencv头文件的路径，需要根据自己的头文件路径进行修改 INCLUDEPATH+=/home/wbyq/work_pc/opencv-3.4.9/_install/install/include\ /home/wbyq/work_pc/opencv-3.4.9/_install/install/include/opencv\ /home/wbyq/work_pc/opencv-3.4.9/_install/install/include/opencv2 LIBS+=/home/wbyq/work_pc/opencv-3.4.9/_install/install/lib/libopencv_* } win32 { message('运行win32版本') #添加opencv头文件的路径，需要根据自己的头文件路径进行修改 INCLUDEPATH+=C:/OpenCV_3.4.7/OpenCV-MinGW-Build-OpenCV-3.4.7/include \ C:/OpenCV_3.4.7/OpenCV-MinGW-Build-OpenCV-3.4.7/include/opencv \ C:/OpenCV_3.4.7/OpenCV-MinGW-Build-OpenCV-3.4.7/include/opencv2 LIBS+=C:/OpenCV_3.4.7/OpenCV-MinGW-Build-OpenCV-3.4.7/x86/mingw/bin/libopencv_*.dll } RESOURCES += \ image.qrc
清除之前编译报错的文件，再次编译即可。
