在编辑器中通过点击 #include "opencv2/opencv.hpp" 头文件就可以看到该头文件的定义如下

#ifndef OPENCV_ALL_HPP
#define OPENCV_ALL_HPP

// File that defines what modules where included during the build of OpenCV
// These are purely the defines of the correct HAVE_OPENCV_modulename values
#include "opencv2/opencv_modules.hpp"

// Then the list of defines is checked to include the correct headers
// Core library is always included --> without no OpenCV functionality available
#include "opencv2/core.hpp"

// Then the optional modules are checked
#ifdef HAVE_OPENCV_CALIB3D
#include "opencv2/calib3d.hpp"
#endif
#ifdef HAVE_OPENCV_FEATURES2D
#include "opencv2/features2d.hpp"
#endif
#ifdef HAVE_OPENCV_DNN
#include "opencv2/dnn.hpp"
#endif
#ifdef HAVE_OPENCV_FLANN
#include "opencv2/flann.hpp"
#endif
#ifdef HAVE_OPENCV_HIGHGUI
#include "opencv2/highgui.hpp"
#endif
#ifdef HAVE_OPENCV_IMGCODECS
#include "opencv2/imgcodecs.hpp"
#endif
#ifdef HAVE_OPENCV_IMGPROC
#include "opencv2/imgproc.hpp"
#endif
#ifdef HAVE_OPENCV_ML
#include "opencv2/ml.hpp"
#endif
#ifdef HAVE_OPENCV_OBJDETECT
#include "opencv2/objdetect.hpp"
#endif
#ifdef HAVE_OPENCV_PHOTO
#include "opencv2/photo.hpp"
#endif
#ifdef HAVE_OPENCV_SHAPE
#include "opencv2/shape.hpp"
#endif
#ifdef HAVE_OPENCV_STITCHING
#include "opencv2/stitching.hpp"
#endif
#ifdef HAVE_OPENCV_SUPERRES
#include "opencv2/superres.hpp"
#endif
#ifdef HAVE_OPENCV_VIDEO
#include "opencv2/video.hpp"
#endif
#ifdef HAVE_OPENCV_VIDEOIO
#include "opencv2/videoio.hpp"
#endif
#ifdef HAVE_OPENCV_VIDEOSTAB
#include "opencv2/videostab.hpp"
#endif
#ifdef HAVE_OPENCV_VIZ
#include "opencv2/viz.hpp"
#endif

// Finally CUDA specific entries are checked and added
#ifdef HAVE_OPENCV_CUDAARITHM
#include "opencv2/cudaarithm.hpp"
#endif
#ifdef HAVE_OPENCV_CUDABGSEGM
#include "opencv2/cudabgsegm.hpp"
#endif
#ifdef HAVE_OPENCV_CUDACODEC
#include "opencv2/cudacodec.hpp"
#endif
#ifdef HAVE_OPENCV_CUDAFEATURES2D
#include "opencv2/cudafeatures2d.hpp"
#endif
#ifdef HAVE_OPENCV_CUDAFILTERS
#include "opencv2/cudafilters.hpp"
#endif
#ifdef HAVE_OPENCV_CUDAIMGPROC
#include "opencv2/cudaimgproc.hpp"
#endif
#ifdef HAVE_OPENCV_CUDAOBJDETECT
#include "opencv2/cudaobjdetect.hpp"
#endif
#ifdef HAVE_OPENCV_CUDAOPTFLOW
#include "opencv2/cudaoptflow.hpp"
#endif
#ifdef HAVE_OPENCV_CUDASTEREO
#include "opencv2/cudastereo.hpp"
#endif
#ifdef HAVE_OPENCV_CUDAWARPING
#include "opencv2/cudawarping.hpp"
#endif

#endif

通过观察代码可知， opencv.hpp 中己经包含了 OpenCV 各模块的头文件，如高层 GUI 图形用户界面模块头文件 highgui.hpp 、图像处理模块头文件 imgproc.hpp 、2D 特征模块头文件 features2d.hpp 等。

所以，我们在编写 core 、 objdetect 、 imgproc 、 photo 、 video 、 features2d 、 calib3d 、 ml 、 highgui 模块的应用程序时，原则上仅写上一句 #include <opencv2/opencv.hpp> 即可，这样可以精简优化代码。