Problem with running c++ opencv program in linux: functions not declared

Question

I'm learning OpenCV and I'm using c++. I installed opencv in my ubuntu using the libopencv-dev and python3-opencv packegs in ubuntu 20.04 from this tutorial: https://linuxize.com/post/how-to-install-opencv-on-ubuntu-20-04/ but when I try to run a sample program using this command g++ -I/usr/local/include/opencv4 Example1.cpp -o Example1 I get this error:

Example1.cpp: In function ‘int main(int, char**)’:

Example1.cpp:5:2: error: ‘IplImage’ was not declared in this scope

    5 |  IplImage* img = cvLoadImage( argv[1] );
      |  ^~~~~~~~

Example1.cpp:5:12: error: ‘img’ was not declared in this scope

    5 |  IplImage* img = cvLoadImage( argv[1] );
      |            ^~~

Example1.cpp:5:18: error: ‘cvLoadImage’ was not declared in this scope

    5 |  IplImage* img = cvLoadImage( argv[1] );
      |                  ^~~~~~~~~~~

Example1.cpp:6:29: error: ‘CV_WINDOW_AUTOSIZED’ was not declared in this scope

    6 |  cvNamedWindow( "Example1", CV_WINDOW_AUTOSIZED );
      |                             ^~~~~~~~~~~~~~~~~~~

Example1.cpp:6:2: error: ‘cvNamedWindow’ was not declared in this scope

    6 |  cvNamedWindow( "Example1", CV_WINDOW_AUTOSIZED );
      |  ^~~~~~~~~~~~~

Example1.cpp:7:2: error: ‘cvShowImage’ was not declared in this scope

    7 |  cvShowImage( "Example1", img );
      |  ^~~~~~~~~~~

Example1.cpp:8:2: error: ‘cvWaitKey’ was not declared in this scope

    8 |  cvWaitKey(0);
      |  ^~~~~~~~~

Example1.cpp:9:2: error: ‘cvReleaseImage’ was not declared in this scope

    9 |  cvReleaseImage( &img );
      |  ^~~~~~~~~~~~~~

Example1.cpp:10:2: error: ‘cvDestroyWindow’ was not declared in this scope

    10 |  cvDestroyWindow( "Example1" );
       |  ^~~~~~~~~~~~~~~

here is the source code:

#include <opencv2/highgui.h>

int main( int argc, char** argv ) {

    IplImage* img = cvLoadImage( argv[1] );
    cvNamedWindow( "Example1", CV_WINDOW_AUTOSIZED );
    cvShowImage( "Example1", img );
    cvWaitKey(0);
    cvReleaseImage( &img );
    cvDestroyWindow( "Example1" );

}

and the pkg-config output(if it's needed):

p4n0@p4n0-desktop:~/Desktop/Example_Project$ pkg-config --cflags opencv

-I/usr/local/include/opencv4

p4n0@p4n0-desktop:~/Desktop/Example_Project$ pkg-config --libs opencv

-L/usr/local/lib -lopencv_gapi -lopencv_stitching -lopencv_alphamat -lopencv_aruco -lopencv_bgsegm -lopencv_bioinspired -lopencv_ccalib -lopencv_dnn_objdetect -lopencv_dnn_superres -lopencv_dpm -lopencv_highgui -lopencv_face -lopencv_freetype -lopencv_fuzzy -lopencv_hfs -lopencv_img_hash -lopencv_intensity_transform -lopencv_line_descriptor -lopencv_mcc -lopencv_quality -lopencv_rapid -lopencv_reg -lopencv_rgbd -lopencv_saliency -lopencv_stereo -lopencv_structured_light -lopencv_phase_unwrapping -lopencv_superres -lopencv_optflow -lopencv_surface_matching -lopencv_tracking -lopencv_datasets -lopencv_text -lopencv_dnn -lopencv_plot -lopencv_videostab -lopencv_videoio -lopencv_xfeatures2d -lopencv_shape -lopencv_ml -lopencv_ximgproc -lopencv_video -lopencv_xobjdetect -lopencv_objdetect -lopencv_calib3d -lopencv_imgcodecs -lopencv_features2d -lopencv_flann -lopencv_xphoto -lopencv_photo -lopencv_imgproc -lopencv_core

Answer 1

To use IplImage , you need to include opencv2/core/types_c.h

Edit:- Since you are using OpenCV4, those includes are not available for you. They were removed.

In C++, like other languages to use a function you need to import it so the compiler can find the function during it's compilation step

Off topic suggestion:- This applies in case you are not forced to use IplImage etc.

If you are allowed to use Modern OpenCV, and are not restricted by old versions, prefer the Modern solutions not the C API.

IplImage etc. were removed from OpenCV4.