i create simple c++ file called test.cpp

#include <iostream>
#include <libavformat\avformat.h>
using namespace std;

int main() 
{
    cout << "Hello, World!";
    return 0;
}

and using g++ in terminal to compile test.cpp file with the command :

g++ test.cpp -o test

i get this error =>

test.cpp:2:10: fatal error: libavformat\avformat.h: No such file or directory
    2 | #include <libavformat\avformat.h>
      |          ^~~~~~~~~~~~~~~~~~~~~~~~
compilation terminated.

Notes:
libavformat\avformat.h path is : /usr/include/x86_64-linux-gnu/libavformat/avformat.h

and using ffmpeg version is :

$ ffmpeg -version

ffmpeg version 4.4.2-0ubuntu0.22.04.1 Copyright (c) 2000-2021 the FFmpeg developers
built with gcc 11 (Ubuntu 11.2.0-19ubuntu1)
configuration: –prefix=/usr –extra-version=0ubuntu0.22.04.1 –toolchain=hardened –libdir=/usr/lib/x86_64-linux-gnu –incdir=/usr/include/x86_64-linux-gnu –arch=amd64 –enable-gpl –disable-stripping –enable-gnutls –enable-ladspa –enable-libaom –enable-libass –enable-libbluray –enable-libbs2b –enable-libcaca –enable-libcdio –enable-libcodec2 –enable-libdav1d –enable-libflite –enable-libfontconfig –enable-libfreetype –enable-libfribidi –enable-libgme –enable-libgsm –enable-libjack –enable-libmp3lame –enable-libmysofa –enable-libopenjpeg –enable-libopenmpt –enable-libopus –enable-libpulse –enable-librabbitmq –enable-librubberband –enable-libshine –enable-libsnappy –enable-libsoxr –enable-libspeex –enable-libsrt –enable-libssh –enable-libtheora –enable-libtwolame –enable-libvidstab –enable-libvorbis –enable-libvpx –enable-libwebp –enable-libx265 –enable-libxml2 –enable-libxvid –enable-libzimg –enable-libzmq –enable-libzvbi –enable-lv2 –enable-omx –enable-openal –enable-opencl –enable-opengl –enable-sdl2 –enable-pocketsphinx –enable-librsvg –enable-libmfx –enable-libdc1394 –enable-libdrm –enable-libiec61883 –enable-chromaprint –enable-frei0r –enable-libx264 –enable-shared
libavutil 56. 70.100 / 56. 70.100
libavcodec 58.134.100 / 58.134.100
libavformat 58. 76.100 / 58. 76.100
libavdevice 58. 13.100 / 58. 13.100
libavfilter 7.110.100 / 7.110.100
libswscale 5. 9.100 / 5. 9.100
libswresample 3. 9.100 / 3. 9.100
libpostproc 55. 9.100 / 55. 9.100

using this command :

g++ `pkg-config --cflags libavformat` test.cpp  `pkg-config --libs libavformat` -o test

>Solution :

You should use a forward slash, since you are apparently on a Unix system:

#include <libavformat/avformat.h>