I have read many posts about this problem on stackoverflow and I have tried them but having no luck. I have removed some files which inherit the interface class but still the error multiple definition for functions in the protocol.h file.

main.cpp

#include "interfaceinterface.h"

interface.h

#pragma once
#ifndef INTERFACE_H
#define INTERFACE_H

#include "..buffer.h"
#include "..enums.h"
#include "..cpu.h"
...More Code

#endif

interface.cpp

#include "interface.h"

buffer.h

#ifndef BUFFER_H
#define BUFFER_H

#include "common.h"
#include "protocol.h"
...More Code

#endif

common.h

#pragma once

...Common include files like <vector> and such

protocol.h

#pragma once
#ifndef PROTOCOL_H
#define PROTOCOL_H
...More Code

#endif

enums.h

Just Code

cpu.h

Just code

command with gcc 13.2.0

"args": [
        "-fdiagnostics-color=always",
        "-I",
        "${fileDirname}",
        "-fcommon",
        "-g",
        "-std=c++20",
        "${workspaceFolder}\*.cpp",
        "${workspaceFolder}\interface\*.cpp"
        "-o",
        // "${fileDirname}\${fileBasenameNoExtension}.exe"
        "${fileDirname}\main.exe"
      ]

Since only two cpp files so pretty much main.cpp and interface.cpp

• I have tried putting extern in front of the functions
• I have put #pragma once and #ifndef on all the files
• The file structure to see if any files gets included twice

interface{
buffer{
common
protocol
}
cpu
enums
}
main

So it looks like common,protocol,buffer,cpu,enums,interface,main in that particular order