Question

Getting a LNK2005 “already defined in GUI.obj” for a function pointer in PAL.h

//GUI.cpp
#include "PAL.h"

//PAL.h
#define PAL_INCLUDE
int (*addPAL)( int, void(*)(), void(*)() );

//main.cpp
#include "GUI.h"
#ifndef PAL_INCLUDE
#include "PAL.h"
#endif

Have I misunderstood the nature of includes and #ifndef?

Answer 1

You’re getting the error probably because the preprocessor is processing main.cpp before GUI.cpp.
Try changing the content of your PAL.h to this:

 #ifndef PAL_INCLUDED
 #define PAL_INCLUDED
 // definitions
 #endif

Reference

Include guard