Hi Gašper,

 

perhaps my answer was too fast.

 

I was trying to go a third way:

Neither content nor local file identity, but using the path given in the #include files.

 

Difficulties arise from the facts

 - that libraries have several entry points for their includes, possibly in subdirectories.

 - that -I include directories may point to one or several libraries

 - that some libraries would need several -I parameters, e.g. for sub-components or plugins

 

Perhaps a working solution would be to create a #once only working for #include "".

After including a library with a search path with <>, all subsequent includes with "" are seen relative to the current file.

 

(Some compilers provide more complicated options to also extend a search path for "" or do so with the paths of all previously included files, but those extended lookup paths are not standard.)

 

With those files at least it should be possible to rely solely on the paths provided in the #include "" directives.

 

 

What to do with

 

main.cpp

#include <mylib/librarydefs.h>

#include <mylib/constants.h>

librarydefs.h:

#include "constants.h"

 

librarytemplates.h:

#include "constants.h"

 

constants.h:
#once

 

 

We would have to know that both includes in main point to the same library.

 

That could get more difficult with

 

-I /includes/alllibs -I /specialincludes/mylibplugins

 

On the other hand, #include "" probably would not work now either between those subcomponents of the library.

 

So we could limit #once to work only within one -I path (or of course in the cpp file, as long as #include "" is used locally)

 

 

 

Not sure, if it is worth it, as it is only a partly solution (only #include "") and is different from all the existing #pragma once implementations. And perhaps I have not seen some of the possible pitfalls.