Can the C preprocessor be used to tell if a file exists?

Question

I have a very large codebase (read: thousands of modules) that has code shared across numerous projects that all run on different operating systems with different C++ compil

Answer 1

So far as I know cpp does not have a directive regarding the existence of a file.

You might be able to accomplish this with a bit of help from the Makefile, if you're using the same make across platforms. You can detect the presence of a file in the Makefile:

foo.o: foo.c
    if [ -f header1.h ]; then CFLAGS+=-DHEADER1_INC

As @Greg Hewgill mentions, you can then make your #includes be conditional:

#ifdef HEADER1_INC
#include <header1.h>
#endif

Answer 2

I had to do something similar for the Symbian OS. This is how i did it: lets say you want to check if the file "file_strange.h" exists and you want to include some headers or link to some libraries depending on the existance of that file.

first creat a small batch file for checking the existence of that file.

autoconf is good but an over kill for many small projects.

----------check.bat

@echo off

IF EXIST [\epoc32\include\domain\middleware\file_strange] GOTO NEW_API
GOTO OLD_API
GOTO :EOF

:NEW_API
echo.#define NEW_API_SUPPORTED>../inc/file_strange_supported.h
GOTO :EOF

:OLD_API
echo.#define OLD_API_SUPPORTED>../inc/file_strange_supported.h
GOTO :EOF

----------check.bat ends

then i created a gnumake file

----------checkmedialist.mk

do_nothing :
    @rem do_nothing

MAKMAKE : 
        check.bat

BLD : do_nothing

CLEAN : do_nothing

LIB : do_nothing

CLEANLIB : do_nothing

RESOURCE : do_nothing

FREEZE : do_nothing

SAVESPACE : do_nothing

RELEASABLES : do_nothing

FINAL : do_nothing

----------check.mk ends

include the check.mk file in your bld.inf file, it MUST be before your MMP files

PRJ_MMPFILES
gnumakefile checkmedialist.mk

now at compile time the file file_strange_supported.h will have an appropriate flag set. you can use this flag in your cpp files or even in the mmp file for example in mmp

#include "../inc/file_strange_supported.h"
#ifdef NEW_API_SUPPORTED
LIBRARY newapi.lib
#else
LIBRARY oldapi.lib
#endif

and in .cpp

#include "../inc/file_strange_supported.h"
#ifdef NEW_API_SUPPORTED
CStrangeApi* api = Api::NewLC();
#else
// ..
#endif

Answer 3

Little Update

Some compilers might support __has_include ( header-name ).

The extension was added to the C++17 standard (P0061R1).

Compiler Support

• Clang
• GCC from 5.X
• Visual Studio from VS2015 Update 2 (?)

Example (from clang website):

// Note the two possible file name string formats.
#if __has_include("myinclude.h") && __has_include(<stdint.h>)
# include "myinclude.h"
#endif

Sources

• SD-6: SG10 Feature Test Recommendations
• Clang Language Extensions