How can I include a file or another, based on the value of a defined string?
This doesn't work:
#define VAR VALUE_A
#if VAR == "VALUE_A"
#include "a.h"
#elif VAR == "VALUE_B"
#include "b.h"
#endif
If it's important, I'm not actually defining VAR, I'm passing it down from the command-line via gcc -D NAME=VALUE.
The == operator does not compare strings. But you have a couple of other options to configure your includes. In addition to the solutions already mentioned in other answers, I like this one because I think it is quite self-explanatory.
/* Constant identifying the "alpha" library. */
#define LIBRARY_ALPHA 1
/* Constant identifying the "beta" library. */
#define LIBRARY_BETA 2
/* Provide a default library if the user does not select one. */
#ifndef LIBRARY_TO_USE
#define LIBRARY_TO_USE LIBRARY_ALPHA
#endif
/* Include the selected library while handling errors properly. */
#if LIBRARY_TO_USE == LIBRARY_ALPHA
#include <alpha.h>
#elif LIBRARY_TO_USE == LIBRARY_BETA
#define BETA_USE_OPEN_MP 0 /* You can do more stuff than simply include a header if needed. */
#include <beta.h>
#else
#error "Invalid choice for LIBRARY_TO_USE (select LIBRARY_ALPHA or LIBRARY_BETA)"
#endif
Your users can now compile with:
$ cc -DLIBRARY_TO_USE=LIBRARY_BETA whatever.c
You can use #ifdef or #ifndef for conditional includes.
#ifdef VALUE_A
#include "a.h"
#endif
#ifdef VALUE_B
#include "b.h"
#endif
The closest possilibility I can think of is to utilize third form of #include directive (C11 §6.10.2/4), namely define VAR with value, that holds actual header filename:
#define VAR "a.h"
then just use the following:
#include VAR
