How to Enforce Exactly One Definition?

Question

Given a C project that needs to support multiple environments, how do I use the preprocessor to enforce that exactly one environment is defined?

I can already do

    #if defined PROJA
    (blah blah blah)
    #elif defined PROJB
    (etc)
    #else
    #error "No project defined"
    #endif

All that does, though, is tell me if 0 projects are defined. If some helpful soul defines both project A and project B, the preprocessor will assume only project A. However, the correct behavior, from my perspective, is to flag an error.

Granted, with only 2 projects defined this problem is trivial. How do I solve it with 200?

Answer 1

Maybe have different files

include_proja.h
include_projc.h

and then use your Makefile or whatever to include the right file. You can then generate 200 distinct files with the code and include the right ones at compile-time.

This kind of thing is what build systems are for. If you are doing something weird like this with macros ... find a better way outside the source code.

Each file can do (Excuse the verbosity here)

#define A_PROJECT_INCLUDE_WAS_INCLUDED

And then do

#ifndef A_PROJECT_INCLUDE_WAS_INCLUDED
    #error "No project include"
#endif

But some missing symbols will break it anyway in all likelihood.

Good Luck

Answer 2

something like this:

#if defined PROJA
  #ifdef HAVE_PROJ
    #error 
  #endif

  #define HAVE_PROJ
#endif

#if defined PROJB
  #ifdef HAVE_PROJ
    #error 
  #endif

  #define HAVE_PROJ
#endif

#ifndef HAVE_PROJ
  #error No project selected (you need to define PROJA, PROJB, or ...)
#endif

Answer 3

The problem with nested ifs is that you'll end up with n^2 different tests for n projects.

You just need some expression that will give a compile-time error in this case. Perhaps:

#ifdef PROJA
#define PROJA-TEST "
#else
#define PROJA-TEST ""
#endif

and so on for B, C, etc.

Then:

const char *test_only_one_project = PROJA-TEST PROJB-TEST PROJC-TEST " "More than one project is defined!";

EDIT: ... of course this only tests that an odd number of projects are defined. But this should work:

#ifdef PROJA
#define PROJA-TEST (
#endif

and so on, then

const char *test_only_one_project = PROJA-TEST PROJB-TEST PROJC-TEST "More than one project is defined!" );

Answer 4

perhaps you can do something like:

#if PROJ==A
(blah blah blah)
#elif PROJ==B
(etc)
#else
#error "No project defined"
#endif

Answer 5

Try this

#define ENV_UNKNOWN 0
#define ENV_MACOSX  1
#define ENV_LINUX   2
#define ENV_WIN32   3
/* and so on */


#ifndef ENVIRONMENT
/* no environment given, default to something (perhaps) */
#define ENVIRONMENT ENV_UNKNOWN
#endif

/* and now the environment specific parts */
#if (ENVIRONMENT == ENV_MACOSX)
#include "macosx_port.h"
#endif

#if (ENVIRONMENT == ENV_LINUX)
#include "linux_port.h"
#endif

#if (ENVIRONMENT == ENV_WIN32)
#include "win32_port.h"
#endif

#if (ENVIRONMENT == ENV_UNKNOWN)
#error You have to specify the ENVIRONMENT.
#endif

Now you can specify the environment you want to compile for on the command line, like this:

cc -DENVIRONMENT=2 ...

Another way is to include/link different modules from your build system depending on the environment you're compiling for.

Answer 6

#if defined PROJA
bool one_project_defined = true;
#endif

#if defined PROJB
bool one_project_defined = true;
#endif

#if defined PROJC
bool one_project_defined = true;
#endif

#if defined PROJD
bool one_project_defined = true;
#endif

one_project_defined; // Won't compile in wrong builds