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From: Tim Rentsch <tr.17687@z991.linuxsc.com>
Newsgroups: comp.lang.c
Subject: Re: Concatenated if and preprocessor
Date: Fri, 14 Mar 2025 09:44:16 -0700
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pozz <pozzugno@gmail.com> writes:

> Il 13/03/2025 16:44, pozz ha scritto:
>
>> Consider this code:
>>
>> if (cond1) {
>> ...
>> } else if (cond2) {
>> ...
>> } else if (cond3) {
>> ...
>> }
>>
>> I want to activate every single if with a macro preprocessor.  All
>> the combinations are possible:  only the first, only the second, only
>> the third, the first and second... and so on.
>>
>> What's the best method to have a clean code that is always compiled
>> without errors?
>
> You're right, a real example is better to define my problem.
>
> I have a project that can be compiled in different ways, depending on
> the final product model (it's an embedded platform, it's a material
> device).
> I have three models with different characteristics:  MODEL_A, MODEL_B,
> MODEL_C.  Three different builds that are managed at compile time by
> defining MODEL macro as MODEL_A, MODEL_B or MODEL_C.
>
> For each model, I could have a different features set.  FEATURE_1,
> FUTURE_2, FUTURE_3, ...
>
> #if MODEL == MODEL_A
> #  define FEATURE_1   1
> #  define FEATURE_2   0
> #  define FEATURE_3   0
> #elif MODEL == MODEL_B
> #  define FEATURE_1   0
> #  define FEATURE_2   1
> #  define FEATURE_3   0
> #elif MODEL == MODEL_C
> #  define FEATURE_1   1
> #  define FEATURE_2   1
> #  define FEATURE_3   1
> #endif
>
> Now, on the full-featured model (MODEL_C) I could write:
>
> if (key == 1) {
>   ...
> } else if (key == 2) {
>   ...
> } else if (key == 3) {
>   ...
> } else {
>   ...
> }
>
> However, for MODEL_A I should have:
>
> if (key == 1) {
>   ...
> } else {
>   ...
> }
>
> For MODEL_B I should have:
>
> if (key == 2) {
>   ...
> } else {
>   ...
> }
>
> This is the scenario.
>
> I thought using if(0) or if(1), but many times I receive some
> warnings from the compiler (condition is always true or condition
> is always false).

So your question is not a language question but how to avoid
implemenation-specific warnings?  It would have been good to
explain that at the beginning.

My first suggestion is to just turn off the damn warning.  Any
compiler that gives a "condition always false" warning on 'if(0)'
is one best avoided.

Having said that, here is an alternative:

  for(  int just_once = 1;  just_once;  just_once = 0  ){
    #if FEATURE_1
      if(  condition_1  ){
        ...
        break;
      }
    #endif

    #if FEATURE_2
      if(  condition_2  ){
        ...
        break;
      }
    #endif

    #if FEATURE_3
      if(  condition_3  ){
        ...
        break;
      }
    #endif

      ... default code goes here ...

  }

Test compiles at optimization level -O1 shows gcc and clang are both
smart enough to optimize the for(...) out of existence, leaving
just the enabled if() sections.