Re: Concatenated if and preprocessor

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.