C Preprocessor Syntax

Question

Is it possible to do something like this with the C preprocessor? If it's possible, what is the correct syntax? I would expect to see "5" as an answer, but I am getting "7". Thank you

#include <stdio.h>

#define ENABLE_FEATURE_1   true
#define ENABLE_FEATURE_2   false
#define ENABLE_FEATURE_3   true

#if (ENABLE_FEATURE_1 == true)
    #define FT_BIT_0    1
#else
    #define FT_BIT_0    0
#endif

#if (ENABLE_FEATURE_2 == true)
    #define FT_BIT_1    2
#else
    #define FT_BIT_1    0
#endif

#if (ENABLE_FEATURE_3 == true)
    #define FT_BIT_2    4
#else
    #define FT_BIT_2    0
#endif

#define ENABLED_FEATURES (FT_BIT_0 + FT_BIT_1 + FT_BIT_2)

int main() {
  printf("Enabled Features: %i", ENABLED_FEATURES);
  return 0;
}

Petr Skocik · Accepted Answer · 2018-10-04 17:45:13Z

2

Yes. But you need defines for true and false (or at least true), otherwise the preprocessor will treat such unresolvable tokens in preprocessor conditionals as 0 (see 6.10.1p4), which is why you're getting 7 not 5 in your output (both true and false are treated as 0 in the conditionals and since 0 == 0, all BIT macro get set to their non-zero versions).

#includeing <stdbool.h> will provide the defines. They are guaranteed to be (7.18):

#define true 1
#define false 0

edited Oct 4, 2018 at 17:45

answered Oct 4, 2018 at 17:32

Petr Skocik

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