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(standards.info)Conditional Compilation

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Conditional Compilation
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   When supporting configuration options already known when building
your program we prefer using `if (... )' over conditional compilation,
as in the former case the compiler is able to perform more extensive
checking of all possible code paths.

   For example, please write

       if (HAS_FOO)
         ...
       else
         ...

   instead of:

       #ifdef HAS_FOO
         ...
       #else
         ...
       #endif

   A modern compiler such as GCC will generate exactly the same code in
both cases, and we have been using similar techniques with good success
in several projects.

   While this is not a silver bullet solving all portability problems,
following this policy would have saved the GCC project alone many person
hours if not days per year.

   In the case of function-like macros like `REVERSIBLE_CC_MODE' in GCC
which cannot be simply used in `if( ...)' statements, there is an easy
workaround.  Simply introduce another macro `HAS_REVERSIBLE_CC_MODE' as
in the following example:

       #ifdef REVERSIBLE_CC_MODE
       #define HAS_REVERSIBLE_CC_MODE 1
       #else
       #define HAS_REVERSIBLE_CC_MODE 0
       #endif