difference between "ifndef" and "if !defined" in C?

c include-guards
onemach picture onemach · Dec 23, 2011 · Viewed 23.3k times · Source

I have seen #ifndef ABC and #if !defined (ABC) in the same C source file.

Is there subtle difference between them? (If it is a matter of style, why would someone use them in the same file)

Answer

Dan Fego picture Dan Fego · Dec 23, 2011

No, there's no difference between the two when used that way. The latter form (using defined()) is useful when the initial #if or one of the subsequent #elif conditions needs a more complex test. #ifdef will still work, but it might be clearer using #if defined() in that case. For example, if it needs to test if more than one macro is defined, or if it equals a specific value.

The variance (using both in a file) could depend on specific subtleties in usage, as mentioned above, or just poor practice, by being inconsistent.

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