What does an empty # mean in a macro definition?

Question

I have the following macro:

#define F(Args, ...) \
   // macro definition
#
   F(()) // looks like usage?
#undef F

What does the line containing only # mean? Is F(()) a usage of the macro?

Petr Skocik · Accepted Answer · 2019-02-02T10:22:36.127

19

Technically, that's not part of the macro (no continuation line before it). It's a directive that comes after the #define directive.

# on its own line is called a null directive and it does nothing (as good as a comment).

It's no longer practically useful (except as a visual marker) but in prehistoric C, the preprocessor only got invoked if a C source file started with a directive and the null directive placed at the very beginning of a C file was a good way to make sure the file was preprocessed (i.e., that later directives worked) without starting with a concrete directive.

edited Feb 02 '19 at 10:22

answered Feb 02 '19 at 10:10

Petr Skocik

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I suppose it can be used to obfuscade code even further – Stack Danny Feb 02 '19 at 12:01
@StackDanny Yeah, it could be seen as unnecessarily obfuscating. – Petr Skocik Feb 02 '19 at 12:08
2

I knew that preprocessing was not always an automatic part of C compilation, but I didn't know that this was how the preprocessor was engaged. Nice. – John Bollinger Feb 02 '19 at 14:54

What does an empty # mean in a macro definition?

1 Answers1