defined(XXXX) macro in C/C++?

Question

I'm going through the BSON source code, and came across something I've never seen before.

#if !defined(BSON_INSIDE) && !defined(BSON_COMPILATION)
#error "Only <bson.h> can be included directly."
#endif

What is the defined(XXXX) macro above? I can guess what it does, but I can't seem to find any documentation about it. Is it specific to some compilers? It gives me a W4 warning on Microsoft Visual C++ (that I'm trying to resolve in my project).

[This preprocessor reference](http://en.cppreference.com/w/cpp/preprocessor) might be helpful. — Some programmer dude, Jan 11 '18 at 05:41
Works like `#ifdef` but allows you to make a multi-predicate condition instead of multiple nested `#ifdef` blocks. — Dave M., Jan 11 '18 at 05:43
@MillieSmith Nope, I put the word 'macro' in! There it is, on the second page when I remove that word > — aaronsnoswell, Jan 11 '18 at 05:55
Hm. It's the first non-featured result for me :S. That's strange. — Millie Smith, Jan 11 '18 at 06:10
For anyone who is interested I've submitted a possible bug to the MongoDB user forum to raise the root issue here. I'll post a link here once it's approved. — aaronsnoswell, Jan 11 '18 at 06:48
Google is working just fine. A search for "c defined" gave this as first hit: https://gcc.gnu.org/onlinedocs/cpp/Defined.html. — Lundin, Jan 11 '18 at 10:31

score 4 · Accepted Answer · answered Jan 11 '18 at 05:41

4

From 6.10.1

The expression that controls conditional inclusion shall be an integer constant expression except that: identifiers (including those lexically identical to keywords) are interpreted as described below;166) and it may contain unary operator expressions of the form
  defined identifier
or
  defined ( identifier )
which evaluate to 1 if the identifier is currently defined as a macro name (that is, if it is predefined or if it has been the subject of a #define preprocessing directive without an intervening #undef directive with the same subject identifier), 0 if it is not.

It is not macro - it is an operator.

answered Jan 11 '18 at 05:41

user2736738

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Wow. Thank you - I had no idea this existed. – aaronsnoswell Jan 11 '18 at 05:54
That's a C reference. Is it also valid for C++? – Jongware Jan 13 '18 at 02:20
1

@aaronsnoswell.: Again we see how tagging leads to these kind of situation. My answer was solely given for C - I am not aware whether the C++11 or C++14 standard alllows it or not...if I find anything useful(absence or presene ) will update. – user2736738 Jan 13 '18 at 02:55

defined(XXXX) macro in C/C++?

1 Answers1