Can #if pre-processor directives be nested in C++?

Question

I have a question about Pre-processor directives in c++:

For example:

#ifndef QUESTION

//some code here

#ifndef QUESTION

//some code here

#endif

#endif

Can we use it in this way, and can the C++ compiler match the ifndef and endif in the right way?

You can nest conditionals, and you can call one macro from another but you can't nest macros directly, e.g. `#define AAA #define XXX` - it won't work correctly. — SF., Jul 13 '11 at 12:46
Since [a C question](http://stackoverflow.com/q/38963852/827263) was just closed as a duplicate of this one, I'll point out that rules for the C and C++ preprocessor are the same, at least as far as this question is concerned. — Keith Thompson, Aug 15 '16 at 22:44

score 132 · Accepted Answer · edited Aug 13 '15 at 11:04

132

Yes, we can. The #endif statement matches to the previous #if #ifdef or #ifndef etc for which there hasn't been a corresponding #endif.

e.g.

#if  ----------|
#if  -----|    |
#endif ---|    |
#endif --------|

edited Aug 13 '15 at 11:04

Zachi Shtain

answered Jul 13 '11 at 11:54

Armen Tsirunyan

we have two #endif here, so the first #endif will match the second #ifndef? – Jul 13 '11 at 11:55
It won't match the previous one; that would mean we *couldn't* nest them. – Fred Foo Jul 13 '11 at 11:56

score 64 · Answer 2 · answered Jul 13 '11 at 11:55

64

Yes, you can nest #if/#endif blocks. Some C coding styles would tell you to write

#ifdef CONDITION1
# ifdef CONDITION2
# endif
#endif

using spaces to denote the level of nesting.

answered Jul 13 '11 at 11:55

Fred Foo

7

+1 for the nesting bit, it helps a lot with clarity sometimes. – André Caron Jul 13 '11 at 11:59
Why would you insert space? why not just indent the `#ifdef` – Jahid Sep 14 '16 at 14:52
3

I didn't even know spaces between `#` and preprocessor directives were allowed. Big thanks for that tip! – AlexR Nov 23 '17 at 15:49

score 0 · Answer 3 · answered Jul 13 '11 at 11:59

0

In your code, the #ifndef QUESTION section will be discarded unless you #undef QUESTION.

Good luck!

answered Jul 13 '11 at 11:59

bert-jan

Sorry, misread the code sample! I thought it started with #ifdef QUESTION – bert-jan Jul 13 '11 at 12:00

3 Answers3