What effect does #define X X have in C?

Question

In the source code of stdbool.h in LLVM project, it reads:

/* Don't define bool, true, and false in C++, except as a GNU extension. */
#ifndef __cplusplus
#define bool _Bool
#define true 1
#define false 0
#elif defined(__GNUC__) && !defined(__STRICT_ANSI__)
/* Define _Bool, bool, false, true as a GNU extension. */
#define _Bool bool
#define bool  bool
#define false false
#define true  true
#endif

In the last 4 lines there are three lines of the from #define X X. Why would you do that? What difference does it make? Wouldn't this force compiler to just replace, say, true with true?

score 19 · Accepted Answer · edited Sep 01 '13 at 11:31

19

The only reason I can think of is, that preprocessor statements like

#ifdef bool
// do some stuff or define bool
#endif

in other c files include afterwards will work proper and not trying to redefine bool in another way like

#define bool int

which would interfere with the first definition

edited Sep 01 '13 at 11:31

Mohsen Nosratinia

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answered Sep 01 '13 at 11:22

Martin Schlott

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alk · Answer 2 · 2013-09-01T13:20:29.060

7

#define X X

has the effect that "the pre-processor conditional"^*:

#ifdef X

~~is "true"~~ "succeeds".^*

^* update

edited Sep 01 '13 at 13:20

answered Sep 01 '13 at 11:18

alk

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Not me, but I would guess saying that a preprocessor directive is "true" got you the down vote. Some people are sticklers for accuracy. – john Sep 01 '13 at 12:20
@john: I was aware of this, that's why I put it in quotes. However it is a boolean expression in the context of pre-processing, isn't it? Do you have an idea for an alternative wording? – alk Sep 01 '13 at 12:25
Ok, I think the edit should fullfill anybodys expectations in accurate wording. – alk Sep 01 '13 at 12:30
And why would anyone chose it instead of just `#define X` ? – vsz Sep 01 '13 at 18:25
3

@vsz in this case it would redefine C++'s bool keyword to expand to nothing. – tab Sep 02 '13 at 04:53

score 2 · Answer 3 · answered Sep 01 '13 at 11:18

2

It would make the difference that true, false etc are now macros. So code like this

#if defined(true)
    ...
#else
    ...
#endif

would be affected.

answered Sep 01 '13 at 11:18

john

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You mean `#ifdef true` – Miles Rout Sep 01 '13 at 12:13
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@MilesRout `#ifdef true` and `#if defined(true)` are the same. – john Sep 01 '13 at 12:18
Well that's opinion, nevertheless I meant what I wrote. – john Sep 01 '13 at 12:22
3

@MilesRout Shorter, yes. Clearer, not necessarily. `#ifdef` is less general. `defined X` can be combined with other expressions. And I personally think `#if !defined X` is less likely to be misread than `#ifndef`. – jamesdlin Sep 01 '13 at 13:22
2

@MilesRout `#if defined` has the advantage over `#ifdef` that it allows you to add `#elif` cases – ouah Sep 01 '13 at 14:43

What effect does #define X X have in C?

3 Answers3