Re: Interesting preprocessor code behavior
Re: Interesting preprocessor code behavior
- Subject: Re: Interesting preprocessor code behavior
- From: "Clark Cox" <email@hidden>
- Date: Thu, 3 Jul 2008 10:24:54 -0700
On Thu, Jul 3, 2008 at 8:55 AM, Howard Moon <email@hidden> wrote:
> Hi all,
>
> I'm not sure this really belongs in an Xcode list, or if it's more of
> a general gcc (or even C++) item. But I thought you might like to hear
> about this, in case you weren't aware or hadn't seen such a thing before.
>
> Preprocessor comparisons (like #if (A==B)) cannot be relied upon to
> consistently pass OR fail if either symbol is undefined.
If this is true, then this is a bug in GCC. If you have a reproducible
case, please file a bug.
>From the C++ standard (16.1 4):
---
After all replacements due to macro expansion and the _defined_ unary operator
have been performed, all remaining identifiers and keywords, except
for true and false, are
replaced with the pp-number 0, and then each preprocessing token is
converted into a token.
---
Therefore, all identifiers and keywords, that are not macros, are not
used with the 'defined' operator are replaced with zero. My experience
bears this out:
[ccox@clarkco:~]% cat test.cpp
#define ZERO 0
#define ONE 1
#if MY_UNDEFINED_SYMBOL == ONE
#error Shouldnt get here
#endif
#if MY_UNDEFINED_SYMBOL != ZERO
#error Shouldnt get here
#endif
#if MY_UNDEFINED_SYMBOL == ZERO
#warning Success
#endif
#if MY_UNDEFINED_SYMBOL != ONE
#warning Success
#endif
#if MY_UNDEFINED_SYMBOL
#error Shouldnt get here
#else
#warning Success
#endif
#if !MY_UNDEFINED_SYMBOL
#warning Success
#else
#error Shouldnt get here
#endif
int main() { return 0; }
[ccox@clarkco:~]% c++ test.cpp
test.cpp:13:2: warning: #warning Success
test.cpp:17:2: warning: #warning Success
test.cpp:23:2: warning: #warning Success
test.cpp:27:2: warning: #warning Success
--
Clark S. Cox III
email@hidden
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