What is the difference (if any) between the two following preprocessor control statements.
#if
and
#ifdef
+12
A:
#ifdef FOO
is a shortcut for:
#if defined(FOO)
#if can also be used for other tests or for more complex preprocessor conditions.
#if defined(FOO) || defined(BAR)
ereOn
2010-09-27 10:35:21
or my favorite #if 0 instead of commenting multiple lines.
Anders K.
2010-09-27 10:37:42
`if (0)` allows the lines to be compiled in Debug mode, so that you can jump in the block when debugging...
pascal
2010-09-27 10:40:42
@pascal: that sounds highly compiler/env specific. C++ doesn't even define a debug mode (though NDEBUG has meaning in the context of assert). ;-P
Tony
2010-09-27 10:46:09
@Tony, pascal: I think Pascal is not talking about #if (0) but if (0). The code won't be executed but compiled i.e. instructions are generated. If optimizations are enabled (typically in "release mode") no CPU instructions are generated.
ur
2010-09-27 10:55:04
If I could have accepted your answer too I would have. +1 instead. :-)
Konrad
2010-09-27 13:42:41
@Konrad: No problem ;) Glad I could help.
ereOn
2010-09-27 13:53:09
Can we also write #if null?
fahad
2010-09-27 20:48:55
+24
A:
You can demonstrate the difference by doing:
#define FOO 0
#if FOO
// won't compile this
#endif
#ifdef FOO
// will compile this
#endif
#if checks for the value of the symbol, while #ifdef checks the existence of the symbol (regardless of its value).
Greg Hewgill
2010-09-27 10:50:33
+1 for explicitly pointing out that defining something to zero will disable #if, but not #ifdef.
DevSolar
2010-09-27 10:52:31
@fahad: `#if` checks the *value* of the symbol to see whether it is non-zero (true) or zero (false). Then the block is compiled if the result is true.
Greg Hewgill
2010-09-27 21:10:22