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C preprocessor and concatenation
can anybody explain with example ?
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1Duplicate of stackoverflow.com/questions/1489932/…codaddict– codaddict2010-09-03 12:29:39 +00:00Commented Sep 3, 2010 at 12:29
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2 Answers
It allows to construct indentifiers from their parts. Eg:
#define CLASS_NAME(name) CLASS__ ## name
would expand CLASS_NAME(alpha) to CLASS__alpha. It is vastly used in tricks used by the boost preprocessor library, eg.
#define IF0(a, b) b
#define IF1(a, b) a
#define IF(cond, a, b) IF ## cond(a, b)
which would expand IF(0, a, b) to a and IF(1, a, b) to b. Also, sometimes, it is used to generate struct and function names (akin to c++ templates).
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user13267
Isn't IF(0, a, b) mapped to b and IF(1, a, b) mapped to a?
I'm not sure what you meen by "##" preprocessor.
C has a preprocessor for expanding macros before compile time. This is a first pass on the source code. There are a few different things it is used for:
- including other source / header files (
#include) - conditionally compiling code (
#ifdefetc.) - expanding macros (
#define)- handy for constants
- simple functions
Note, though, that this is not really "C" (though part of the spec) and can cause headaches if you get it wrong. I believe new languages would not do it this way anymore.
1 Comment
Hemanth
Hi Daren jpalecek's answer is what i was expecting. Thanks for the heads up.