Question

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C preprocessor and concatenation

can anybody explain with example ?

Answer 1

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).