Combining two #defined characters in a C ++ preprocessor

Question

I want to make:

#define VERSION XY123 #define PRODUCT MyApplication_VERSION

so PRODUCT is actually MyApplication_XY123. I tried to play with the merge operator ##, but with limited success ...

 #define VERSION XY123 #define PRODUCT MyApplication_##VERSION

=> MyApplication_VERSION

 #define VERSION XY123 #define PRODUCT MyApplication_##(VERSION)

=> MyApplication_ (XY123) - close, but not quite

I want this to be possible?

+8

c ++ c-preprocessor

Mr. Boy May 16, '13 at 15:30

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3 answers

The ## operator is valid until accepting a placeholder argument. The classic solution is to use a helper:

 #define CONCAT2(a, b) a ## b #define CONCAT(a, b) CONCAT2(a, b) CONCAT(MyApplication_, VERSION)

+3

James kanze May 16, '13 at 15:37

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All problems in computer science can be solved by an additional level of indirection:

 #define JOIN_(X,Y) X##Y #define JOIN(X,Y) JOIN_(X,Y) #define VERSION XY123 #define PRODUCT JOIN(MyApplication_,VERSION)

+1

Jonathan wakely May 16, '13 at 15:37

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brian beuning · Accepted Answer · 2013-05-16T15:37:40+0000

Label insertion works with arguments for macros. So try this

 #define VERSION XY123 #define PASTE(x) MyApplication_ ## x #define PRODUCT PASTE(VERSION)