C macro expand in another C macro

Question

I have something like :

#define NBR 42
#define THE_ANS_IS theAnsIsNBR

Currently the second macro is expand as 'theAnsIsNBR' as expected, but i want it to be expand as 'theAnsIs42' i am not sure if it is even possible !?

I think we need a canonical duplicate for the `##` operator, as this is quite a common FAQ. Anyone? — Lundin, May 04 '18 at 13:14

score 2 · Accepted Answer · answered May 04 '18 at 12:50

2

#define Paste(x, y)  x##y
#define Expand(x, y) Paste(x, y)

#define NBR 42
#define THE_ANS_IS Expand(theAnsIs, NBR)

answered May 04 '18 at 12:50

Eric Postpischil

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score 1 · Answer 2 · answered May 04 '18 at 12:51

1

#define _CONCAT(x,y) x ## y
#define CONCAT(x,y) _CONCAT(x,y)

#define NBR 42
#define THE_ANS_IS CONCAT(theAnsIs, NBR)

This works because ## concatenates two tokens. The problem is, they aren't expanded first. But calling another macro on them expands them, therefore you need to nest two function-like macros here.

answered May 04 '18 at 12:51

thx for the explanation bro. – Welgriv May 04 '18 at 13:27

C macro expand in another C macro

2 Answers2