How to trick Preprocessor

Question

I have some macros in code

#define NUM_IN 3

#define NUM_1 1
#define NUM_2 2
#define NUM_3 3

#define MY(NUMBER) NUM_##NUMBER

so if I'm calling the macro

MY(NUM_IN)

I'm expecting after preprocessing result as 3 but I'm getting NUM_NUM_IN

So, what I should do so preprocessor will take input as

step 1

MY(NUM_IN)

step 2

MY(3)

step 3

NUM_3

step 4

Please let me know what I'm missing. I am new to stackoverflow so please guide me to the proper channel if I am wrong.
Thanks

score 2 · Answer 1 · answered Aug 24 '17 at 09:58

2

Add one layer of indirection:

#define CAT_(a, b) a ## b     /* Pastes  */
#define CAT(a, b)  CAT_(a, b) /* Expands */

#define MY(NUMBER) CAT(NUM_, NUMBER)

answered Aug 24 '17 at 09:58

Quentin

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This is two layer of indirection. – haccks Aug 24 '17 at 10:26
@haccks it is. And I'm so used to this method that I can't remember why there are two... – Quentin Aug 24 '17 at 11:06

score 2 · Answer 2 · answered Aug 24 '17 at 09:59

2

Add a layer of indirection:

#define NUM_IN 3

#define NUM_1 1
#define NUM_2 
#define NUM_3 3

#define CAT_(X,Y) X##Y
#define MY(NUMBER) CAT_(NUM_,NUMBER)

MY(NUM_IN)

answered Aug 24 '17 at 09:59

Petr Skocik

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haccks · Accepted Answer · 2017-08-24T10:20:21.050

1

Macro arguments are completely macro-expanded before they are substituted into a macro body, unless they are stringized or pasted with other tokens.¹

So, in MY(NUM_IN), NUM_IN will not expanded. You need to define an another macro for concatenation

#define PASTER(n) NUM_ ## n
#define MY(NUMBER) PASTER(NUMBER)

_{1. GCC docs: 3.5 Concatenation}

edited Aug 24 '17 at 10:20

answered Aug 24 '17 at 09:53

haccks

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Thanks for the explanation ... Your four line made every thing clear to me clear. – Bebhav Soni Aug 25 '17 at 10:31

How to trick Preprocessor

3 Answers3