Macro for creating variables in C

Question

I want to create several variables of the form:

static char fooObjectKey;
static char bazObjectKey;
static char wthObjectKey;
static char myObjectObjectKey;
...

So I wrote

#define defineVar(x) static char #x ObjectKey

defineVar(foo);
defineVar(baz);
defineVar(wth);
defineVar(myObject);

but I get the error: Expected identifier or }

What am I doing wrong here? :) Any help is appreciated

Most compilers can show you the result of the pre-processor. This is invaluable when trying to fully understand complex macros. — Ohio Enthusiast
– Ohio Enthusiast, Commented Dec 26, 2012 at 10:22
I am using LLVM in Xcode, Do you know how can I see the result? :) — nacho4d
– nacho4d, Commented Dec 26, 2012 at 10:28
@nacho4d with gcc is gcc -E source.c I think that with clang it's the same since clang offers a gcc-compatible driver so clang -E source.c. — user1797612
– user1797612, Commented Dec 26, 2012 at 10:35
Found how to do it in Xcode! stackoverflow.com/questions/5937031/xcode-4-preprocessor-output See @Steven Hepting's answer — nacho4d
– nacho4d, Commented Dec 26, 2012 at 10:47

MByD · Accepted Answer · 2012-12-26 11:36:12Z

7

You need to concatenate them:

#define defineVar(x) static char x##ObjectKey

Explanation:

The preprocessor operator ## provides a way to concatenate actual arguments during macro expansion. If a parameter in the replacement text is adjacent to a ##, the parameter is replaced by the actual argument, the ## and surrounding white space are removed, and the result is re-scanned. For example, the macro paste concatenates its two arguments:

#define paste(front, back) front ## back

so paste(name, 1) creates the token name1.

edited Dec 26, 2012 at 11:36

answered Dec 26, 2012 at 10:20

MByD

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Comments

Adeel Ahmed · Accepted Answer · 2012-12-26 10:26:36Z

3

# in macro is used to stringify argument, ## is used for concatenation in macro... in your case, following is the correct syntax..

#define defineVar(arg) static char arg##ObjectKey

if you use this,

#define defineVar(x) static char #x ObjectKey

variable declaration become...

static char "foo" ObjectKey;

answered Dec 26, 2012 at 10:26

Adeel Ahmed

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Comments

milleniumbug · Accepted Answer · 2012-12-26 10:23:36Z

1

Use double hash for concatenation

#define defineVar(x) static char x##ObjectKey

answered Dec 26, 2012 at 10:23

milleniumbug

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Comments

Gaurav · Accepted Answer · 2012-12-26 10:27:57Z

0

The ## operator concatenates two tokens into one token
Hence 
defineVar(foo) will be replace with static char fooObjectKey

answered Dec 26, 2012 at 10:27

Gaurav

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