C Preprocessor concatenation with variable

Question

Possible Duplicate:
C preprocessor and concatenation

Is it possible to concatenate a C preprocessor with a variable name?

#define  WIDTH 32

int dataWIDTH;


// dataWIDTH should be interpreted as 'data32'

printf("%d",dataWIDTH);

Solution 1

Your use case requires a double-unescaping; using the token pasting (##) operator by itself will just append the name of the preprocessor directive.

#define WIDTH 32

#define _MAKEDATA(n) data##n
#define MAKEDATA(n) _MAKEDATA(n)

int MAKEDATA(WIDTH) = 7;
int _MAKEDATA(WIDTH) = 8;

int main(int argc, char *argv[]) {
    printf("%d\n", data32);
    return 0;
}

yields

$ gcc -E foo.c 
int data32 = 7;
int dataWIDTH = 8;

int main(int argc, char *argv[]) {
    printf("%d\n", data32);
    return 0;
}

Solution 2

There's a token pasting operator called ##, read about it for example here:

http://msdn.microsoft.com/en-us/library/09dwwt6y(v=vs.80).aspx

Comments

[user295691]

Also, see stackoverflow.com/questions/1489932/… for an excellent discussion of the double-pasting "trick"

[user295691]

The preprocessor expands this to printf("%d", data##32), which yields a compiler error. My understanding is that token pasting only works inside of macros.

[Per Ersson]

That example was added by another user, the provided link shows how to use token-pasting from within a macro.