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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);
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2 Answers
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;
}
1 Comment
user295691
Also, see stackoverflow.com/questions/1489932/… for an excellent discussion of the double-pasting "trick"
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
2 Comments
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.