c token pasting (concatenation) with empty string - c

Question

I'm trying to use macro for c code.
However, I stuck in using token concatenation
I have below variables.
A, aA, bA, cA ...
And, all of these variables used for same function (situation is a bit complicate, so just passing variable is not enough).
If I have only
aA, bA, cA
Then, I can do using the below macro.
#define CALL_FUNCTION(GROUP) \
FUNCTION(GROUP##A);
However, because of
A
I can't use the mentioned macro anymore.
I tried,
#define CALL_FUNCTION(GROUP) \
FUNCTION(GROUP##A);
FUNCTION(NULL);
FUNCTION(a);
FUNCTION(b);
FUNCTION(c);
But actually, NULL is not empty string, it didn't work.
FUNCTION("");
Also didn't work.
There are alternative way like,
#define CALL_FUNCTION(GROUP) \
if(GROUP == NULL)\
FUNCTION(A);\
else\
FUNCTION(GROUP##A);
However, in this case, I need to write few lines more.
In my case, it cause much more codes.
I googled a lot, but I couldn't solution.
Is there anyone who knows how to token paste with empty string?
Thanks

Answer 1

Pass an empty argument and it will effectively concatenate nothing with A, producing just A:
#define CALL_FUNCTION(GROUP) \
FUNCTION(GROUP##A);
CALL_FUNCTION() // expands to FUNCTION(A);
CALL_FUNCTION(a) // expands to FUNCTION(aA);
You can see this work live.
The thing to note is that when you call CALL_FUNCTION(), you do not pass zero arguments, as you do with functions. Instead, you pass a single argument - empty. Similarly, MACRO(,) calls MACRO with two empty arguments. It just so happens that the concatenation behaviour with empty is what you want.

Answer 2

You just need a second macro that takes no parameters:
#define CALL_FUNCTION() \
FUNCTION(A);
#define CALL_FUNCTION(GROUP) \
FUNCTION(GROUP##A);
Example:
#include <stdio.h>
#define FUNCTION(x) printf( # x "\n" )
#define CALL_FUNCTION() FUNCTION(A)
#define CALL_FUNCTION(GROUP) FUNCTION(GROUP##A)
int main(void) {
CALL_FUNCTION();
CALL_FUNCTION(a);
CALL_FUNCTION(b);
CALL_FUNCTION(c);
return 0;
}
Output:
A
aA
bA
cA