How to have a macro expand into the arg name of another macro

Question

for example:

#define Y (b * 2)
#define X(b) Y

int main()
{
    printf("%d", X(4)); // Want it to print 8, but doesn't work
}

Is something like this possible with C macros?

Answer 1

As mentioned in comments, the macro parameter name has local scope to the specific function-like macro, so you can't access it from outside the macro.

Something similar is possible with the design pattern known as "X macros" though. It is based on providing a list (or a single item) of data in a macro, than have that list macro take a different macro as parameter, in order to apply something to the list of data.

Example:

#include <stdio.h>

// here X will be an external function-like macro with 1 parameter:
#define LIST(X) \
  X(4)          \
  X(8)

// some macros with 1 parameter that should print and possibly also multiplicate
#define PRINTx1(a) printf("%d ", (a));  // note the semicolon here
#define PRINTx2(a) printf("%d ", (a)*2);

int main (void)
{
  LIST(PRINTx1)  // note the lack of semicolon here
  puts("");
  LIST(PRINTx2)
}

Output:

4 8 
8 16 

---

All of the above will expand like this:

PRINTx1(4) PRINTx1(8)
puts("");
PRINTx2(4) PRINTx2(8)

->

printf("%d ", (4)); printf("%d ", (8));
puts("");
printf("%d ", (4)*2); printf("%d ", (8)*2);