Macros inside of macros

Hey, I am trying to #define something inside of one of my macros. Here is an example of my code:

Code:

---

int unused;

#define REGISTER(a) a ## _REGISTER

#define REGISTER_OUT(a)          \
  #ifndef (REGISTER(a))          \
      #define REGISTER(a) unused  \
  #endif // REGISTER_OUT

REGISTER_OUT(A1)

---

So I want to check if A1_REGISTER is defined, and if it is not, define it as the int unused. How can I do this, as the above code does not work.

Unfortunately -- you can't do that, per se. That is, you can't use preprocessor directives like #define inside macros. I've often wanted to, but you can't. :)

However, you might be able to solve your problem, with something like this.

Code:

---

#ifndef A1_REGISTER
  #define REGISTER(a) unused
#else
  #define REGISTER(a) a ## _REGISTER
#endif // A1_REGISTER

---

Wait some people at the http://www.ioccc.org have used shamelesly defines within macros. Maybe you should check out their source even though it's a bit obfuscated...