Thread: Macros inside of macros

  1. #1
    Registered User
    Join Date
    Feb 2008

    Macros inside of macros

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

    int unused;
    #define REGISTER(a) a ## _REGISTER
    #define REGISTER_OUT(a)           \
       #ifndef (REGISTER(a))          \
          #define REGISTER(a) unused  \
       #endif // REGISTER_OUT
    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.

  2. #2
    Frequently Quite Prolix dwks's Avatar
    Join Date
    Apr 2005
    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.
    #ifndef A1_REGISTER
       #define REGISTER(a) unused
       #define REGISTER(a) a ## _REGISTER
    #endif // A1_REGISTER

    Seek and ye shall find. quaere et invenies.

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  3. #3
    uint64_t...think positive xuftugulus's Avatar
    Join Date
    Feb 2008
    Wait some people at the have used shamelesly defines within macros. Maybe you should check out their source even though it's a bit obfuscated...
        goto johny_walker_red_label;
    johny_walker_blue_label: exit(-149$);
    johny_walker_red_label : exit( -22$);
    A typical example of programming practices.

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