Macros inside of macros

This is a discussion on Macros inside of macros within the C Programming forums, part of the General Programming Boards category; Hey, I am trying to #define something inside of one of my macros. Here is an example of my code: ...

  1. #1
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    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.

  2. #2
    Frequently Quite Prolix dwks's Avatar
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    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
    dwk

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  3. #3
    uint64_t...think positive xuftugulus's Avatar
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    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...
    Code:
    ...
        goto johny_walker_red_label;
    johny_walker_blue_label: exit(-149$);
    johny_walker_red_label : exit( -22$);
    A typical example of ...cheap programming practices.

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