Thread: #define and #undef

  1. #1
    Registered User
    Join Date
    Feb 2003

    #define and #undef

    I have following code in .h file

    #define ABC 1

    #define PQR 1

    #if (ABC == 1)
    #define PQR 0

    Is it ok to say so or do I need to say undef PQR?

    What is difference between two?

    Apart from this, Any link for good pre-processor tutorails is appreciated...


  2. #2
    ATH0 quzah's Avatar
    Join Date
    Oct 2001
    You never need to #undef. You can, but redefinition is allowed (as far as macros are concerned anyway).
    #define FOO 1
    #define FOO 0
    Perfectly legal.

    Hope is the first step on the road to disappointment.

  3. #3
    and the hat of int overfl Salem's Avatar
    Join Date
    Aug 2001
    The edge of the known universe
    Well if you don't use #undef, you'll probably get "macro redeclared" errors.

    Like goto's, you can create a huge mess very quickly if you keep definining and undefining things, so it's use needs to be pretty exceptional.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.

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