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Editorial Team
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Asked: 2026-06-18T16:19:53+00:00

Is there a preprocessor directive that checks whether a constant is not defined. I

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Is there a preprocessor directive that checks whether a constant is not defined. I am aware of the #ifndef directive but I am also looking for a #elif not defined directive. Does #elif not defined exist?

This is how I would use it:

#define REGISTER_CUSTOM_CALLBACK_FUNCTION(callbackFunctName) \
    #ifndef CUSTOM_CALLBACK_1 \
        #define CUSTOM_CALLBACK_1 \
        FORWARD_DECLARE_CALLBACK_FUNCTION(callbackFunctName) \
    #elif not defined CUSTOM_CALLBACK_2 \
        #define CUSTOM_CALLBACK_2  \
        FORWARD_DECLARE_CALLBACK_FUNCTION(callbackFunctName) \
    #elif not not defined CUSTOM_CALLBACK_3 \
        #define CUSTOM_CALLBACK_3  \
        FORWARD_DECLARE_CALLBACK_FUNCTION(callbackFunctName) \
    #endif
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1 Answer

  1. Editorial Team

    How about the

    #elif !defined(...)

    But you’ve got bigger problems – the trailing \ exclude the other directives – or rather make them illegal. So, even with the valid syntax, your definitions won’t do what you want.

    You’ll need to move the initial define inside the conditions.

    #ifndef CUSTOM_CALLBACK_1
    #define CUSTOM_CALLBACK_1 
    #define REGISTER_CUSTOM_CALLBACK_FUNCTION(callbackFunctName) \
    FORWARD_DECLARE_CALLBACK_FUNCTION(callbackFunctName) 
#elif !defined(CUSTOM_CALLBACK_2)
    //.....
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