Store Preprocessor Constants Value then 'overwrite' through undef/define

Question

I am trying to store a Preprocessor Constant's value then 'overwrite' it.

My Problem: The code below attempts to store a preprocessor constant's value in variable 'A' then the code undefines that variable then redefines it so it has a new value. The problem is that variable 'A' has the newly defined value and not the old one, if that makes sense. Can I store a Preprocessor Constants value and not its reference(Which is what seems like is happening)?

#ifdef CUSTOM_EVENT_CALLBACK_DEFINED
  #define SUB_CUSTOM_EVENT_CALLBACK_DEFINED CUSTOM_EVENT_CALLBACK_DEFINED
  #undef CUSTOM_EVENT_CALLBACK_DEFINED
#endif
#define CUSTOM_EVENT_CALLBACK_DEFINED "def"

int main()
{
    printf(CUSTOM_EVENT_CALLBACK_DEFINED);     // prints out "def". 
    printf("\n");
    printf(SUB_CUSTOM_EVENT_CALLBACK_DEFINED); // prints out "def". I was hoping this would be "abc"
    printf("\n");

    system("PAUSE");
    return 0;
}

// Usage: where the function def is a callback function for a window in a civil engineering program that runs ontop of windows
int def(int id, int cmd, int type)
{
    #ifdef SUB_CUSTOM_EVENT_CALLBACK_DEFINED
      SUB_CUSTOM_EVENT_CALLBACK_DEFINED(id, cmd, type);
    #endif

    // perform my custom code here
}

macroses in C, C++ are used by preprocessor which acts like text editor, it generates text which later parsed by compiler. Therefore macroses can`t be used in run time which you expect from them. I think the verb "to store" is not applicable to macroses, because no memory in run time && compile time associated with them. — spin_eight
– spin_eight, Commented Nov 20, 2012 at 6:52

Luchian Grigore · Accepted Answer · 2012-11-20 05:05:57Z

2

Short answer - no, it's not possible. Macros don't work like that.

But I doubt you really need to do this. A workaround, for example, would be to store the value in a variable before you overwrite it:

#ifdef CUSTOM_EVENT_CALLBACK_DEFINED
  std::string SUB_CUSTOM_EVENT_CALLBACK_DEFINED = CUSTOM_EVENT_CALLBACK_DEFINED;
  #undef CUSTOM_EVENT_CALLBACK_DEFINED
#else
  std::string SUB_CUSTOM_EVENT_CALLBACK_DEFINED = "";
#endif

#define CUSTOM_EVENT_CALLBACK_DEFINED "def"

int main()
{
    printf(CUSTOM_EVENT_CALLBACK_DEFINED);     // prints out "def". 
    printf("\n");
    printf(SUB_CUSTOM_EVENT_CALLBACK_DEFINED); // prints out "def". I was hoping this would be "abc"
    printf("\n");

    system("PAUSE");
    return 0;
}

Or not use macros at all for this purpose.

answered Nov 20, 2012 at 5:05

Luchian Grigore

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3 Comments

sazr Over a year ago

I cant use a string because I am actually coding in .4dm not pure C which is similar but doesn't allow usage of variables outside main or functions - except for Preprocessor defines

sazr Over a year ago

is there a way I can copy/store a define by value not reference? Maybe a bubble sort type approach with a temporary variable may be my only option.

Luchian Grigore Over a year ago

@JakeM no. Just think of how it works - #define x y everywhere the preprocessor sees x after this, it will replace it with y - not with what y is defined as.

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