Why use define instead of a variable

Question

What is the point of  define in C    I ve only seen examples where it s used in place of a  magic number  but I don t see the point in just giving that value to a variable instead

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Most common use  other than to declare constants  is an include guard

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The  define is part of the preprocessor language for C and C     When they re used in code  the compiler just replaces the  define statement with what ever you want   For example  if you re sick of writing for  int i 0  i lt  10  i    all the time  you can do the following    define fori10 for  int i 0  i lt  10  i        some code     fori10          do stuff to i     If you want something more generic  you can create preprocessor macros    define fori x  for  int i 0  i lt  x  i       the x will be replaced by what ever is put into the parenthesis  such as    20 here fori 20           do more stuff to i     It s also very useful for conditional compilation  the other major use for  define  if you only want certain code used in some particular build      compile the following if debugging is turned on and defined  ifdef DEBUG    some code  endif   Most compilers will allow you to define a macro from the command line  e g  g   -DDEBUG something cpp   but you can also just put a define in your code like so    define DEBUG   Some resources    Wikipedia article C   specific site Documentation on GCC s preprocessor Microsoft reference C specific site  I don t think it s different from the C   version though

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The  define allows you to establish a value in a header that would otherwise compile to size-greater-than-zero   Your headers should not compile to size-greater-than-zero      File   MyFile h     This header will compile to size-zero   define TAX RATE 0 625     NO   static const double TAX RATE   0 625     NO   extern const double TAX RATE      WHAT IS THE VALUE    EDIT   As Neil points out in the comment to this post  the explicit definition-with-value in the header would work for C    but not C

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I got in trouble at work one time   I was accused of using  magic numbers  in array declarations     Like this   int Marylyn 256   Ann 1024     The company policy was to avoid these magic numbers because  it was explained to me  that these numbers were not portable  that they impeded easy maintenance   I argued that when I am reading the code  I want to know exactly how big the array is  I lost the argument and so  on a Friday afternoon I replaced the offending  magic numbers  with  defines  like this     define TWO FIFTY SIX 256   define TEN TWENTY FOUR 1024   int Marylyn TWO FIFTY SIX   Ann TEN TWENTY FOUR     On the following Monday afternoon I was called in and accused of having passive defiant tendencies

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C didn t use to have consts  so  defines were the only way of providing constant values  Both C and C   do have them now  so there is no point in using them  except when they are going to be tested with  ifdef ifndef

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Mostly stylistic these days   When C was young  there was no such thing as a const variable   So if you used a variable instead of a  define  you had no guarantee that somebody somewhere wouldn t change the value of it  causing havoc throughout your program   In the old days  FORTRAN passed even constants to subroutines by reference  and it was possible  and headache inducing  to change the value of a constant like  2  to be something different   One time  this happened in a program I was working on  and the only hint we had that something was wrong was we d get an ABEND  abnormal end  when the program hit the STOP 999 that was supposed to end it normally

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define can accomplish some jobs that normal C   cannot  like guarding headers and other tasks  However  it definitely should not be used as a magic number- a static const should be used instead

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Define is evaluated before compilation by the pre-processor  while variables are referenced at run-time  This means you control how your application is built  not how it runs   Here are a couple examples that use define which cannot be replaced by a variable     define min i  j     i   lt   j      i     j   note this is evaluated by the pre-processor  not during runtime http   msdn microsoft com en-us library 8fskxacy aspx

[c++] Why use #define instead of a variable

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