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 val

Answer 1

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

Answer 2

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:

1. #define min(i, j) (((i) < (j)) ? (i) : (j))
   note this is evaluated by the pre-processor, not during runtime

2. http://msdn.microsoft.com/en-us/library/8fskxacy.aspx

Answer 3

#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.

Answer 4

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.

Answer 5

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.

Answer 6

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.