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.

Answer 1

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<=10; i++) all the time, you can do the following:

#define fori10 for (int i=0; i<=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<=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:

1. Wikipedia article
2. C++ specific site
3. Documentation on GCC's preprocessor
4. Microsoft reference
5. C specific site (I don't think it's different from the C++ version though)

Answer 2

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.