Why are preprocessor macros evil and what are the alternatives?

Question

I have always asked this but I have never received a really good answer; I think that almost any programmer before even writing the first \"Hello World\" had encountered a p

Answer 1

A common trouble is this :

#define DIV(a,b) a / b

printf("25 / (3+2) = %d", DIV(25,3+2));

It will print 10, not 5, because the preprocessor will expand it this way:

printf("25 / (3+2) = %d", 25 / 3 + 2);

This version is safer:

#define DIV(a,b) (a) / (b)