Is there a good reason for always enclosing a define in parentheses in C?

Question

Clearly, there are times where #define statements must have parentheses, like so:

#define WIDTH 80+20

int a = WIDTH * 2; // expect a==200 but a         


        

Answer 1

As Blagovest Buyukliev said:

The define consists of a single token (one operand only, no operators), the parentheses are not needed because a single token (such as 100) is an indivisible atom when lexing and parsing.

But I would recommend the following rules when it comes to macros:

1. Avoid function like macros @see Lundin's comment.

If you want to use function like macros badly consider the following 2 rules:

1. Always use brackets for arguments in macros
2. Only use an macro argument once

Why rule 1.? (To keep the order of the operations correct)

#define quad(x) (x*x)
int a = quad(2+3);

will expand to:

int a = (2+3*2+3);

Why rule 2.? (To ensure a side effect is only applied once)

#define quad(x) (x*x)
int i = 1;
int a = quad(i++);

will expand to:

int a = i++ * i++;